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

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Message-ID: <17888.14958.85897.289141@gargle.gargle.HOWL>
Date:	Sat, 24 Feb 2007 16:15:26 +0300
From:	Nikita Danilov <nikita@...sterfs.com>
To:	Tomoki Sekiyama <tomoki.sekiyama.qu@...achi.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	miklos@...redi.hu, 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
Subject: Re: [RFC][PATCH 0/3] VM throttling: avoid blocking occasional writers

Tomoki Sekiyama writes:
 > Hi,

Hello,

 > 

[...]

 > 
 > While Dirty+Writeback pages get more than 40% of memory, process-B is
 > blocked in balance_dirty_pages() until writeback of some (`write_chunk',
 > typically = 1536) dirty pages on disk-b is started.

May be the simpler solution is to use separate variables to control
ratelimit and write chunk?

writeback_set_ratelimit() adjusts ratelimit_pages to avoid too frequent
calls to balance_dirty_pages(), but once we are inside of
writeback_inodes(), there is no need to write especially many pages in
one go: overhead of any additional looping is negligible, when compared
with the cost of writing.

Speaking of which, now that expensive get_writeback_state() is gone from
page-writeback.c why do we need adjustable ratelimiting at all? It looks
like writeback_set_ratelimit() can be dropped, and fixed ratelimit used
instead.

Nikita.

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