Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: 2.6.19-rc5: known regressions | Date | Wed, 08 Nov 2006 22:10:33 -0700 |
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Tim Chen <tim.c.chen@linux.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...
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