Tim Chen <[email protected]> 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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