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    SubjectRe: unexpected paging during large file reads in 2.1.127
    On Mon, 16 Nov 1998, Stephen C. Tweedie wrote:
    > On Mon, 16 Nov 1998 21:48:35 +0100 (CET), Rik van Riel
    > <H.H.vanRiel@phys.uu.nl> said:
    > > On Mon, 16 Nov 1998, Stephen C. Tweedie wrote:
    > >> The real cure is to disable page aging in the page cache completely.
    > >> Now that we have disabled it for swap, it makes absolutely no sense at
    > >> all to keep it in the page cache.
    >
    > > This is not entirely true. There is a major difference
    > > between pages in the page cache and pages that can go
    > > into swap. The latter kind will always be mapped inside
    > > the address space of a program (where it gets proper
    > > aging and stuff)
    >
    > No it doesn't, that's what I'm saying. Linus removed swap page aging in
    > the recent kernels. That throws the balance between swap and cache
    > completely out of the window: removing the page cache aging is necessary
    > to restore balance. There are many many reports of massive cache growth
    > on the latest kernels as a result of this.

    I meant the page aging that occurs in vmscan.c, where we
    decide on which page to unmap from a program's address
    space. There we do aging while we don't age pages from
    files that are read().

    > > Now we can get severe problems with readahead when we
    > > are evicting just read-in data because it isn't mapped,
    >
    > No, we don't. We don't evict just-read-in data, because we mark such
    > pages as PG_Referenced. It takes two complete shrink_mmap() passes
    > before we can evict such pages.

    OK, I can (and have for quite a while) agree with this.
    Kernels with this feature and enough memory will run great,
    maybe small machines (<16M) will have a bit of trouble
    keeping up readahead performance (since kswapd will have
    made it's round a bit fast) but those machines will have
    sucky performance anyway :)

    Rik -- slowly getting used to dvorak kbd layout...
    +-------------------------------------------------------------------+
    | Linux memory management tour guide. H.H.vanRiel@phys.uu.nl |
    | Scouting Vries cubscout leader. http://www.phys.uu.nl/~riel/ |
    +-------------------------------------------------------------------+


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