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    SubjectRe: [PATCH] mm: do not drain pagevecs for mlock
    2011/12/30 Tao Ma <tm@tao.ma>:
    > On 12/30/2011 04:11 PM, KOSAKI Motohiro wrote:
    >> 2011/12/30 Tao Ma <tm@tao.ma>:
    >>> In our test of mlock, we have found some severe performance regression
    >>> in it. Some more investigations show that mlocked is blocked heavily
    >>> by lur_add_drain_all which calls schedule_on_each_cpu and flush the work
    >>> queue which is very slower if we have several cpus.
    >>>
    >>> So we have tried 2 ways to solve it:
    >>> 1. Add a per cpu counter for all the pagevecs so that we don't schedule
    >>>   and flush the lru_drain work if the cpu doesn't have any pagevecs(I
    >>>   have finished the codes already).
    >>> 2. Remove the lru_add_drain_all.
    >>>
    >>> The first one has some problems since in our product system, all the cpus
    >>> are busy, so I guess there is very little chance for a cpu to have 0 pagevecs
    >>> except that you run several consecutive mlocks.
    >>>
    >>> From the commit log which added this function(8891d6da), it seems that we
    >>> don't have to call it. So the 2nd one seems to be both easy and workable and
    >>> comes this patch.
    >>
    >> Could you please show us your system environment and benchmark programs?
    >> Usually lru_drain_** is very fast than mlock() body because it makes
    >> plenty memset(page).
    > The system environment is: 16 core Xeon E5620. 24G memory.
    >
    > I have attached the program. It is very simple and just uses mlock/munlock.

    Because your test program is too artificial. 20sec/100000times =
    200usec. And your
    program repeat mlock and munlock the exact same address. so, yes, if
    lru_add_drain_all() is removed, it become near no-op. but it's
    worthless comparision.
    none of any practical program does such strange mlock usage.

    But, 200usec is much than I measured before. I'll dig it a bit more.
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