linux-kernel - Re: Page Cache writeback too slow, SSD/noop scheduler/ext2

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Date:	Tue, 31 Mar 2009 20:31:12 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jos Houtman <jos@...es.nl>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeff Layton <jlayton@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: Page Cache writeback too slow,   SSD/noop scheduler/ext2

On Tue, Mar 31, 2009 at 08:16:52PM +0800, Jos Houtman wrote:
> > 
> > Next to that I was wondering if there are any plans to make sure that not
> > all dirty-files are written back in the same interval.
> > 
> > In my case all database files are written back each 30 seconds, while I
> > would prefer them to be more divided over the interval.
> 
> There another question I have: does the writeback go through the io
> scheduler? Because no matter the io scheduler or the tuning done, the
> writeback algorithm totally starves the reads.

I noticed this annoying writes-starve-reads problem too. I'll look into it.

> See the url below for an example with CFQ, but deadline or noop all show
> this behaviour:
> http://94.100.113.33/535450001-535500000/535451701-535451800/535451800_6_L7g
> t.jpeg
> 
> Is there anything I can do about this behaviour by creating a better
> interleaving of the reads and writes?

I guess it should be handled in the generic block io layer.  Once we
solved the writes-starve-reads problem, the bursty-writeback behavior
becomes a no-problem for SSD.

Thanks,
Fengguang

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