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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