[PATCH] no need to check for NULL before calling kfree() - fs/ext2/

[linux-os]

Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed? I suggest that the check
for a NULL before the call is much less expensive than calling
kfree() and doing the check there. The resulting "double check"
is cheap, compared to the call.

(please keep me on CC)
kfree() handles NULL fine, to check is redundant.
--- linux-2.6.12-rc1-mm3-orig/fs/ext2/acl.c 2005-03-02 08:38:18.000000000 +0100
+++ linux-2.6.12-rc1-mm3/fs/ext2/acl.c 2005-03-25 22:41:07.000000000 +0100
@@ -194,8 +194,7 @@ ext2_get_acl(struct inode *inode, int ty
acl = NULL;
else
acl = ERR_PTR(retval);
- if (value)
- kfree(value);
+ kfree(value);
if (!IS_ERR(acl)) {
switch(type) {
@@ -262,8 +261,7 @@ ext2_set_acl(struct inode *inode, int ty
error = ext2_xattr_set(inode, name_index, "", value, size, 0);
- if (value)
- kfree(value);
+ kfree(value);
if (!error) {
switch(type) {
-
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
Notice : All mail here is now cached for review by Dictator Bush.
98.36% of all statistics are fiction.
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[Jesper Juhl]

Post by linux-os
Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed? I suggest that the check
for a NULL before the call is much less expensive than calling
kfree() and doing the check there. The resulting "double check"
is cheap, compared to the call.

I've been looking at some of the actual code gcc generates for those
checks, and it's quite bloated. My guess is that the reduced memory
footprint, one less branch, and the fact that kfree is probably already in
cache (since it's called often all over the place) outweighs the cost of a
function call - especially in the cases where the pointer is rarely NULL
and we'll end up doing the call in any case.
And the reduced use of screen real-estate is nice as well :)
--
Jesper juhl

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[Pekka Enberg]

Hi,

On Fri, 25 Mar 2005 17:29:56 -0500 (EST), linux-os

Post by linux-os
Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed? I suggest that the check
for a NULL before the call is much less expensive than calling
kfree() and doing the check there. The resulting "double check"
is cheap, compared to the call.

Resource release paths are usually not performance critical. However,
if removing the redundant checks introduce a _measurable_ regressions
in terms of performance, we can make kfree() inline which will take
care of it.

Pekka
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[Arjan van de Ven]

Post by linux-os
Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed?

nope

a call instruction is effectively half a cycle or less, the branch
predictor of the cpu can predict perfectly where the next instruction is
from. The extra if() you do in front is a different matter, that can
easily cost 100 cycles+. (And those are redundant cycles because kfree
will do the if again anyway). So what you propose is to spend 100+
cycles to save half a cycle. Not a good tradeoff ;)

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[Jesper Juhl]

Post by Arjan van de Ven

Post by linux-os
Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed?

nope
a call instruction is effectively half a cycle or less, the branch

Wrong!

Post by Arjan van de Ven
predictor of the cpu can predict perfectly where the next instruction is
from. The extra if() you do in front is a different matter, that can
easily cost 100 cycles+. (And those are redundant cycles because kfree
will do the if again anyway). So what you propose is to spend 100+
cycles to save half a cycle. Not a good tradeoff ;)

Wrong!

[snip]

Always, always, a call will be more expensive than a branch
on condition. It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).
If somebody said; "I think that the code will look better
and the few cycles lost will not be a consequence with modern
CPUs...", then there is a point. But coming up with this
disingenuous bullshit is something else.

I tried to create a test to see what the actual impact of this sort of
change is, the result I reached is below (as well as the code used to
obtain the numbers):

Each test is run 10000000 times, and the number of jiffies spent doing the
kfree();
or
if (p)
kfree(p);
is meassured. Total number of jiffies used for that for all 10000000 runs
is reported.

test 0:
Pointer is NULL half the time, value returned by kmalloc half the
time.
kfree() is called on the pointer without checking for NULL first.

test 1:
Pointer is NULL half the time, value returned by kmalloc half the
time.
The pointer is checked for NULL and kfree() is called on the
pointer only if it is != NULL.

test 2:
Pointer is NULL the majority of the time, only in 1 out of 50
cases is it assigned a real value by kmalloc().
kfree() is called on the pointer without checking for NULL first.

test 3:
Pointer is NULL the majority of the time, only in 1 out of 50
cases is it assigned a real value by kmalloc().
The pointer is checked for NULL and kfree() is called on the
pointer only if it is != NULL.

test 4:
Pointer is rarely NULL - only in 1 out of 50 cases.
kfree() is called on the pointer without checking for NULL first.

test 5:
Pointer is rarely NULL - only in 1 out of 50 cases.
The pointer is checked for NULL and kfree() is called on the
pointer only if it is != NULL.

Here are the numbers from 5 runs on my box - the numbers naturally
differ a bit between each run, but they are quite similar each time :

[ 1395.059375] test 0 used up 235 kfree related jiffies
[ 1395.059385] test 1 used up 195 kfree related jiffies
[ 1395.059389] test 2 used up 66 kfree related jiffies
[ 1395.059392] test 3 used up 20 kfree related jiffies
[ 1395.059395] test 4 used up 366 kfree related jiffies
[ 1395.059398] test 5 used up 428 kfree related jiffies

[ 1412.994705] test 0 used up 231 kfree related jiffies
[ 1412.994744] test 1 used up 209 kfree related jiffies
[ 1412.994748] test 2 used up 68 kfree related jiffies
[ 1412.994751] test 3 used up 12 kfree related jiffies
[ 1412.994754] test 4 used up 362 kfree related jiffies
[ 1412.994757] test 5 used up 392 kfree related jiffies

[ 1423.734356] test 0 used up 245 kfree related jiffies
[ 1423.734366] test 1 used up 179 kfree related jiffies
[ 1423.734370] test 2 used up 78 kfree related jiffies
[ 1423.734373] test 3 used up 30 kfree related jiffies
[ 1423.734376] test 4 used up 384 kfree related jiffies
[ 1423.734379] test 5 used up 385 kfree related jiffies

[ 1434.390194] test 0 used up 242 kfree related jiffies
[ 1434.390203] test 1 used up 179 kfree related jiffies
[ 1434.390207] test 2 used up 70 kfree related jiffies
[ 1434.390210] test 3 used up 16 kfree related jiffies
[ 1434.390214] test 4 used up 365 kfree related jiffies
[ 1434.390217] test 5 used up 397 kfree related jiffies

[ 1446.529856] test 0 used up 231 kfree related jiffies
[ 1446.530046] test 1 used up 232 kfree related jiffies
[ 1446.530117] test 2 used up 79 kfree related jiffies
[ 1446.530211] test 3 used up 16 kfree related jiffies
[ 1446.530278] test 4 used up 360 kfree related jiffies
[ 1446.530362] test 5 used up 412 kfree related jiffies

The conclusions I draw from those numbers are that when NULL pointers are
rare (tests 4 & 5) then it pays off to not have the if() check. When NULL
pointers are common, then there's a small bennefit to having the if()
check, but we are talking ~50 jiffies (or less) over 10 million runs pr
test, which is pretty insignificant unless the code is in a very hot path.
When pointers are NULL 50% of the time there's a bennefit to the if(), but
it's small.
So, unless the code is extremely performance critical *and* the pointer
is NULL more often than not, having the if(pointer != NULL) check before
calling kfree() is pointless and may even be degrading performance if the
pointer is most commonly != NULL. I'd say that the general rule should
be "don't check for NULL first unless you *know* the pointer will be NULL

50% of the time"...

I ran these tests on a 1.4GHz AMD Athlon (T-bird), and with a HZ setting
of 1000.

Am I drawing flawed conclusions here?

If someone could check the sanity of my code used to obtain these numbers
(below), then I'd appreciate it - if the numbers are wrong, then any
conclusion is also wrong of course.

Here's the tiny module I wrote to get the numbers above :

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>

#define NR_TESTS 10000000

void do_work(void *data);

DECLARE_WORK(work, do_work, NULL);

static int test_time[] = {0, 0, 0, 0, 0, 0};

void do_work(void *data)
{
unsigned long j;
static int what_test = 0;
unsigned long start;
void *tmp;

switch (what_test) {
case 0:
for (j = 0; j < NR_TESTS; j++) {
if (j%2 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
kfree(tmp);
test_time[0] += jiffies - start;
}
break;
case 1:
for (j = 0; j < NR_TESTS; j++) {
if (j%2 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (tmp)
kfree(tmp);
test_time[1] += jiffies - start;
}
break;
case 2:
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
kfree(tmp);
test_time[2] += jiffies - start;
}
break;
case 3:
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (tmp)
kfree(tmp);
test_time[3] += jiffies - start;
}
break;
case 4:
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = NULL;
else
tmp = kmalloc(1, GFP_KERNEL);
start = jiffies;
kfree(tmp);
test_time[4] += jiffies - start;
}
break;
case 5:
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = NULL;
else
tmp = kmalloc(1, GFP_KERNEL);
start = jiffies;
if (tmp)
kfree(tmp);
test_time[5] += jiffies - start;
}
break;
default:
break;
}
printk(KERN_ALERT "test %d done.\n", what_test);

if (what_test < 5)
schedule_delayed_work(&work, 1);
else
printk(KERN_ALERT "All tests done...\n");

what_test++;
}

static int kfreetest_init(void)
{
schedule_work(&work);
return 0;
}

static void kfreetest_exit(void)
{
int i;

cancel_delayed_work(&work);
flush_scheduled_work();
for (i = 0; i < 6; i++)
printk(KERN_ALERT "test %d used up %d kfree related jiffies\n", i, test_time[i]);
}

module_init(kfreetest_init);
module_exit(kfreetest_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Jesper Juhl");
--
Jesper Juhl <juhl-***@dif.dk>

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[Lee Revell]

Post by Jesper Juhl
I'd say that the general rule should
be "don't check for NULL first unless you *know* the pointer will be NULL

50% of the time"...

How about running the same tests but using likely()/unlikely() for the
'1 in 50' cases?

Lee

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[Jesper Juhl]

Post by Lee Revell

Post by Jesper Juhl
I'd say that the general rule should
be "don't check for NULL first unless you *know* the pointer will be NULL

50% of the time"...

How about running the same tests but using likely()/unlikely() for the
'1 in 50' cases?

I added likely() and unlikely() to all tests, here are the results from 3
runs on my box :

[ 4379.223858] starting test : 50/50 NULL pointers, kfree(p)
[ 4379.785541] Test done. This test used up 240 kfree related jiffies
[ 4379.785543] -------------------------
[ 4379.884863] starting test : 50/50 NULL pointers, if(p) kfree(p)
[ 4380.609537] Test done. This test used up 221 kfree related jiffies
[ 4380.609539] -------------------------
[ 4380.709285] starting test : 50/50 NULL pointers, if(likely(p)) kfree(p)
[ 4381.241958] Test done. This test used up 237 kfree related jiffies
[ 4381.241961] -------------------------
[ 4381.341843] starting test : 50/50 NULL pointers, if(unlikely(p)) kfree(p)
[ 4381.874492] Test done. This test used up 261 kfree related jiffies
[ 4381.874495] -------------------------
[ 4381.974396] starting test : 49 out of 50 pointers == NULL, kfree(p)
[ 4382.239784] Test done. This test used up 87 kfree related jiffies
[ 4382.239787] -------------------------
[ 4382.339138] starting test : 49 out of 50 pointers == NULL, if(p) kfree(p)
[ 4382.519165] Test done. This test used up 22 kfree related jiffies
[ 4382.519167] -------------------------
[ 4382.618944] starting test : 49 out of 50 pointers == NULL, if(likely(p)) kfree(p)
[ 4382.798832] Test done. This test used up 18 kfree related jiffies
[ 4382.798834] -------------------------
[ 4382.898746] starting test : 49 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)
[ 4383.079062] Test done. This test used up 26 kfree related jiffies
[ 4383.079065] -------------------------
[ 4383.178549] starting test : 1 out of 50 pointers == NULL, kfree(p)
[ 4384.187707] Test done. This test used up 365 kfree related jiffies
[ 4384.187710] -------------------------
[ 4384.286769] starting test : 1 out of 50 pointers == NULL, if(p) kfree(p)
[ 4385.351731] Test done. This test used up 438 kfree related jiffies
[ 4385.351733] -------------------------
[ 4385.450951] starting test : 1 out of 50 pointers == NULL, if(likely(p)) kfree(p)
[ 4386.480408] Test done. This test used up 378 kfree related jiffies
[ 4386.480410] -------------------------
[ 4386.580161] starting test : 1 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)
[ 4387.630779] Test done. This test used up 432 kfree related jiffies
[ 4387.630782] -------------------------

[ 4614.027356] starting test : 50/50 NULL pointers, kfree(p)
[ 4614.589174] Test done. This test used up 258 kfree related jiffies
[ 4614.589177] -------------------------
[ 4614.688741] starting test : 50/50 NULL pointers, if(p) kfree(p)
[ 4615.409793] Test done. This test used up 252 kfree related jiffies
[ 4615.409795] -------------------------
[ 4615.509165] starting test : 50/50 NULL pointers, if(likely(p)) kfree(p)
[ 4616.041816] Test done. This test used up 200 kfree related jiffies
[ 4616.041818] -------------------------
[ 4616.141720] starting test : 50/50 NULL pointers, if(unlikely(p)) kfree(p)
[ 4616.678002] Test done. This test used up 223 kfree related jiffies
[ 4616.678005] -------------------------
[ 4616.777275] starting test : 49 out of 50 pointers == NULL, kfree(p)
[ 4617.042512] Test done. This test used up 91 kfree related jiffies
[ 4617.042514] -------------------------
[ 4617.142017] starting test : 49 out of 50 pointers == NULL, if(p) kfree(p)
[ 4617.322044] Test done. This test used up 24 kfree related jiffies
[ 4617.322047] -------------------------
[ 4617.421820] starting test : 49 out of 50 pointers == NULL, if(likely(p)) kfree(p)
[ 4617.601710] Test done. This test used up 29 kfree related jiffies
[ 4617.601713] -------------------------
[ 4617.701625] starting test : 49 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)
[ 4617.882083] Test done. This test used up 27 kfree related jiffies
[ 4617.882085] -------------------------
[ 4617.981427] starting test : 1 out of 50 pointers == NULL, kfree(p)
[ 4618.990599] Test done. This test used up 355 kfree related jiffies
[ 4618.990601] -------------------------
[ 4619.089646] starting test : 1 out of 50 pointers == NULL, if(p) kfree(p)
[ 4620.154737] Test done. This test used up 388 kfree related jiffies
[ 4620.154740] -------------------------
[ 4620.253829] starting test : 1 out of 50 pointers == NULL, if(likely(p)) kfree(p)
[ 4621.283279] Test done. This test used up 372 kfree related jiffies
[ 4621.283282] -------------------------
[ 4621.383035] starting test : 1 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)
[ 4622.442580] Test done. This test used up 468 kfree related jiffies
[ 4622.442583] -------------------------

[ 4673.948568] starting test : 50/50 NULL pointers, kfree(p)
[ 4674.513874] Test done. This test used up 257 kfree related jiffies
[ 4674.513877] -------------------------
[ 4674.613603] starting test : 50/50 NULL pointers, if(p) kfree(p)
[ 4675.338429] Test done. This test used up 256 kfree related jiffies
[ 4675.338432] -------------------------
[ 4675.438022] starting test : 50/50 NULL pointers, if(likely(p)) kfree(p)
[ 4675.970685] Test done. This test used up 209 kfree related jiffies
[ 4675.970687] -------------------------
[ 4676.070575] starting test : 50/50 NULL pointers, if(unlikely(p)) kfree(p)
[ 4676.603217] Test done. This test used up 233 kfree related jiffies
[ 4676.603219] -------------------------
[ 4676.703132] starting test : 49 out of 50 pointers == NULL, kfree(p)
[ 4676.968502] Test done. This test used up 82 kfree related jiffies
[ 4676.968504] -------------------------
[ 4677.067877] starting test : 49 out of 50 pointers == NULL, if(p) kfree(p)
[ 4677.247945] Test done. This test used up 17 kfree related jiffies
[ 4677.247948] -------------------------
[ 4677.347675] starting test : 49 out of 50 pointers == NULL, if(likely(p)) kfree(p)
[ 4677.527564] Test done. This test used up 29 kfree related jiffies
[ 4677.527566] -------------------------
[ 4677.627480] starting test : 49 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)
[ 4677.807935] Test done. This test used up 19 kfree related jiffies
[ 4677.807937] -------------------------
[ 4677.907282] starting test : 1 out of 50 pointers == NULL, kfree(p)
[ 4678.916434] Test done. This test used up 404 kfree related jiffies
[ 4678.916437] -------------------------
[ 4679.015503] starting test : 1 out of 50 pointers == NULL, if(p) kfree(p)
[ 4680.087547] Test done. This test used up 423 kfree related jiffies
[ 4680.087549] -------------------------
[ 4680.186681] starting test : 1 out of 50 pointers == NULL, if(likely(p)) kfree(p)
[ 4681.209136] Test done. This test used up 373 kfree related jiffies
[ 4681.209139] -------------------------
[ 4681.308891] starting test : 1 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)
[ 4682.366623] Test done. This test used up 449 kfree related jiffies
[ 4682.366625] -------------------------

And here's the source for the module that generated the above :

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/compiler.h>
#include <linux/workqueue.h>

#define NR_TESTS 10000000

void do_kfreetest_work(void *data);

DECLARE_WORK(kfree_work, do_kfreetest_work, NULL);

static int test_time[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

void do_kfreetest_work(void *data)
{
unsigned long j;
static int what_test = 0;
unsigned long start;
void *tmp;

printk(KERN_ALERT "starting test : ");
switch (what_test) {
case 0:
printk("50/50 NULL pointers, kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%2 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 1:
printk("50/50 NULL pointers, if(p) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%2 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (tmp)
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 2:
printk("50/50 NULL pointers, if(likely(p)) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%2 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (likely(tmp))
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 3:
printk("50/50 NULL pointers, if(unlikely(p)) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%2 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (unlikely(tmp))
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 4:
printk("49 out of 50 pointers == NULL, kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 5:
printk("49 out of 50 pointers == NULL, if(p) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (tmp)
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 6:
printk("49 out of 50 pointers == NULL, if(likely(p)) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (likely(tmp))
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 7:
printk("49 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = kmalloc(1, GFP_KERNEL);
else
tmp = NULL;
start = jiffies;
if (unlikely(tmp))
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 8:
printk("1 out of 50 pointers == NULL, kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = NULL;
else
tmp = kmalloc(1, GFP_KERNEL);
start = jiffies;
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 9:
printk("1 out of 50 pointers == NULL, if(p) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = NULL;
else
tmp = kmalloc(1, GFP_KERNEL);
start = jiffies;
if (tmp)
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 10:
printk("1 out of 50 pointers == NULL, if(likely(p)) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = NULL;
else
tmp = kmalloc(1, GFP_KERNEL);
start = jiffies;
if (likely(tmp))
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
case 11:
printk("1 out of 50 pointers == NULL, if(unlikely(p)) kfree(p)\n");
for (j = 0; j < NR_TESTS; j++) {
if (j%50 == 0)
tmp = NULL;
else
tmp = kmalloc(1, GFP_KERNEL);
start = jiffies;
if (unlikely(tmp))
kfree(tmp);
test_time[what_test] += jiffies - start;
}
break;
default:
printk(KERN_ALERT "hit default\n");
break;
}
printk(KERN_ALERT "Test done. This test used up %d kfree related jiffies\n-------------------------\n", test_time[what_test]);

if (what_test < 11)
schedule_delayed_work(&kfree_work, 100);
else
printk(KERN_ALERT "All tests done.....\n-----------------------------------\n");

what_test++;
}

static int kfreetest_init(void)
{
schedule_work(&kfree_work);
return 0;
}

static void kfreetest_exit(void)
{
cancel_delayed_work(&kfree_work);
flush_scheduled_work();
}

module_init(kfreetest_init);
module_exit(kfreetest_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Jesper Juhl");

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[Paul Jackson]

Post by Jesper Juhl
I added likely() and unlikely() to all tests, here are the results from 3

Any chance you could summarize what these results are, for those
of us too lazy to parse it all out? The time spent by one author
to summarize in English what the numbers state can save the time of
a hundred readers each individually having to parse the numbers.

Just looking at the third run, it seems to me that "if (likely(p))
kfree(p);" beats a naked "kfree(p);" everytime, whether p is half
NULL's, or very few NULL's, or almost all NULL's.

If I'm reading this right, and if these results are valid, then we are
going about this optimization all wrong, at least if your CPU is an
AMD Athlon (T-bird). Weird. Instead of stripping the "if (p)" test, we
should be changing it to "if (likely(p))", regardless of whether it
is very likely, or unlikely, or in between. That is not what I would
call intuitive.
--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <***@engr.sgi.com> 1.650.933.1373, 1.925.600.0401
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[Jan Engelhardt]

Post by Paul Jackson
Just looking at the third run, it seems to me that "if (likely(p))
kfree(p);" beats a naked "kfree(p);" everytime, whether p is half
NULL's, or very few NULL's, or almost all NULL's.

Well, kfree inlined was already mentioned but forgotten again.
What if this was used:

inline static void kfree_WRAP(void *addr) {
if(likely(addr != NULL)) {
kfree_real(addr);
}
return;
}

And remove the NULL-test in kfree_real()? Then we would have:

test eax, eax
jz afterwards;
<some more stuff for call>
call kfree_real;
.afterwards:
<continue execution>

The two cases then:
ptr==NULL: test-jmp
ptr!=NULL: test-call(freeit-return)

Looks like the least expensive way to me.

Jan Engelhardt
--
No TOFU for me, please.
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[Dave Jones]

Post by Jan Engelhardt
Well, kfree inlined was already mentioned but forgotten again.
inline static void kfree_WRAP(void *addr) {
if(likely(addr != NULL)) {
kfree_real(addr);
}
return;
}

Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats
almost never in a path that needs optimising ?
I'd be amazed if any of this masturbation showed the tiniest
blip on a real workload, or even on a benchmark other than
one crafted specifically to test kfree in a loop.

That each occurance of this 'optimisation' also saves a handful
of bytes in generated code is it's only real benefit afaics.
Even then, if a functions cache performance is better off because
we trimmed a few bytes from the tail of a function, I'd be
completely amazed.

I guess April 1st came early this year.

Dave

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[Jan Engelhardt]

Post by Dave Jones
Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats
almost never in a path that needs optimising ?
I'd be amazed if any of this masturbation showed the tiniest
blip on a real workload, or even on a benchmark other than
one crafted specifically to test kfree in a loop.
[...]

Was not me who started it :P

Post by Dave Jones
I guess April 1st came early this year.

DST change for Europe and Australia is a day ahead- hm, does not suffice to
get onto April 1.

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[Pekka Enberg]

Post by Dave Jones
Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats
almost never in a path that needs optimising ?
I'd be amazed if any of this masturbation showed the tiniest
blip on a real workload, or even on a benchmark other than
one crafted specifically to test kfree in a loop.

Indeed. The NULL checks are redundant and thus need to go. If someone
can show a measurable performance regression in the kernel for a
realistic workload, kfree() can be turned into an inline function
which will take care of it. The microbenchmarks are fun but should not
stand in the way of merging the kfree() cleanups from Jesper and
others.

Pekka
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[Jesper Juhl]

Post by Dave Jones
Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats
almost never in a path that needs optimising ?
I'd be amazed if any of this masturbation showed the tiniest
blip on a real workload, or even on a benchmark other than
one crafted specifically to test kfree in a loop.
That each occurance of this 'optimisation' also saves a handful
of bytes in generated code is it's only real benefit afaics.
Even then, if a functions cache performance is better off because
we trimmed a few bytes from the tail of a function, I'd be
completely amazed.

I agree, it's amazing that this is treated like a big issue, it's not, and
I never meant for it to be a matter of such debate.

The whole thing (viewed from where I'm sitting) started when I noticed a
few of those redundant NULL checks while reading code, thought I'd clean
them up since they were clearly not needed and submit those patches. When
those patches then got merged I thought "ok, so this is something that's
actually appreciated, guess I might as well do some more when I come
across them or maybe even seek them out and get rid of them once and for
all"... So I started doing that and more of the patches got merged which
(at least to me) confirmed that it was a worthwhile activity, until at
some point voices were raised in objection.

At that point I felt I needed to explain the "why" of why I was doing it
and try and show that it might actually be a bennefit (and I believe the
small test I wrote shows it to be either a bennefit in the usual case or
at worst a trivial performance hit in the not-so-common case).
What I'm trying to find out now is if there's a general consensus that
these patches are worthwile and wanted or if they are unwanted and I'm
wasting my time. If the patches are wanted I don't mind doing them, but
if they are not wanted I don't want to waste my time (nor anyone elses) on
them. So, if I could just get peoples comment on that "wanted vs
not-wanted" then I could get on with either producing some patches for
people or get on with other things and drop this... Or I guess I could
just go on making those patches, submit them and then just leave it in the
hands of the individual maintainers (which was more or less how I started
out)...
--
Jesper Juhl

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[Paul Jackson]

Post by Jesper Juhl
What I'm trying to find out now is if there's a general consensus that
these patches are worthwile and wanted or if they are unwanted and I'm
wasting my time.

Thanks for your good work so far, and your good natured tolerance of
the excessively detailed discussions resulting.

I don't have a big preference either way - but a couple of questions:

1) Do you have any wild guess of how many of these you've done so far,
and how many potentially remain that could be done? Tens, hundreds,
thousands?

2) Any idea what was going on with the numbers you posted yesterday,
which, at least from what I saw at first glance, seemed to show
"if (likely(p)) kfree(p);" to be a good or best choice, for all
cases, including (p != NULL) being very unlikely? That seemed
like a weird result.

If the use of "likely(p)" is almost always a winner, then the changes
you've been doing, followed by a header file change:

static inline void kfree(const void *p)
{
if (likely(p))
__kfree(p); /* __kfree(p) doesn't check for NULL p */
}

along the lines that Jan considered a few posts ago, might be a winner.

But since this "likely(p)" result seems so bizarre, it seems unlikely that
the above kfree wrapper would be a winner.
--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <***@engr.sgi.com> 1.650.933.1373, 1.925.600.0401
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[Paul Jackson]

Post by Dave Jones
Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats

Eh .. it's well known behaviour. Bring two questions before a
committee, one for millions of dollars and the other for pocket change,
and watch the committee spend more time discussing the second question.
--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <***@engr.sgi.com> 1.650.933.1373, 1.925.600.0401
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[Geert Uytterhoeven]

Post by Dave Jones

Post by Jan Engelhardt
Well, kfree inlined was already mentioned but forgotten again.
inline static void kfree_WRAP(void *addr) {
if(likely(addr != NULL)) {
kfree_real(addr);
}
return;
}

Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats
almost never in a path that needs optimising ?
I'd be amazed if any of this masturbation showed the tiniest
blip on a real workload, or even on a benchmark other than
one crafted specifically to test kfree in a loop.

The benchmarks were started when someone noticed one of the tests was (a) not
in a cleanup path and (b) very unlikely to be true.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ***@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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[Lee Revell]

Post by Dave Jones

Post by Jan Engelhardt
Well, kfree inlined was already mentioned but forgotten again.
inline static void kfree_WRAP(void *addr) {
if(likely(addr != NULL)) {
kfree_real(addr);
}
return;
}

Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats
almost never in a path that needs optimising ?
I'd be amazed if any of this masturbation showed the tiniest
blip on a real workload, or even on a benchmark other than
one crafted specifically to test kfree in a loop.

I see kfree used in several hot paths. Check out
this /proc/latency_trace excerpt:

(T1/#147) ksoftirqd/0 2 0 2 00000002 00000093 [0038603486826120] 0.133ms (+0.000ms): preempt_schedule+0x11/0x80 <c02b5181> (rpc_wake_up_task+0x6c/0x80 <c02a802c>)
(T1/#148) ksoftirqd/0 2 0 2 00000001 00000094 [0038603486826375] 0.133ms (+0.000ms): preempt_schedule+0x11/0x80 <c02b5181> (udp_data_ready+0x1ca/0x270 <c02a5b8a>)
(T1/#149) ksoftirqd/0 2 0 2 00000001 00000095 [0038603486826527] 0.133ms (+0.000ms): skb_free_datagram+0xb/0x30 <c0257ecb> (udp_data_ready+0x19c/0x270 <c02a5b5c>)
(T1/#150) ksoftirqd/0 2 0 2 00000001 00000096 [0038603486826686] 0.133ms (+0.000ms): __kfree_skb+0xe/0xf0 <c0255bce> (udp_data_ready+0x19c/0x270 <c02a5b5c>)
(T1/#151) ksoftirqd/0 2 0 2 00000001 00000097 [0038603486826924] 0.133ms (+0.000ms): sock_rfree+0x8/0x20 <c0254648> (__kfree_skb+0x6b/0xf0 <c0255c2b>)
(T1/#152) ksoftirqd/0 2 0 2 00000001 00000098 [0038603486827082] 0.134ms (+0.000ms): kfree_skbmem+0xe/0x30 <c0255b9e> (__kfree_skb+0x76/0xf0 <c0255c36>)
(T1/#153) ksoftirqd/0 2 0 2 00000001 00000099 [0038603486827189] 0.134ms (+0.000ms): skb_release_data+0xd/0xd0 <c0255acd> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#154) ksoftirqd/0 2 0 2 00000001 0000009a [0038603486827444] 0.134ms (+0.000ms): skb_drop_fraglist+0xc/0x50 <c0255a4c> (skb_release_data+0xa3/0xd0 <c0255b63>)
(T1/#155) ksoftirqd/0 2 0 2 00000001 0000009b [0038603486827573] 0.134ms (+0.000ms): __kfree_skb+0xe/0xf0 <c0255bce> (skb_drop_fraglist+0x35/0x50 <c0255a75>)
(T1/#156) ksoftirqd/0 2 0 2 00000001 0000009c [0038603486827733] 0.134ms (+0.000ms): kfree_skbmem+0xe/0x30 <c0255b9e> (__kfree_skb+0x76/0xf0 <c0255c36>)
(T1/#157) ksoftirqd/0 2 0 2 00000001 0000009d [0038603486827861] 0.134ms (+0.000ms): skb_release_data+0xd/0xd0 <c0255acd> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#158) ksoftirqd/0 2 0 2 00000001 0000009e [0038603486828121] 0.134ms (+0.000ms): kfree+0x14/0x70 <c0143514> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#159) ksoftirqd/0 2 0 2 00000001 0000009f [0038603486828671] 0.135ms (+0.000ms): kmem_cache_free+0x14/0x60 <c0143444> (kfree_skbmem+0x2a/0x30 <c0255bba>)
(T1/#160) ksoftirqd/0 2 0 2 00000001 000000a0 [0038603486829127] 0.135ms (+0.000ms): __kfree_skb+0xe/0xf0 <c0255bce> (skb_drop_fraglist+0x35/0x50 <c0255a75>)
(T1/#161) ksoftirqd/0 2 0 2 00000001 000000a1 [0038603486829341] 0.135ms (+0.000ms): kfree_skbmem+0xe/0x30 <c0255b9e> (__kfree_skb+0x76/0xf0 <c0255c36>)
(T1/#162) ksoftirqd/0 2 0 2 00000001 000000a2 [0038603486829469] 0.135ms (+0.000ms): skb_release_data+0xd/0xd0 <c0255acd> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#163) ksoftirqd/0 2 0 2 00000001 000000a3 [0038603486829644] 0.135ms (+0.000ms): kfree+0x14/0x70 <c0143514> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#164) ksoftirqd/0 2 0 2 00000001 000000a4 [0038603486829944] 0.136ms (+0.000ms): kmem_cache_free+0x14/0x60 <c0143444> (kfree_skbmem+0x2a/0x30 <c0255bba>)
(T1/#165) ksoftirqd/0 2 0 2 00000001 000000a5 [0038603486830243] 0.136ms (+0.000ms): __kfree_skb+0xe/0xf0 <c0255bce> (skb_drop_fraglist+0x35/0x50 <c0255a75>)
(T1/#166) ksoftirqd/0 2 0 2 00000001 000000a6 [0038603486830463] 0.136ms (+0.000ms): kfree_skbmem+0xe/0x30 <c0255b9e> (__kfree_skb+0x76/0xf0 <c0255c36>)
(T1/#167) ksoftirqd/0 2 0 2 00000001 000000a7 [0038603486830589] 0.136ms (+0.000ms): skb_release_data+0xd/0xd0 <c0255acd> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#168) ksoftirqd/0 2 0 2 00000001 000000a8 [0038603486830800] 0.136ms (+0.000ms): kfree+0x14/0x70 <c0143514> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#169) ksoftirqd/0 2 0 2 00000001 000000a9 [0038603486831083] 0.137ms (+0.000ms): kmem_cache_free+0x14/0x60 <c0143444> (kfree_skbmem+0x2a/0x30 <c0255bba>)
(T1/#170) ksoftirqd/0 2 0 2 00000001 000000aa [0038603486831394] 0.137ms (+0.000ms): __kfree_skb+0xe/0xf0 <c0255bce> (skb_drop_fraglist+0x35/0x50 <c0255a75>)
(T1/#171) ksoftirqd/0 2 0 2 00000001 000000ab [0038603486831608] 0.137ms (+0.000ms): kfree_skbmem+0xe/0x30 <c0255b9e> (__kfree_skb+0x76/0xf0 <c0255c36>)
(T1/#172) ksoftirqd/0 2 0 2 00000001 000000ac [0038603486831736] 0.137ms (+0.000ms): skb_release_data+0xd/0xd0 <c0255acd> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#173) ksoftirqd/0 2 0 2 00000001 000000ad [0038603486831985] 0.137ms (+0.000ms): kfree+0x14/0x70 <c0143514> (kfree_skbmem+0x19/0x30 <c0255ba9>)
(T1/#174) ksoftirqd/0 2 0 2 00000001 000000ae [0038603486832443] 0.138ms (+0.000ms): kmem_cache_free+0x14/0x60 <c0143444> (kfree_skbmem+0x2a/0x30 <c0255bba>)
(T1/#175) ksoftirqd/0 2 0 2 00000001 000000af [0038603486832744] 0.138ms (+0.000ms): __kfree_skb+0xe/0xf0 <c0255bce> (skb_drop_fraglist+0x35/0x50 <c0255a75>)
(T1/#176) ksoftirqd/0 2 0 2 00000001 000000b0 [0038603486832880] 0.138ms (+0.000ms): kfree_skbmem+0xe/0x30 <c0255b9e> (__kfree_skb+0x76/0xf0 <c0255c36>)

Lee

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[Pekka Enberg]

Post by Lee Revell
I see kfree used in several hot paths. Check out

Yes, but is the pointer being free'd NULL most of the time? The
optimization does not help if you are releasing actual memory.

Pekka
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[Jan Engelhardt]

Post by Pekka Enberg

Post by Lee Revell
I see kfree used in several hot paths. Check out

Yes, but is the pointer being free'd NULL most of the time?

"[...]In general, you should prefer to use actual profile feedback for this
(`-fprofile-arcs'), as programmers are NOTORIOUSLY BAD AT PREDICTING how
their programs actually perform." --gcc info pages.

Post by Pekka Enberg
The optimization does not help if you are releasing actual memory.

It does not turn the real case (releasing memory) worse, but just improves the
unreal case (releasing NULL).

Jan Engelhardt
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[Pekka J Enberg]

Hi,

Post by Jan Engelhardt
"[...]In general, you should prefer to use actual profile feedback for this
(`-fprofile-arcs'), as programmers are NOTORIOUSLY BAD AT PREDICTING how
their programs actually perform." --gcc info pages.

Indeed.

Post by Jan Engelhardt

Post by Pekka Enberg
The optimization does not help if you are releasing actual memory.

It does not turn the real case (releasing memory) worse, but just improves the
unreal case (releasing NULL).

You don't know that until you profile! Please note that it _can_ turn the
real case worse as the generated code will be bigger (assuming we inline
kfree() to optimize the special case). To summarize:

(1) The optimization only helps when the passed pointer is NULL.
(2) Most of the time, kfree() _should_ be given a real pointer.
Anything else but sounds quite broken.
(3) We don't know if inlining kfree() hurts the common case.
(4) The cleanups Jesper and others are doing are to remove the
_redundant_ NULL checks (i.e. it is now checked twice).

Therefore please keep merging the cleanup patches and don't inline kfree()
unless someone can show a _globally visible_ performance regression (i.e. p%
slowdown in XYZ benchmark).

Pekka

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[Paul Jackson]

Jan - please don't trim the CC lists when responding on lkml.
Thanks.
--
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[Marcin Dalecki]

Always, always, a call will be more expensive than a branch
on condition. It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

Needless to say that there are enough architectures out there, which
don't even
have something like an explicit call as separate assembler
instruction...

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[linux-os]

Post by Marcin Dalecki

Always, always, a call will be more expensive than a branch
on condition. It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

Needless to say that there are enough architectures out there, which
don't even
have something like an explicit call as separate assembler
instruction...

Yes, they break the 'call' into seperate expensive operations like
loading the IP address that will exist after the call into a register
storing that in a dedicated register, used as a "stack", then
branching to the called procedure with another indirection, etc.
Cheers,
Dick Johnson
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[linux-os]

Post by Arjan van de Ven

Post by linux-os
Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed?

nope
a call instruction is effectively half a cycle or less, the branch

Wrong!

Post by Arjan van de Ven
predictor of the cpu can predict perfectly where the next instruction is
from. The extra if() you do in front is a different matter, that can
easily cost 100 cycles+. (And those are redundant cycles because kfree
will do the if again anyway). So what you propose is to spend 100+
cycles to save half a cycle. Not a good tradeoff ;)

Wrong!

Pure unmitigated bull-shit. I measure (with hardware devices)
the execution time of real code in modern CPUs. I do this for
a living so you don't have to stand in line for a couple of
hours to have your baggage scanned at the airport.

Always, always, a call will be more expensive than a branch
on condition. It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

If somebody said; "I think that the code will look better
and the few cycles lost will not be a consequence with modern
CPUs...", then there is a point. But coming up with this
disingenuous bullshit is something else.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
Notice : All mail here is now cached for review by Dictator Bush.
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[Arjan van de Ven]

Post by Arjan van de Ven

Post by linux-os
Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed?

nope
a call instruction is effectively half a cycle or less, the branch

Wrong!

oh? a call is "push eip + a new eip" effectively. the new eip is
entirely free, the push eip takes half a cycle (or 1 full cycle but only
one of the two/three pipelines).

Post by Arjan van de Ven
predictor of the cpu can predict perfectly where the next instruction is
from. The extra if() you do in front is a different matter, that can
easily cost 100 cycles+. (And those are redundant cycles because kfree
will do the if again anyway). So what you propose is to spend 100+
cycles to save half a cycle. Not a good tradeoff ;)

Wrong!

Is it wrong that the cpu can predict the target perfectly? No. Unless
you use function pointers (then it's a whole different ballgame).

Pure unmitigated bull-shit. I measure (with hardware devices)
the execution time of real code in modern CPUs. I do this for
a living so you don't have to stand in line for a couple of
hours to have your baggage scanned at the airport.

Ok I used to do this kind of performance work for a living too and
measuring it to death as well.

Always, always, a call will be more expensive than a branch
on condition.

It is not on modern Out of order cpus.

It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

and it's a so common pattern that it's optimized to death. Internally a
call gets transformed to 2 uops or so, one is push eip, the other is the
jmp (which gets then just absorbed by the "what is the next eip" logic,
just as a "jmp"s are 0 cycles)

If somebody said; "I think that the code will look better
and the few cycles lost will not be a consequence with modern
CPUs...", then there is a point. But coming up with this
disingenuous bullshit is something else.

I don't have to take this from you and I don't. You're calling me a liar
with zero evidence. Lets get some facts straight
1) On a modern cpu, a miss of the branch predictor is quite expensive.
The entire pipeline needs flushing if this happens, and on a p4 this
will be in the order of 50 to 100 cycles at minimum.
2) absolute "jmp" is free on modern OOO cpus. Instead of taking an
actual execution slot, all that happens is that the "what is the next
EIP" logic gets a different value. (you can argue what happens if you
have a sequence of jmps and that it's not free then, and I'll grant
you that, but that corner case is not relevant here)
3) a "call" instruction gets translated into what basically is
"push EIP" and "jmp" uops.
4) modern processors have special logic to optimize push/pop
instructions; for example a "push eax ; push ebx" sequence will
execute in parallel in the same cycle even though there is a data
dependency on esp, the cpu can perfectly predict the esp effect and
will do so.
5) modern processors have a call/ret fifo cache they use to do branch
prediction for the target of "ret" instructions. Unless you do
misbalanced call/ret pairs the prediction will be perfect.

Based on this the conclusion "a function call is really cheap versus a
conditional branch" is justified imo. Now you better come with proof
about which of the 5 things above I'm totally lying to you or you better
come with an apology.



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[Denis Vlasenko]

Post by Arjan van de Ven

It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

and it's a so common pattern that it's optimized to death. Internally a
call gets transformed to 2 uops or so, one is push eip, the other is the
jmp (which gets then just absorbed by the "what is the next eip" logic,
just as a "jmp"s are 0 cycles)

Arjan, you overlook the fact that kfree() contains 'if(!p) return;' too.
call + test-and-branch can never be faster than test+and+branch.
Maybe on the really clever CPU it can take the same time, but not faster...
--
vda

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[Arjan van de Ven]

Post by Denis Vlasenko

Post by Arjan van de Ven

It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

and it's a so common pattern that it's optimized to death. Internally a
call gets transformed to 2 uops or so, one is push eip, the other is the
jmp (which gets then just absorbed by the "what is the next eip" logic,
just as a "jmp"s are 0 cycles)

Arjan, you overlook the fact that kfree() contains 'if(!p) return;' too.
call + test-and-branch can never be faster than test+and+branch

ok so for the non-null case you have

test-nbranch-call-test-nbranch
vs
call-test-nbranch

vs the null case where you get
test-branch
vs
call-test-branch

(I'm using nbranch here as a non-taken branch; it's also a conditional
branch and it has the same misprediction possibility)

in the non-null case with if you have *two* chances for the branch
predictor to go wrong. (and "wrong" can also mean "cold, eg unknown"
here) and always an extra "test-nbranch" sequence, which is probably a
cycle at least

the offset for that is the null-case-without-if where you have an extra
"call", which is also half to a whole cycle.

even in the null case it's dubious if there is gain, it depends on how
the branch predictor happens to feel that day ;)

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[linux-os]

Post by Arjan van de Ven

Post by Arjan van de Ven

Post by linux-os
Isn't it expensive of CPU time to call kfree() even though the
pointer may have already been freed?

nope
a call instruction is effectively half a cycle or less, the branch

Wrong!

oh? a call is "push eip + a new eip" effectively. the new eip is
entirely free, the push eip takes half a cycle (or 1 full cycle but only
one of the two/three pipelines).

Post by Arjan van de Ven
predictor of the cpu can predict perfectly where the next instruction is
from. The extra if() you do in front is a different matter, that can
easily cost 100 cycles+. (And those are redundant cycles because kfree
will do the if again anyway). So what you propose is to spend 100+
cycles to save half a cycle. Not a good tradeoff ;)

Wrong!

Is it wrong that the cpu can predict the target perfectly? No. Unless
you use function pointers (then it's a whole different ballgame).

Pure unmitigated bull-shit. I measure (with hardware devices)
the execution time of real code in modern CPUs. I do this for
a living so you don't have to stand in line for a couple of
hours to have your baggage scanned at the airport.

Ok I used to do this kind of performance work for a living too and
measuring it to death as well.

Always, always, a call will be more expensive than a branch
on condition.

It is not on modern Out of order cpus.

It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

and it's a so common pattern that it's optimized to death. Internally a
call gets transformed to 2 uops or so, one is push eip, the other is the
jmp (which gets then just absorbed by the "what is the next eip" logic,
just as a "jmp"s are 0 cycles)

If somebody said; "I think that the code will look better
and the few cycles lost will not be a consequence with modern
CPUs...", then there is a point. But coming up with this
disingenuous bullshit is something else.

I don't have to take this from you and I don't. You're calling me a liar
with zero evidence. Lets get some facts straight
1) On a modern cpu, a miss of the branch predictor is quite expensive.
The entire pipeline needs flushing if this happens, and on a p4 this
will be in the order of 50 to 100 cycles at minimum.

No. It depends upon how the branch prediction is done. If there are
two virtual logic-units, one that has already taken the result of
the TRUE condition the other that has already taken the result of
the FALSE condition, then the IP is set to the address of one
or the other, regardless of whatever is taken. Both of these
IPs are already in the cache because the branch logic makes
sure they are before the conditional test occurs. The possible loss
of performance occurs if there is another conditional branch that
must occur before the logic-units pipelines can be refilled.

This can happen if the tests are conducted like:

cmpl $NUMBER, %eax
jbe 1f # First branch ZF or CF
...... # Other code
...... # Other code
1: jc 2f # Immediate second branch on CF

Most compilers don't produce code like that, but they could.
The loss of performance is only the loss of the branch predictor,
for the second branch.

Post by Arjan van de Ven
2) absolute "jmp" is free on modern OOO cpus. Instead of taking an
actual execution slot, all that happens is that the "what is the next
EIP" logic gets a different value. (you can argue what happens if you
have a sequence of jmps and that it's not free then, and I'll grant
you that, but that corner case is not relevant here)

So? I never even mentioned a jump and, falling through after
a conditional test is not a jump, it's just executing the
next instruction.

Post by Arjan van de Ven
3) a "call" instruction gets translated into what basically is
"push EIP" and "jmp" uops.

No, It can't be on machines that have a "call" instruction.
This is because, in cases where you use the same stack for
access to data, one needs to maintain the coherency of the
stack. If I call 'kfree', I do:

pushl (pointer)
call kfree
addl $4, %esp # Sizeof the pointer

... or

movl (pointer), %eax
pushl %eax
call kfree
addl $4, %esp

(pointer) is a memory operand, could be on the stack or in
other data. It is the pointer you want to free.

When kfree() gets called, the value passed __must__ be in its
final place on the stack, which means that the return address
must be there and the stack-pointer value adjusted to its
final resting place. This is necessary so that code will
get the correct values. On ix86, the called function will
find the first passed parameter at 0x04(%esp). The return
address will be at 0x00(%esp).

With the test for NULL, a decent compiler will produce code
like:
movl (pointer), %eax
orl %eax,%eax
jz 1f
pushl %eax
call kfree
addl $4, %esp
1:

So you add a one-byte instruction and branch on condition.

Post by Arjan van de Ven
4) modern processors have special logic to optimize push/pop
instructions; for example a "push eax ; push ebx" sequence will
execute in parallel in the same cycle even though there is a data
dependency on esp, the cpu can perfectly predict the esp effect and
will do so.

So? BTW, it's not a data-dependency, it's a size of register
dependency. The call-frame optimization can load several
register values onto the stack and adjust the stack-pointer
value only once. That's what the advertising hype is all about.

Post by Arjan van de Ven
5) modern processors have a call/ret fifo cache they use to do branch
prediction for the target of "ret" instructions. Unless you do
misbalanced call/ret pairs the prediction will be perfect.

The Intel CPUs don't have return-on-condition instructions like
the old Z80 and they don't have call-on-condition. If they did
you wouldn't have to jump around the calls as shown above.

It would be nice to have a call-non-zero instruction.

Post by Arjan van de Ven
Based on this the conclusion "a function call is really cheap versus a
conditional branch" is justified imo. Now you better come with proof
about which of the 5 things above I'm totally lying to you or you better
come with an apology.

There is a vast difference between the perception that advertising
hype produces, and the actual hardware designs. At one time, such
hype remained with the definition of megabyte. Now, they have
advertising agencies writing hype about technical buzz-words like
"branch prediction" and some engineers assume that the chip
contains a bunch of wonderful logic that will allow them to write
"pretty" or sloppy code. It just isn't so. There is much logic
that needs to be executed sequentially of else it won't work,
some needs to be coherent only occasionally. Stack operations
aren't in that category. The called procedures really need to
get the correct parameter values.

This whole advertising hype thing is a sore-point with
me because a server-box company went all the way to the
president of this company, trying to get me fired, when
I discovered that their box didn't work. Kill the messenger.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
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[Horst von Brand]

Post by Marcin Dalecki

Always, always, a call will be more expensive than a branch
on condition.

Wrong.

Post by Marcin Dalecki

It's impossible to be otherwise.

Many, many counterexamples say otherwise...

Post by Marcin Dalecki

A call requires
that the return address be written to memory (the stack),

Not necesarily right now, it can be done at leisure later on while doing
other stuff.

Post by Marcin Dalecki

using register indirection (the stack-pointer).

So what? The stack pointer is surely special. Modern programming languages
(and programming styles) encourage many calls, so this is very heavily
optimized.

Post by Marcin Dalecki
Needless to say that there are enough architectures out there, which
don't even have something like an explicit call as separate assembler
instruction...

The mechanism exists somehow.
--
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[Marcin Dalecki]

Post by Horst von Brand

Post by Marcin Dalecki
Needless to say that there are enough architectures out there, which
don't even have something like an explicit call as separate assembler
instruction...

The mechanism exists somehow.

Most RISC architectures are claiming a huge register set advantage over
IA32.
However in reality it's normal that:

1. Some of the register take roles as declared by the ABI. One is stack
one
is basis pointer and so no.
2. Only a subset of register is declared to be guaranteed to be
preserved by
system calls.

Thus the mechanisms are simple calling conventions.

Compilers can frequently see what a subroutine does and can flatten out
the cost
of function calls to something very much resembling just two jumps
instead of
a single jump around a condition.

On the other hand most modern IA32 implementation (since cyrix 486) are
very
efficient at mapping stack operations to a special cache between the
CPU and
L1 cache. I could even imagine them to be more efficient then plain
jumps, which
simply don't carry the same information for cache prefetch and branch
predition.

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[Horst von Brand]

Post by Dave Jones

Post by Jan Engelhardt
Well, kfree inlined was already mentioned but forgotten again.
inline static void kfree_WRAP(void *addr) {
if(likely(addr != NULL)) {
kfree_real(addr);
}
return;
}

Am I the only person who is completely fascinated by the
effort being spent here micro-optimising something thats
almost never in a path that needs optimising ?
I'd be amazed if any of this masturbation showed the tiniest
blip on a real workload, or even on a benchmark other than
one crafted specifically to test kfree in a loop.

Right.

Post by Dave Jones
That each occurance of this 'optimisation' also saves a handful
of bytes in generated code is it's only real benefit afaics.

No. It clears up the calls to kfree() a bit too in the source. Not really
important, sure.
.

Post by Dave Jones
Even then, if a functions cache performance is better off because
we trimmed a few bytes from the tail of a function, I'd be
completely amazed.
I guess April 1st came early this year.

Got (at) you! ;-)
--
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