Re: cpufreq and memory intensive benchmarks

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Monday, August 05, 2013 12:40:13 PM Viresh Kumar wrote:
> Please remember adding cc'd lists/people in your next mail
> 
> On Sat, Aug 3, 2013 at 1:15 AM, Guru Prasad <gurupras@buffalo.edu> wrote:
> > I was recently running some SPEC benchmarks to test the way in which
> > cpufreq module adjusts the CPU frequency for various benchmarks and I
> > noticed that cpufreq always tends to rest at the maximum frequency
> > regardless of the benchmark on ondemand / conservative.
> 
> They will stay there if cpu is busy doing some activity..
> 
> > I then tried to identify memory intensive benchmarks
> > (http://users.elis.ugent.be/~leeckhou/papers/isca12-2.pdf) and upon
> > running milc / libquantum, I noticed that the same trend continued.
> >
> > Does this mean that memory operation latencies are attributed as CPU busy time?
> > Could someone explain the inner workings behind this? What does the
> > CPU *really* do while the load/store is executing? Is it busy-waiting
> > / idle?
> 
> We actually take cpu's busy vs idle time for all calculations here..
> idle time is calculated by the time cpu was running its idle thread..
> i.e. when CPU doesn't have any process to run and is idle..
> 
> But in your case not sure how cpu would behave.. If cpu is doing
> word-by-word copy of some stuff, it would be busy most of the
> time and isn't really running its idle thread. And so would be counted
> as busy I suppose..
> 
> But if some DMA is taking care of copying stuff then it can be idle,
> depending on what else is getting scheduled on it.
> 
> @Rafael: Am I right?

All depends on whether or not the CPU is idle from the scheduler viewpoint,
so basically this is correct.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.