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Date: Mon, 19 Feb 2007 05:39:27 +0100
From: Willy Tarreau <w@1wt.eu>
To: Phy Prabab <phyprabab@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Question about setting affinity in 2.4
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Hi Phy !

On Sun, Feb 18, 2007 at 06:02:17PM -0800, Phy Prabab wrote:
> Hello everyone,
> 
> I am trying to set affinity on a program to make sure I can get the
> best use of the cache as possible and to eliminate as much noise as
> possible with running my program.  I have tried unsuccessfully to set
> affinity using sched_set/getaffinity and the CPU_SET macros.  In
> particular, I can not seem to get the process to use affinity I
> specify as in:
> 
> int main () {
>  cpu_set_t mask;
>  cpu_set_t setmask;
>  long long x;
>  double foo;
>  printf("Mask set to %08lx\n", sched_getaffinity(0, &mask));
>  CPU_SET(1, &mask);
>  if (sched_setaffinity(0, &mask) == -1) {
>     printf("error setting affinity.");
>  }
> 
>  printf("Mask is reset to %08lx\n", sched_getaffinity(0, &mask));
>  // do some long lasting calculations ....
> 
>  return;
> }
> 
> No mater what I do, the process is never bond to the processor in
> question as exposed by this little appy and by cat /proc/<pid>/cpu.
> Is this just impossible to do?
> 
> Ah, the kernel I am using is RH 2.4.21-37.ELsmp.

I don't know for RH kernel, but the affinity syscalls are not in 2.4
mainline. You can apply the patch without much hassle if you want. It's
in Robert Love's directory :

 http://www.kernel.org/pub/linux/kernel/people/rml/cpu-affinity/v2.4/

I believe you'll have to use an -ac patch for RHEL.

> TIA!
> Phy

regards,
Willy