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    SubjectRe: Random file I/O regressions in 2.6
    On Tue, May 04, 2004 at 01:47:29AM -0700, Andrew Morton wrote:
    > Arjan van de Ven <> wrote:
    > >
    > >
    > > >
    > > > That would cause the kernel to perform lots of pointless pagecache lookups
    > > > when the file is already 100% cached.
    > >
    > > well surely the read itself will do those AGAIN anyway, so in the fully
    > > cached case this is just warming up the cpu cache ;) (and thus really
    > > cheap as nett cost I suspect)
    > Probably true for x86, but the cost is noticeable on ppc64, for example.
    > Anton fixed some things in there shortly after it went in, but it's still
    > apparent on profiles.

    well do the profiles also show that the actual later lookup becomes near
    free due to a warm cpu cache?

    > We could perhaps speed things up a little bit by using gang lookup in both
    > __do_page_cache_readahead() and in do_generic_file_read().

    or go into the readahead path only when the first miss occurs; for the fully
    cached case you can then avoid the cost while when you're doing IO, well, a
    few premature cache misses...

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