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    SubjectRe: [PATCH] Cpuset: rcu optimization of page alloc hook
    pj wrote:
    > Time for me to learn more about rcu.

    Well, that was easy.

    Directly using RCU to guard that task->cpuset pointer instead of
    cheating via the RCU hooks in the slab cache was just a few lines of

    But, boy oh boy, that synchronize_rcu() call sure takes it time.

    My cpuset torture test was creating, destroying and abusing about 2600
    cpusets/sec before this change, and now it does about 144 cpusets/sec.

    That cost 95% of the performance. This only hits on the cost of
    attaching a task to a different cpuset (by writing its <pid> to
    some other cpuset 'tasks' file.)

    Just commenting out the synchronize_cpu() restores the previous speed.

    Nothing surprising here - this is just how rcu claims it behaves,
    heavily favoring the read side performance. These delays are waiting
    for the other cpus doing rcu_read_lock() to go through a quite period
    (reschedule, idle or return to user code).

    Patches coming soon, to remove the cpuset_cache slab cache, and to
    use RCU directly instead.

    Thanks, Eric.

    I won't rest till it's the best ...
    Programmer, Linux Scalability
    Paul Jackson <> 1.925.600.0401
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      Last update: 2005-12-14 04:57    [W:0.020 / U:11.524 seconds]
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