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    SubjectRe: [rfc][patch] swap: virtual swap readahead
    On Mon, Jun 08, 2009 at 03:52:46PM +0800, Wu Fengguang wrote:
    > On Wed, May 27, 2009 at 05:05:46PM +0200, Johannes Weiner wrote:
    > > The current swap readahead implementation reads a physically
    > > contiguous group of swap slots around the faulting page to take
    > > advantage of the disk head's position and in the hope that the
    > > surrounding pages will be needed soon as well.
    > >
    > > This works as long as the physical swap slot order approximates the
    > > LRU order decently, otherwise it wastes memory and IO bandwidth to
    > > read in pages that are unlikely to be needed soon.
    > >
    > > However, the physical swap slot layout diverges from the LRU order
    > > with increasing swap activity, i.e. high memory pressure situations,
    > > and this is exactly the situation where swapin should not waste any
    > > memory or IO bandwidth as both are the most contended resources at
    > > this point.
    > >
    > > This patch makes swap-in base its readaround window on the virtual
    > > proximity of pages in the faulting VMA, as an indicator for pages
    > > needed in the near future, while still taking physical locality of
    > > swap slots into account.
    > >
    > > This has the advantage of reading in big batches when the LRU order
    > > matches the swap slot order while automatically throttling readahead
    > > when the system is thrashing and swap slots are no longer nicely
    > > grouped by LRU order.
    >
    > Hi Johannes,
    >
    > You may want to test the patch against a real desktop :)
    > The attached scripts can do that. I also have the setup to
    > test it out conveniently, so if you send me the latest patch..

    Thanks a bunch for the offer! I'm just now incorporating Hugh's
    feedback and hope I will be back soon with the next version. I will
    let you know, for sure.

    Hannes


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