Re: Possible bug from kernel 2.6.22 and above, 2.6.24-rc4

[Jie Chen <chen@jlab.org>]

Ingo Molnar wrote:
> * Jie Chen <chen@jlab.org> wrote:
>
> > > and then you use this in the measurement loop:
> > >
> > >    for (k=0; k<=OUTERREPS; k++){
> > >      start  = getclock();
> > >      for (j=0; j<innerreps; j++){
> > >  #ifdef _QMT_PUBLIC
> > >        delay((void *)0, 0);
> > >  #else
> > >        delay(0, 0, 0, (void *)0);
> > >  #endif
> > >      }
> > >      times[k] = (getclock() - start) * 1.0e6 / (double) innerreps;
> > >    }
> > >
> > > the problem is, this does not take the overhead of gettimeofday into 
> > > account - which overhead can easily reach 10 usecs (the observed 
> > > regression). Could you try to eliminate the gettimeofday overhead from 
> > > your measurement?
> > >
> > > gettimeofday overhead is something that might have changed from .21 to .22 
> > > on your box.
> > >
> > > 	Ingo
> > Hi, Ingo:
> >
> > In my pthread_sync code, I first call refer () subroutine which 
> > actually establishes the elapsed time (reference time) for 
> > non-synchronized delay() using the gettimeofday. Then each 
> > synchronization overhead value is obtained by subtracting the 
> > reference time from the elapsed time with introduced synchronization. 
> > The effect of gettimeofday() should be minimal if the time difference 
> > (overhead value) is the interest here. Unless the gettimeofday behaves 
> > differently in the case of running 8 threads .vs. running 2 threads.
> >
> > I will try to replace gettimeofday with a lightweight timer call in my 
> > test code. Thank you very much.
>
> gettimeofday overhead is around 10 usecs here:
>
>  2740  1197359374.873214 gettimeofday({1197359374, 873225}, NULL) = 0 <0.000010>
>  2740  1197359374.970592 gettimeofday({1197359374, 970608}, NULL) = 0 <0.000010>
>
> and that's the only thing that is going on when computing the reference 
> time - and i see a similar syscall pattern in the PARALLEL and BARRIER 
> calculations as well (with no real scheduling going on).
>
> 	Ingo

Hi, Ingo:

I guess it is a good news. I did patch 2.6.21.7 kernel using your cfs 
patch. The results of pthread_sync is the same as the non-patched 2.6.21 
kernel. This means the performance of is not related to the scheduler. 
As for overhead of the gettimeofday, there is no difference between 
2.6.21 and 2.6.24-rc4. The reference time is around 10.5 us for both 
kernel.

So what is changed between 2.6.21 and 2.6.22? Any hints :-). Thank you 
very much for all your help.

--
###############################################
Jie Chen
Scientific Computing Group
Thomas Jefferson National Accelerator Facility
12000, Jefferson Ave.
Newport News, VA 23606

(757)269-5046 (office) (757)269-6248 (fax)
chen@jlab.org
###############################################

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