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    SubjectRe: [PATCH 0/3] readahead drop behind and size adjustment
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    On Sun, 2007-07-22 at 18:33 +1000, Rusty Russell wrote:
    > On Sun, 2007-07-22 at 16:10 +0800, Fengguang Wu wrote:
    > > - It will avoid large-file-reads-thrashing-my-desktop problem,
    > > so most desktop users should like it. But sure there will be counter
    > > cases when a user want to keep the data cached.
    > > - File servers may hurt from it. Imagine a mp3/png file server. The
    > > files are large enough to trigger drop-behind, but small (and hot)
    > > enough to be cached. Also when a new fedora DVD iso is released, it
    > > may be cached for some days. These are only the obvious cases.
    >
    > I'm still not convinced of the latter case. If a second user accesses
    > the file before it pages are thrown out, from my understanding there
    > won't be readahead but the pages will be put back on the active list
    > (ie. the dropbehind is undone).
    >
    > AFAICT this patch truly biasses towards releasing pages from
    > sequentially accessed files. So file servers where memory pressure
    > comes from lots of such files should be fine (bias will work correctly
    > against least-served files).
    >
    > The main problem I can see is a low-memory server where we are currently
    > swapping out pages from non-sequential files (eg. gigantic running java
    > apps), and now performance might decline because we'll discard file
    > pages first.
    >
    > > So I opt for it being made tunable, safe, and turned off by default.
    >
    > I'd like to see it turned on by default in -mm, and try to come up with
    > some server-like workload to measure the effect. Should be easy to
    > simulate something (eg. apache server, where clients grab some files in
    > preference, and apache server where clients grab different files).
    >
    > Peter?

    I guess we could do that. Just populate the server with a lot of
    randomly sized files, and make a client do a random pull or one with a
    poisson distribution or something like that.



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