linux-kernel - Re: 2.6.19-rc5: known regressions

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Date:	Wed, 08 Nov 2006 22:10:33 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	tim.c.chen@...ux.intel.com
Cc:	Adrian Bunk <bunk@...sta.de>, Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.19-rc5: known regressions

Tim Chen <tim.c.chen@...ux.intel.com> writes:

> On Wed, 2006-11-08 at 15:11 -0800, Tim Chen wrote:
>> On Wed, 2006-11-08 at 17:22 +0100, Adrian Bunk wrote:
>> 
>> With CONFIG_NR_CPUS increased from 8 to 64:
>> 2.6.18     see no change in fork time measured.
CONFIG_NR_CPUS has no affect on NR_IRQS in 2.6.18.
So this test unfortunately told us nothing.

>> 2.6.19-rc5 see a 138% increase in fork time.
>> 
>
> Lmbench is broken in its fork time measurement.
> It includes overhead time when it is pinning processes onto
> specific cpu. The actual fork time is not affected by NR_IRQS.
>
> Lmbench calls the following C library function to determine the 
> number of processors online before it pin the processes: 
> 	sysconf(_SC_NPROCESSORS_ONLN);
>
> This function takes the same order of time to run as
> fork itself.  In addition, runtime of this function 
> increases with NR_IRQS.  This resulted in the change in
> time measured.
>
> After hardcoding the number of online processors in lmbench,
> the fork time measured now does not change with CONFIG_NR_CPUS
> for both 2.6.18 and 2.6.19-rc5.  So we can now conclude that
> NR_IRQS does not affect fork.  We can remove this particular
> issue from the known regression.

Cool.  I'm glad to know it was simply a buggy lmbench.

What is sysconf(_SN_NPROCESSORS_ONLN) doing that it slows down as the
number of irqs increase?  It is a slow path certainly but possibly
something we should fix.  My hunch is cat /proc/cpuinfo...

Eric
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