Messages in this thread | | | Date | Thu, 12 Jul 2007 19:13:17 -0700 | From | Andrew Morton <> | Subject | Re: [patch] x86_64: Avoid too many remote cpu references due to /proc/stat |
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On Thu, 12 Jul 2007 17:06:16 -0700 Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
> Too many remote cpu references due to /proc/stat. > > On x86_64, with newer kernel versions, kstat_irqs is a bit of a problem. > On every call to kstat_irqs, the process brings in per-cpu data from all > online cpus. Doing this for NR_IRQS, which is now 256 + 32 * NR_CPUS > results in (256+32*63) * 63 remote cpu references on a 64 cpu config. > /proc/stat is parsed by common commands like top, who etc, causing > lots of cacheline transfers
(256+32*63) * 63 = 143136
Do you have any actual numbers for how much this hurts?
> This statistic seems useless. Other 'big iron' arches disable this. > Can we disable computing/reporting this statistic? This piece of > statistic is not human readable on x86_64 anymore,
Did you consider using percpu_counters (or such) at interrupt-time? (warning: percpu_counters aren't presently interrupt safe).
> If not, can we optimize computing this statistic so as to avoid > too many remote references (patch to follow)
You other patch is a straightforward optimisation and should just be merged.
But afaict it will only provide a 2x speedup which I doubt is sufficient?
Another thought is: how many of the NR_IRQS counters are actually non-zero? Because a pretty obvious optimisation would be to have a global bitmap[NR_IRQS] and do
if (!bitmap[irq]) bitmap[irq] = 1;
at interrupt-time, then just print a "0" for the interrupts which have never occurred within show_stats().
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