linux-kernel - Re: cleancache can lead to serious performance degradation

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Message-ID: <4E579E9F.9090906@linux.vnet.ibm.com>
Date:	Fri, 26 Aug 2011 08:24:47 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Nebojsa Trpkovic <trx.lists@...il.com>,
	linux-kernel@...r.kernel.org, Konrad Wilk <konrad.wilk@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>
Subject: Re: cleancache can lead to serious performance degradation

On 08/25/2011 11:56 AM, Dan Magenheimer wrote:

> Third, zcache is relatively new and can certainly benefit from
> the input of other developers.  The lzo1x compression in the kernel
> is fairly slow; Seth Jennings (cc'ed) is looking into alternate
> compression technologies.  Perhaps there is a better compression
> choice more suitable for older-slower processors, probably with a
> poorer compression ratio.  Further, zcache currently does compression
> and decompression with interrupts disabled, which may be a
> significant factor in the slowdowns you've observed.  This should
> be fixable.

This was something I've meaning to ask about.  Why are compression
and decompression done with interrupts disabled?  What would need
to change so that we don't have to disable interrupts?

<cut>
>>> I guess that possible workaround could be to implement some kind of
>>> compression throttling valve for cleancache/zcache:
>>>
>>> - if there's available CPU time (idle cycles or so), then compress
>>> (maybe even with low CPU scheduler priority);
> 
> Agreed, and this low-priority kernel thread ideally would also
> solve the "compress while irqs disabled" problem!

--
Seth
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