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    SubjectRe: [PATCH] mm: disallow direct reclaim page writeback
    On Fri, Apr 16, 2010 at 04:14:03PM +0100, Mel Gorman wrote:
    > > > Your patch fixes 2, avoids 1, breaks 3 and haven't thought about 4 but I
    > > > guess dirty pages can cycle around more so it'd need to be cared for.
    > >
    > > Well, you keep saying that they break #3, but I haven't seen any
    > > test cases or results showing that. I've been unable to confirm that
    > > lumpy reclaim is broken by disallowing writeback in my testing, so
    > > I'm interested to know what tests you are running that show it is
    > > broken...
    > >
    > Ok, I haven't actually tested this. The machines I use are tied up
    > retesting the compaction patches at the moment. The reason why I reckon
    > it'll be a problem is that when these sync-writeback changes were
    > introduced, it significantly helped lumpy reclaim for huge pages. I am
    > making an assumption that backing out those changes will hurt it.
    > I'll test for real on Monday and see what falls out.

    One machine has completed the test and the results are as expected. When
    allocating huge pages under stress, your patch drops the success rates
    significantly. On X86-64, it showed

    stress-highalloc stress-highalloc
    enable-directreclaim disable-directreclaim
    Under Load 1 89.00 ( 0.00) 73.00 (-16.00)
    Under Load 2 90.00 ( 0.00) 85.00 (-5.00)
    At Rest 90.00 ( 0.00) 90.00 ( 0.00)

    So with direct reclaim, it gets 89% of memory as huge pages at the first
    attempt but 73% with your patch applied. The "Under Load 2" test happens
    immediately after. With the start kernel, the first and second attempts
    are usually the same or very close together. With your patch applied,
    there are big differences as it was no longer trying to clean pages.

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

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