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

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Date:	Sat, 21 Apr 2007 22:25:38 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	akpm@...ux-foundation.org
CC:	miklos@...redi.hu, a.p.zijlstra@...llo.nl, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, neilb@...e.de, 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

> > The other deadlock, in throttle_vm_writeout() is still to be solved.
> 
> Let's go back to the original changelog:
> 
> Author: marcelo.tosatti <marcelo.tosatti>
> Date:   Tue Mar 8 17:25:19 2005 +0000
> 
>     [PATCH] vm: pageout throttling
>     
>     With silly pageout testcases it is possible to place huge amounts of memory
>     under I/O.  With a large request queue (CFQ uses 8192 requests) it is
>     possible to place _all_ memory under I/O at the same time.
>     
>     This means that all memory is pinned and unreclaimable and the VM gets
>     upset and goes oom.
>     
>     The patch limits the amount of memory which is under pageout writeout to be
>     a little more than the amount of memory at which balance_dirty_pages()
>     callers will synchronously throttle.
>     
>     This means that heavy pageout activity can starve heavy writeback activity
>     completely, but heavy writeback activity will not cause starvation of
>     pageout.  Because we don't want a simple `dd' to be causing excessive
>     latencies in page reclaim.
>     
>     Signed-off-by: Andrew Morton <akpm@...l.org>
>     Signed-off-by: Linus Torvalds <torvalds@...l.org>
> 
> (A good one!  I wrote it ;))
> 
> 
> I believe that the combination of dirty-page-tracking and its calls to
> balance_dirty_pages() mean that we can now never get more than dirty_ratio
> of memory into the dirty-or-writeback condition.
> 
> The vm scanner can convert dirty pages into clean, under-writeback pages,
> but it cannot increase the total of dirty+writeback.

What about swapout?  That can increase the number of writeback pages,
without decreasing the number of dirty pages, no?

Miklos
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