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    SubjectRe: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_pages
    Jamie Lokier wrote:
    > Christoph Hellwig wrote:
    > > On Tue, Sep 01, 2009 at 08:38:55PM +0200, Peter Zijlstra wrote:
    > > > Do we really need a tunable for this?
    > >
    > > It will make increasing it in the field a lot easier. And having deal
    > > with really large systems I have the fear that there are I/O topologies
    > > outhere for which every "reasonable" value is too low.
    > >
    > > > I guess we need a limit to avoid it writing out everything, but can't we
    > > > have something automagic?
    > >
    > > Some automatic adjustment would be nice. But finding the right auto
    > > tuning will be an interesting exercise.
    > I have embedded systems with 32MB RAM and no MMU, where I deliberately
    > make the equivalent of max_writeback_pages *smaller* to limit the
    > number of dirty pages causing fragmentation and preventing allocation
    > of high-order pages... Write performance is less important than being
    > able to allocate contiguous memory for reads.
    > They are still using 2.4 kernels, but the principle still applies.
    > maybe even more on 2.6 which is more prone to fragmentation on small
    > no-MMU devices.

    Sorry, I must get more sleep. I confused max_writeback_pages with the
    limit on dirty pages in the system, which is completely different.

    So please ignore my previous mail.

    -- Jamie

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