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Date: Fri, 19 Jan 2001 13:51:35 -0800
From: Mike Kravetz <mkravetz@sequent.com>
To: Andrea Arcangeli <andrea@suse.de>,
        Davide Libenzi <davidel@xmail.virusscreen.com>
Cc: lse-tech@lists.sourceforge.net, linux-kernel@vger.kernel.org
Subject: Re: [Lse-tech] Re: multi-queue scheduler update
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In-Reply-To: <20010119124921.G26968@w-mikek.des.sequent.com>; from mkravetz@sequent.com on Fri, Jan 19, 2001 at 12:49:21PM -0800
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On Fri, Jan 19, 2001 at 12:49:21PM -0800, Mike Kravetz showed his lack
of internet slang understanding and wrote:
> 
> It was my intention to post IIRC numbers for small thread counts today.
> However, the benchmark (not the system) seems to hang on occasion.  This
> occurs on both the unmodified 2.4.0 kernel and the one which contains
> my multi-queue patch.  Therefore, I'm pretty sure it is not something
> I did. :)
> 
> Anyone else see anything like this before?  I'll look into the reason
> for the hang, but it will delay my posting of these numbers.

I think I have found the problem.  Here is a code snippet from the
benchmark Andrea posted.

void            oneatwork(int thr)
{
    int             i;
    while (!start)              /* don't disturb pthread_create() */
        usleep(10000);                                                          

    actthreads++;
    while (!stop)
    {
        if (count)
            totalwork[thr]++;

        syscall(158); /* sys_sched_yield() */                                   
    }                                                                           
    actthreads--;                                                               
    pthread_exit(0);
}                                                                               

Note that actthreads is a global variable which is being updated
by multiple threads without any form of synchronization.  Because
of this actthreads sometimes never goes to zero after all worker
threads have finished.  I changed actthreads to be an atomic and
used atomic operations to manipulate it.  With this change, I was
able to complete one round of testing which I had not been able to
do in the past.

Does anyone maintain this benchmark code?  The changes I indicate
above should be made.  If you need more specifics I can provide
them.

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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