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    SubjectRe: [PATCH 12/14] Add a tunable that decides when memory should be compacted and when it should be reclaimed
    On Tue, Apr 06, 2010 at 05:06:13PM -0700, Andrew Morton wrote:
    > On Fri, 2 Apr 2010 17:02:46 +0100
    > Mel Gorman <mel@csn.ul.ie> wrote:
    >
    > > The kernel applies some heuristics when deciding if memory should be
    > > compacted or reclaimed to satisfy a high-order allocation. One of these
    > > is based on the fragmentation. If the index is below 500, memory will
    > > not be compacted. This choice is arbitrary and not based on data. To
    > > help optimise the system and set a sensible default for this value, this
    > > patch adds a sysctl extfrag_threshold. The kernel will only compact
    > > memory if the fragmentation index is above the extfrag_threshold.
    >
    > Was this the most robust, reliable, no-2am-phone-calls thing we could
    > have done?
    >
    > What about, say, just doing a bit of both until something worked?

    I guess you could but that is not a million miles away from what
    currently happens.

    This heuristic is basically "based on free memory layout, how likely is
    compaction to succeed?". It makes a decision based on that. A later
    patch then checks if the guess was right. If not, just try direct
    reclaim for a bit before trying compaction again.

    > For
    > extra smarts we could remember what worked best last time, and make
    > ourselves more likely to try that next time.
    >

    With the later patch, this is essentially what we do. Granted we
    remember the opposite "If the kernel guesses wrong, then don't compact
    for a short while before trying again".

    > Or whatever, but extfrag_threshold must die! And replacing it with a
    > hardwired constant doesn't count ;)
    >

    I think what you have in mind is "just try compaction every time" but my
    concern about that is we'll hit a corner case where a lot of CPU time is
    taken scanning zones uselessly. That is what this heuristic and the
    back-off logic in a later patch was meant to avoid. I haven't thought of
    a better alternative :/

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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