linux-kernel - Re: [PATCH 01/45] writeback: reduce calls to global_page_state in balance_dirty

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Message-Id: <1255260317.8967.204.camel@laptop>
Date:	Sun, 11 Oct 2009 13:25:17 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Myklebust Trond <Trond.Myklebust@...app.com>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Richard Kennedy <richard@....demon.co.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/45] writeback: reduce calls to global_page_state in
 balance_dirty_pages()

On Sun, 2009-10-11 at 18:50 +0800, Wu Fengguang wrote:
> 
> Sorry for the confusion, but I mean, filesystems have to limit
> nr_writeback (directly or indirectly via the block io queue),
> otherwise it either hit nr_dirty to 0 (with the loop), or let
> nr_writeback grow out of control (without the loop).

Doesn't this require the writeback queue to have a limit < dirty_thresh?

Or more specifically, for the bdi case:

 bdi_dirty + bdi_writeback + bdi_unstable <= bdi_thresh

we require that the writeback queue be smaller than bdi_thresh, which
could be quite difficult, since bdi_thresh can easily be 0.

Without observing the bdi_thresh constraint we can have:

  \Sum_(over bdis) writeback_queue_size

dirty pages outstanding, which could be significantly higher than
dirty_thresh.

Or am I confused again?

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