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    SubjectRe: [resent PATCH] Re: very slow parallel read performance
    On Sun, 26 Aug 2001, Marc A. Lehmann wrote:
    > On Sat, Aug 25, 2001 at 10:33:36PM +0100, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
    > > > exactly this is a point: my disk can do 5mb/s with almost random seeks,
    > > > and linux indeed reads 5mb/s from it. but the userpsace process doing
    > > > read() only ever sees 2mb/s because the kernel throes away all the nice
    > > > pages.
    > >
    > > Which means the VM in the relevant kernel is probably crap or your working
    > > set exceeds ram.
    >
    > The relevant kernel is linux (all 2.4 versions I tested), and no,
    > working set exceeding ram should never result in such excessive
    > thrashing. So yes, the VM in the kernel is crap (in this particular
    > case, namely high-volume fileserving) ;)

    This is because the readahead windows are too large so the
    kernel ends up evicting data before its needed and has to
    re-read the data.

    Also see http://linux-mm.org/wiki/moin.cgi/StreamingIo
    in the Linux-MM wiki.

    This problem should be relatively easy to fix for 2.4.

    regards,

    Rik
    --
    IA64: a worthy successor to i860.

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