Messages in this thread | | | Date | Wed, 16 Jul 2003 16:00:02 +0200 | From | Jens Axboe <> | Subject | Re: RFC on io-stalls patch |
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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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