How to measure time accurately.

[Lee Revell]

Hi All,
Can any one tell me how to measure time accurately for a block of C code
in device drivers.
For example, If I want to measure the time duration of firmware download.

rdtsc()

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[Chris Friesen]

Hi All,
Can any one tell me how to measure time accurately for a block of C code
in device drivers.
For example, If I want to measure the time duration of firmware download.

Most cpus have some way of getting at a counter or decrementer of
various frequencies. Usually it requires low-level hardware knowledge
and often it needs assembly code.

On ppc you'd use the mftbu/mftbl instructions, as suggested by Lee on
x86 you'd use the rdtsc instruction.

Chris
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[Jan Engelhardt]

In some cases you can simply count jiffies - depending on how accurate you

These "some cases" exclude this one:
If interrupts are disabled, a jiffy might be missed. Take care.

If you are on UP and want to measure within
- a tick [when using preempt]
- kernel space [no preempt]
disabled interrupts should usually not be the case.

Jan Engelhardt
--
No TOFU for me, please.
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[Jesper Juhl]

Date: Mon, 28 Mar 2005 23:07:14 -0600
Subject: Re: How to measure time accurately.

Hi All,
Can any one tell me how to measure time accurately for a block of C code in
device drivers.
For example, If I want to measure the time duration of firmware download.

Most cpus have some way of getting at a counter or decrementer of various
frequencies. Usually it requires low-level hardware knowledge and often it
needs assembly code.
On ppc you'd use the mftbu/mftbl instructions, as suggested by Lee on x86
you'd use the rdtsc instruction.

In some cases you can simply count jiffies - depending on how accurate you
need to time things I'd say that often something like this is adequate :

unsigned long start, time_spent;

start = jiffies;
/* do stuff */
time_spent = jiffies - start;
printk("stuff took %d jiffies (%d seconds)\n", time_spent, time_spent/HZ);
--
Jesper Juhl

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