From: "David Schwartz" <davids@webmaster.com>
To: "Mike Black" <mblack@csi-inc.com>,
"linux-kernel" <linux-kernel@vger.kernel.org>
Subject: RE: Pthread performance
Date: Sun, 13 Jul 2003 13:11:44 -0700 [thread overview]
Message-ID: <MDEHLPKNGKAHNMBLJOLKEEEOEFAA.davids@webmaster.com> (raw)
In-Reply-To: <022401c3479e$2b1cd2e0$c8de11cc@black>
> I found a benchmark program for threads and found a major
> difference between a single CPU and dual CPU system.
> Here's the code:
> http://www-124.ibm.com/pipermail/pthreads-users/2002-April/000176.html
> Is this showing context switches going between CPUs??
No.
> Wouldn't one expect the dual CPU to run twice as fast instead of
> ten times slower?
Not necessarily. The code doesn't do much other than modified shared data
without a lock. Not only does this invoke undefined behavior, but it
penalizes SMP machines because only they are subject to cache ping-ponging.
> As you can see from the timing user time increases by a factor of
> 4 and system time by a factor of 10.
> I seem to remember something about gettimeofday() possibly being
> a problem but couldn't find a reference to it.
> Anybody have an explanation/fix for this?
The program doesn't seem to measure anything useful and invokes undefined
behavior.
If you would like a fix, try this:
#define sched_yield() while(false)
DS
prev parent reply other threads:[~2003-07-13 19:57 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-07-11 11:18 Pthread performance Mike Black
2003-07-13 20:11 ` David Schwartz [this message]
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