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    SubjectRe: [PATCH] vmscan: evict use-once pages first (v2)
    On Thu, 30 Apr 2009 21:50:34 -0400 Rik van Riel <riel@redhat.com> wrote:

    > > Which would cause exactly the problem Elladan saw?
    >
    > Yes. It was not noticable in the initial split LRU code,
    > but after we decided to ignore the referenced bit on active
    > file pages and deactivate pages regardless, it has gotten
    > exacerbated.
    >
    > That change was very good for scalability, so we should not
    > undo it. However, we do need to put something in place to
    > protect the working set from streaming IO.
    >
    > > > Currently the kernel has no effective code to protect the
    > > > page cache working set from streaming IO. Elladan's bug
    > > > report shows that we do need some kind of protection...
    > >
    > > Seems to me that reclaim should treat swapcache-backed mapped mages in
    > > a similar fashion to file-backed mapped pages?
    >
    > Swapcache-backed pages are not on the same set of LRUs as
    > file-backed mapped pages.

    yup.

    > Furthermore, there is no streaming IO on the anon LRUs like
    > there is on the file LRUs. Only the file LRUs need (and want)
    > use-once replacement, which means that we only need special
    > protection of the working set for file-backed pages.

    OK.

    > When we implement working set protection, we might as well
    > do it for frequently accessed unmapped pages too. There is
    > no reason to restrict this protection to mapped pages.

    Well. Except for empirical observation, which tells us that biasing
    reclaim to prefer to retain mapped memory produces a better result.



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