linux-kernel - Re: [PATCH 0/3] VM throttling: avoid blocking occasional writers

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Date:	Thu, 15 Mar 2007 11:07:45 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tomoki Sekiyama <tomoki.sekiyama.qu@...achi.com>
Cc:	linux-kernel@...r.kernel.org, yumiko.sugita.yf@...achi.com,
	masami.hiramatsu.pt@...achi.com, hidehiro.kawai.ez@...achi.com,
	yuji.kakutani.uw@...achi.com, soshima@...hat.com, haoki@...hat.com,
	kamezawa.hiroyu@...fujitsu.com, nikita@...sterfs.com,
	leroy.vanlogchem@...elft.nl
Subject: Re: [PATCH 0/3] VM throttling: avoid blocking occasional writers

> On Wed, 14 Mar 2007 21:42:46 +0900 Tomoki Sekiyama <tomoki.sekiyama.qu@...achi.com> wrote:
>
> ...
>
> 
> -Solution:
> 
> I consider that all of the dirty pages for the disk have been written
> back and that the disk is clean if a process cannot write 'write_chunk'
> pages in balance_dirty_pages().
> 
> To avoid using up the free memory with dirty pages by passing blocking,
> this patchset adds a new threshold named vm.dirty_limit_ratio to sysctl.
> 
> It modifies balance_dirty_pages() not to block when the amount of
> Dirty+Writeback is less than vm.dirty_limit_ratio percent of the memory.
> In the other cases, writers are throttled as current Linux does.
> 
> 
> In this patchset, vm.dirty_limit_ratio, instead of vm.dirty_ratio, is
> used as the clamping level of Dirty+Writeback. And, vm.dirty_ratio is
> used as the level at which a writers will itself start writeback of the
> dirty pages.

Might be a reasonable solution - let's see what Peter comes up with too.

Comments on the patch:

- Please don't VM_DIRTY_LIMIT_RATIO: just use CTL_UNNUMBERED and leave
  sysctl.h alone.

- The 40% default is already too high.  Let's set this new upper limit to
  40% and decrease he non-blocking ratio.

- Please update the procfs documentation in ./Docmentation/

- I wonder if dirty_limit_ratio is the best name we could choose. 
  vm_dirty_blocking_ratio, perhaps?  Dunno.

-
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