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    SubjectRe: [PATCH -mm] mm: more likely reclaim MADV_SEQUENTIAL mappings
    On Tuesday 22 July 2008 13:43, Nick Piggin wrote:
    > On Tuesday 22 July 2008 13:04, Rik van Riel wrote:
    > > On Tue, 22 Jul 2008 12:54:28 +1000

    > > > But we are not doing nothing because we already know and have coded
    > > > for the fact that the mapping will be accessed once, sequentially.
    > > > Now that we have gone this far, we should actually do it properly and
    > > > 1. unmap after use, 2. POSIX_FADV_DONTNEED after use. This will give
    > > > you much better performance and cache behaviour than any automatic
    > > > detection scheme, and it doesn't introduce any regressions for existing
    > > > code.
    > >
    > > If you run just one instance of the application!
    > >
    > > Think about something like an ftp server or a media server,
    > > where you want to cache the data that is served up many
    > > times, while evicting the data that got served just once.
    > >
    > > The kernel has much better knowledge of what the aggregate
    > > of all processes in the system are doing than any individual
    > > process has.
    > That's true, but this case isn't really very good anyway. The information
    > goes away after you drop the mapping anyway. Or did you hope that the
    > backup program or indexer keeps all those mappings open until all the pages
    > have filtered through? Or maybe we can add yet more branches into the unmap
    > path to test for this flag as well?
    > I don't think it is a good idea to add random things just because they seem
    > at first glance like a good idea.

    BTW. in the backup of a busy fileserver or some case like that, I'd
    bet that even using FADV_DONTNEED would be much faster than leaving
    these mappings around to try to drop them due to the decreased churn
    on the LRUs overall anyway.

     \ /
      Last update: 2008-07-22 05:53    [W:0.042 / U:3.536 seconds]
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