Re: Change in default vm_dirty_ratio

Peter Zijlstra
2007-06-20T09:20:00Z

On Wed, 2007-06-20 at 11:14 +0200, Jens Axboe wrote:
> On Wed, Jun 20 2007, Andrew Morton wrote:
> > Perhaps our queues are too long - if the VFS _does_ back off, it'll take
> > some time for that to have an effect.
> >
> > Perhaps the fact that the queue size knows nothing about the _size_ of the
> > requests in the queue is a problem.
>
> It's complicated, the size may not matter a lot. 128 sequential 512kb IO
> may complete faster than 128 random 4kb IO's.

Yes, is there any way a queue could be limited to a certain amount of
'completion time' ?

> > Back away even further here.
> >
> > What user-visible problem(s) are we attemping to fix?
>
> I'd like innocent-app-doing-little-write-or-fsync not being stalled by
> big-bad-app-doing-lots-of-dirtying.

Could you please try this per BDI dirty limit -v7 patch series, the very
last patch tries to address this by taking the per task dirty rate into
account.

Although, on the fsync, ext3 seems to want to do a global fsync, which
will still make the experience suck. :-(