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    SubjectRe: RFC on io-stalls patch
    On Wed, Jul 16 2003, Andrea Arcangeli wrote:
    > On Wed, Jul 16, 2003 at 03:21:39PM +0200, Jens Axboe wrote:
    > > On Wed, Jul 16 2003, Andrea Arcangeli wrote:
    > > > On Wed, Jul 16, 2003 at 03:04:42PM +0200, Jens Axboe wrote:
    > > > > On Wed, Jul 16 2003, Andrea Arcangeli wrote:
    > > > > > On Wed, Jul 16, 2003 at 02:46:56PM +0200, Jens Axboe wrote:
    > > > > > > Well it's a combined problem. Threshold too high on dirty memory,
    > > > > > > someone doing a read well get stuck flushing out as well.
    > > > > >
    > > > > > a pure read not. the write throttling should be per-process, then there
    > > > > > will be little risk.
    > > > >
    > > > > A read from user space, dirtying data along the way.
    > > >
    > > > it doesn't necessairly block on dirty memory. We even _free_ ram clean
    > > > if needed, exactly because of that. You can raise the amount of _free_
    > > > ram up to 99% of the whole ram in your box to be almost guaranteed to
    > > > never wait on dirty memory freeing. Of course the default tries to
    > > > optimize for writeback cache and there's a reasonable margin to avoid
    > > > writing dirty stuff. the sysctl is there for special usages where you
    > > > want to never block in a read from userspace regardless whatever the
    > > > state of the system.
    > >
    > > That may be so, but no user will ever touch that sysctl. He just
    > > experiences what Alan outlined, system grinds to a complete halt. Only
    > > much later does it get going again.
    >
    > and on the small boxes that will happen much less now since on the small
    > boxes the biggest vm overhead could been generated by the uncontrolled
    > size of the I/O queue that previously could grow as big as 32M.

    That is true, however noone runs 32MB boxes anymore :). So I doubt that
    would be the case.

    --
    Jens Axboe

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