Messages in this thread | | | Subject | Re: 2.6.19-rc5: known regressions | From | Tim Chen <> | Date | Wed, 08 Nov 2006 18:49:41 -0800 |
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On Wed, 2006-11-08 at 15:11 -0800, Tim Chen wrote: > On Wed, 2006-11-08 at 17:22 +0100, Adrian Bunk wrote: > > > There's perhaps one thing that might help us to see whether it's just a > > benchmark effekt or a real problem: > > > > With Tim's CONFIG_NR_CPUS=8, NR_IRQS only increases from 224 in 2.6.18 > > to 512 in 2.6.19-rc. > > > > With CONFIG_NR_CPUS=255, NR_IRQS increases from 224 in 2.6.18 > > to 8416 in 2.6.19-rc. > > > > @Tim: > > Can you try CONFIG_NR_CPUS=255 with both 2.6.18 and 2.6.19-rc5? > > > > With CONFIG_NR_CPUS increased from 8 to 64: > 2.6.18 see no change in fork time measured. > 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.
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