Re: [PATCH] randomize benchtests

[chrubis at suse dot cz]

Hi!
> > You can use 'cpufreq-info -f' on Linux, on BSD's you need to play with
> > sysctl. There is a sysctl 'emulation' on Linux that uses /proc, but that
> > doesn't work for CPU speed.
> > 
> It looks that reading file is saner than exec followed by reading
> output.

The actual code is pretty simple:

int speed;

FILE *f = popen("cpufreq-info -f", "r");

if (f == NULL)
	//ERROR

if (fscanf(f, "%i", &speed) != 1)
	//ERROR

pclose(f);

> > But be wary that CPU speed is subject to change when CPU scaling is
> > active, which is on on most of the modern systems.
> > 
> This with other factors is reason why we should test all implementations
> together and select implementations randomly. They will be affected by
> same amount so their speed differences will not be affected. 
> 
> When we test them sequentialy overhead will be different each time
> making comparing them almost worthless.

Not sure if I follow you correctly. What I wanted to say is that the
actuall CPU frequency changes depending on the system load. If you read
it before you start the benchmarks, it will likely be incorrect as the
system was possibly idle and the benchmarks made the cpu freq governor
to switch to higher speeds as the system load increased. So converting
CPU cycles to time and back is meaningless unless you fixed the CPU
speed to certain value manually. 

-- 
Cyril Hrubis
chrubis@suse.cz