linux-ext4 - Re: [PATCH] Improve buffered streaming write ordering

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Message-Id: <20081002124433.f6612036.akpm@linux-foundation.org>
Date:	Thu, 2 Oct 2008 12:44:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] Improve buffered streaming write ordering

On Thu, 2 Oct 2008 23:48:56 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:

> > 
> > For a 4.5GB streaming buffered write, this printk inside
> > ext4_da_writepage shows up 37,2429 times in /var/log/messages.
> > 
> 
> Part of that can happen due to shrink_page_list -> pageout -> writepagee
> call back with lots of unallocated buffer_heads(blocks).

That workload shouldn't be using that code path much at all.  It's
supposed to be the case that pdflush and balance_dirty_pages() do most
of the writeback work.

And that _used_ to be the case, but we broke it.  It happened several
years ago and I wasn't able to provoke anyone into finding out why. 
iirc the XFS guys noticed it because their throughput was fairly badly
affected.

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