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    SubjectRe: VFS: file-max limit 50044 reached


    On Mon, 17 Oct 2005, Eric Dumazet wrote:
    >
    > Thats strange, because on my tests it seems that I dont have one reschedule
    > for 'maxbatch' items. Doing 'grep filp /proc/slabinfo' it seems I have one
    > 'schedule' then filp count goes back to 1000.

    Hmm.

    I think you're right, but for all the wrong reasons.

    "maxbatch" ends up not actually having any real effect in the end: after
    the tasklet ends up running in softirqd, softirqd will actually keep on
    calling the tasklet code until it doesn't get rescheduled any more ;)

    So it will do "maxbatch" RCU entries, reschedule itself, return, and
    immediately get called again.

    Heh.

    The _good_ news is that since it ends up running in softirqd (after the
    first ten times - the softirq code in kernel/softirq.c will start off
    calling it ten times _first_), it can be scheduled away, so it actually
    ends up helping latency.

    Which means that we actually end up doing exactly the right thing,
    although for what appears to be the wrong reasons (or very lucky ones).

    The _bad_ news is that softirqd is running at nice +19, so I suspect that
    with some unlucky patterns it's probably pretty easy to make sure that
    ksoftirqd doesn't actually run very often at all!

    Gaah. So close, yet so far. I'm _almost_ willing to just undo my "make
    maxbatch huge" patch, and apply your patch, because now that I see how it
    all happens to work together I'm convinced that it _almost_ works. Even if
    it seems to be mostly by luck(*) rather than anything else.

    Linus

    (*) Not strictly true. It may not be by design of the RCU code itself, but
    it's definitely by design of the softirq's being designed to be robust and
    have good latency behaviour. So it does work by design, but it works by
    softirq design rather than RCU design ;)
    -
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