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    SubjectRe: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages in kswapd except in high priority
    On Thu, Aug 11, 2011 at 02:18:54PM -0400, Rik van Riel wrote:
    > On 08/10/2011 06:47 AM, Mel Gorman wrote:
    > >It is preferable that no dirty pages are dispatched for cleaning from
    > >the page reclaim path. At normal priorities, this patch prevents kswapd
    > >writing pages.
    > >
    > >However, page reclaim does have a requirement that pages be freed
    > >in a particular zone. If it is failing to make sufficient progress
    > >(reclaiming< SWAP_CLUSTER_MAX at any priority priority), the priority
    > >is raised to scan more pages. A priority of DEF_PRIORITY - 3 is
    > >considered to be the point where kswapd is getting into trouble
    > >reclaiming pages. If this priority is reached, kswapd will dispatch
    > >pages for writing.
    > >
    > >Signed-off-by: Mel Gorman<mgorman@suse.de>
    > >Reviewed-by: Minchan Kim<minchan.kim@gmail.com>
    >
    > My only worry with this patch is that maybe we'll burn too
    > much CPU time freeing pages from a zone.

    The throttling patch prevents too much CPU being used if pages under
    writeback are being encountered during scanning. That said, I shared
    your concern and recorded kswapd CPU usage over time.

    > However, chances
    > are we'll have freed pages from other zones when scanning
    > one zone multiple times (the page cache dirty limit is global,
    > the clean pages have to be _somewhere_).
    >
    > Since the bulk of the allocators are not too picky about
    > which zone they get their pages from, I suspect this patch
    > will be an overall improvement pretty much all the time.
    >

    This is roughly similar to my own reasoning.

    I uploaded all the kswapd CPU usage charts to
    http://www.csn.ul.ie/~mel/postings/riel-20110811

    These are smoothened as the raw figures are barely readable. If you
    go through them, you'll see that kswapd CPU usage is sometimes higher
    but generally within 2-3%.

    > Acked-by: Rik van Riel <riel@redhat.com>

    Thanks.

    --
    Mel Gorman
    SUSE Labs


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