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    SubjectRe: [PATCH] mm: make VM_MAX_READAHEAD configurable
    Hi Martin,

    On Fri, Oct 09, 2009 at 09:49:50PM +0800, Martin Schwidefsky wrote:
    > On Fri, 9 Oct 2009 14:29:52 +0200
    > Jens Axboe <jens.axboe@oracle.com> wrote:
    >
    > > On Fri, Oct 09 2009, Peter Zijlstra wrote:
    > > > On Fri, 2009-10-09 at 13:19 +0200, Ehrhardt Christian wrote:
    > > > > From: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
    > > > >
    > > > > On one hand the define VM_MAX_READAHEAD in include/linux/mm.h is just a default
    > > > > and can be configured per block device queue.
    > > > > On the other hand a lot of admins do not use it, therefore it is reasonable to
    > > > > set a wise default.
    > > > >
    > > > > This path allows to configure the value via Kconfig mechanisms and therefore
    > > > > allow the assignment of different defaults dependent on other Kconfig symbols.
    > > > >
    > > > > Using this, the patch increases the default max readahead for s390 improving
    > > > > sequential throughput in a lot of scenarios with almost no drawbacks (only
    > > > > theoretical workloads with a lot concurrent sequential read patterns on a very
    > > > > low memory system suffer due to page cache trashing as expected).
    [snip]
    >
    > The patch from Christian fixes a performance regression in the latest
    > distributions for s390. So we would opt for a larger value, 512KB seems
    > to be a good one. I have no idea what that will do to the embedded
    > space which is why Christian choose to make it configurable. Clearly
    > the better solution would be some sort of system control that can be
    > modified at runtime.

    May I ask for more details about your performance regression and why
    it is related to readahead size? (we didn't change VM_MAX_READAHEAD..)

    Thanks,
    Fengguang


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