linux-kernel - Re: [PATCH 2/6] writeback: reduce calls to global_page_state in balance_dirty

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Date:	Wed, 28 Jul 2010 10:04:07 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Richard Kennedy <richard@....demon.co.uk>,
	Dave Chinner <david@...morbit.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] writeback: reduce calls to global_page_state in
 balance_dirty_pages()

> > The global threshold check is added in place of clip_bdi_dirty_limit()
> > for safety and not intended as a behavior change. If ever leading to
> > big behavior change and regression, that it would be indicating some
> > too permissive per-bdi threshold calculation.
> > 
> > Did you see the global dirty threshold get exceeded when writing to 2+
> > devices? Occasional small exceeding should be OK though. I tried the
> > following debug patch and see no warnings when doing two concurrent cp
> > over local disk and NFS.
>   Oops, sorry. I've misread the code. You're right. There shouldn't be a big
> change in the behavior.

It does indicate a missing point in the changelog. The paragraph is
updated to:

        We now set and clear dirty_exceeded not only based on bdi dirty limits,
        but also on the global dirty limit. The global limit check is added in
        place of clip_bdi_dirty_limit() for safety and not intended as a
        behavior change. The bdi limits should be tight enough to keep all dirty
        pages under the global limit at most time; occasional small exceeding
        should be OK though. The change makes the logic more obvious: the global
        limit is the ultimate goal and shall be always imposed.

Thanks,
Fengguang
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