Re: [PING] [PATCH] Preheat CPU in benchtests

["Carlos O'Donell" <carlos at redhat dot com>]

On 05/01/2013 03:23 PM, OndÅej BÃlka wrote:
> 
> Ping, 
> 
> I need something like this for following patches. This is better than
> nothing until environment setting utility is implemented.
> 
> OK to commit?
> 
> On Tue, Apr 23, 2013 at 08:10:28AM +0200, OndÅej BÃlka wrote:
>> Hi, 
>>
>> Benchmarks now are affected by cpu scaling when initialy run at low
>> frequency.
>>
>> Following benchmark runs nonsensial loop first to ensure that benchmark
>> are measured at maximal frequency. This greatly cuts time needed to
>> get accurate results.
>>
> 
> ---
>  benchtests/bench-skeleton.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/benchtests/bench-skeleton.c b/benchtests/bench-skeleton.c
> index 7359184..2c728bf 100644
> --- a/benchtests/bench-skeleton.c
> +++ b/benchtests/bench-skeleton.c
> @@ -22,6 +22,8 @@
>  #include <time.h>
>  #include <inttypes.h>
>  
> +volatile double dontoptimize = 0.0;
> +
>  #define TIMESPEC_AFTER(a, b) \
>    (((a).tv_sec == (b).tv_sec) ?						      \
>       ((a).tv_nsec > (b).tv_nsec) :					      \
> @@ -32,6 +34,13 @@ main (int argc, char **argv)
>    unsigned long i, k;
>    struct timespec start, end, runtime;
>  

Needs a comment explaining why we don't disable requency
scaling e.g. needs root (Andi Kleen's comment).

Needs a comment explaining future work around CPU_CLK_UNHALTED.

Needs a comment explaining why we run for a fixed number
of cycles instead of a fixed amount of time (Petr Baudis' comment).

> +  /* This loop should cause CPU switch to maximal freqency.  This makes
> +      subsequent measurement more accurate.  We need side effect to avoid loop
> +      being deleted by compiler.  */
> + for(k = 0; k < 1000000; k++)
> +   dontoptimize += 23.0 * dontoptimize + 2.1;
> +
> +

Make it a function.

Can we avoid using the fpu here and avoid possible
double overflow? Does a volatile unsigned int work just
as well? If we need to heat the CPU and FPU 

>    memset (&runtime, 0, sizeof (runtime));
>    memset (&start, 0, sizeof (start));
>    memset (&end, 0, sizeof (end));
> 

Cheers,
Carlos.