linux-kernel - Re: Likley stupid question on "throttle_vm

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Message-ID: <20091110020858.GA5749@localhost>
Date:	Tue, 10 Nov 2009 10:08:58 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Martin Knoblauch <spamtrap@...bisoft.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Likley stupid question on "throttle_vm_writeout"

On Mon, Nov 09, 2009 at 04:26:33PM +0100, Peter Zijlstra wrote:
> On Mon, 2009-11-09 at 07:15 -0800, Martin Knoblauch wrote:
> > Hi, (please CC me on replies)
> > 
> >  I have a likely stupid question on the function "throttle_vm_writeout". Looking at the code I find:
> > 
> >                 if (global_page_state(NR_UNSTABLE_NFS) +
> >                         global_page_state(NR_WRITEBACK) <= dirty_thresh)
> >                                 break;
> >                 congestion_wait(WRITE, HZ/10);
> > 
> > Shouldn't the NR_FILE_DIRTY pages be considered as well?
> 
> Ha, you just trod onto a piece of ugly I'd totally forgotten about ;-)
> 
> The intent of throttle_vm_writeout() is to limit the total pages in
> writeout and to wait for them to go-away.

Like this:

        vmscan fast => large NR_WRITEBACK => throttle vmscan based on it

> Everybody hates the function, nobody managed to actually come up with
> anything better.

btw, here is another reason to limit NR_WRITEBACK: I saw many
throttle_vm_writeout() waits if there is no wait queue to limit
NR_WRITEBACK (eg. NFS). In that case the (steadily) big NR_WRITEBACK
is _not_ caused by fast vmscan..

Thanks,
Fengguang
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