linux-kernel - Re: [RFC] [PATCH] A clean approach to writeout throttling

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Message-ID: <20071206105242.3e289a21@cuia.boston.redhat.com>
Date:	Thu, 6 Dec 2007 10:52:42 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Daniel Phillips <phillips@...nq.net>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC] [PATCH] A clean approach to writeout throttling

On Thu, 6 Dec 2007 03:55:11 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> - We a-priori decide to limit a particular stack's peak memory usage to
>   1MB
> 
> - We empirically discover that the maximum amount of memory which is
>   allocated by that stack on behalf of a single BIO is 16kb.  (ie: that's
>   the most it has ever used for a single BIO).
> 
> - Now, we refuse to feed any more BIOs into the stack when its
>   instantaneous memory usage exceeds (1MB - 16kb).
> 
> Of course, the _average_ memory-per-BIO is much less than 16kb.  So there
> are a lot of BIOs in flight - probably hundreds, but a minimum of 63.

There is only one problem I can see with this.  With network block
IO, some memory will be consumed upon IO completion.  We need to
make sure we reserve (number of in flight BIOs * maximum amount of
memory consumed upon IO completion) memory, in addition to the
memory you're accounting in your example above.

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