On Thu, 12 Jul 2007 17:06:16 -0700 Ravikiran G Thirumalai <[email protected]> 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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