Re: RFC on io-stalls patch

[Jens Axboe <axboe@suse.de>]

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.

-- 
Jens Axboe