linux-kernel - Re: Change in default vm_dirty

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Message-ID: <20070620092058.GP18863@kernel.dk>
Date:	Wed, 20 Jun 2007 11:20:59 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, davej@...hat.com,
	tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: Change in default vm_dirty_ratio

On Wed, Jun 20 2007, Peter Zijlstra wrote:
> 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' ?

Not easily, we'd need some sort of disk profile for that to be remotely
reliable.

> > > 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.

Yeah, I've been watching your patchset with interesting. Hope it'll get
merged some time soon, I think it's a real problem.

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

Yeah well, extX sucks on many levels :-)

-- 
Jens Axboe

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