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

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Message-ID: <20071210105943.GA5370@elte.hu>
Date:	Mon, 10 Dec 2007 11:59:44 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jie Chen <chen@...b.org>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Possible bug from kernel 2.6.22 and above, 2.6.24-rc4


* Jie Chen <chen@...b.org> wrote:

> I did patch the header file and recompiled the kernel. I observed no 
> difference (two threads overhead stays too high). Thank you.

ok, i think i found it. You do this in your qmt/pthread_sync.c 
test-code:

 double get_time_of_day_()
 {
 ...
   err = gettimeofday(&ts, NULL);
 ...
 }

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
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