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    SubjectRe: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count
    On Thu, 30 Apr 2009 15:11:15 +0900 (JST)
    KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

    >
    > Changelog:
    > since v1
    > - use percpu_counter_sum() instead percpu_counter_read()
    >
    >
    > -------------------------------------
    > Subject: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count
    >
    > cpuacct_update_stats() is called at every tick updating. and it use percpu_counter
    > for avoiding performance degression.
    >
    > For archs which define VIRT_CPU_ACCOUNTING, every tick would result
    > in >1000 units of cputime updates and since this is much much greater
    > than percpu_batch_counter, we end up taking spinlock on every tick.
    >
    > This patch change batch rule. now, any cpu can store "percpu_counter_bach * jiffies"
    > cputime in per-cpu cache.
    > it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n.

    Does this actually matter?

    If we're calling cpuacct_update_stats() with large values of `cputime'
    then presumably we're also calling cpuacct_update_stats() at a low
    frequency, so the common lock-taking won't cause performance problems?



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