linux-kernel - Re: [PATCH 10/10] mm: per device dirty threshold

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Date:	Tue, 24 Apr 2007 14:07:21 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	akpm@...ux-foundation.org
CC:	miklos@...redi.hu, a.p.zijlstra@...llo.nl, neilb@...e.de,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 10/10] mm: per device dirty threshold

> > No, we _start_ writeback for 1.5*ratelimit_pages pages, but do not
> > wait for those writebacks to finish.
> > 
> > So for a slow device and a fast writer, dirty+writeback can indeed
> > increase beyond the dirty threshold.
> > 
> 
> Nope, try it.
> 
> If a process dirties 1000 pages it'll then go into balance_dirty_pages()
> and start writeback against 1,500 pages.  When we hit dirty_ratio that
> process will be required to write back 1,500 pages for each eight pages
> which it dirtied.  We'll quickly reach the stage where there are no longer
> 1,500 pages to be written back and the process will block in
> balance_dirty_pages() until the dirty+writeback level subsides.

OK.  I was confused by this:

static long ratelimit_pages = 32;

and didn't realize, that that 32 is totally irrelevant.

So I'm still right, that for N dirty pages, the writer is allowed to
dirty N/1500*8 more dirty pages, but I agree, that this isn't really
an issue.

Miklos
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