linux-kernel - Re: [PATCH 0/8] zcache: page cache compression support

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Date:	Sun, 18 Jul 2010 13:42:50 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Rik van Riel <riel@...hat.com>, Avi Kivity <avi@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/8] zcache: page cache compression support

On 07/18/2010 01:20 PM, Pekka Enberg wrote:
> Nitin Gupta wrote:
>> Frequently accessed filesystem data is stored in memory to reduce access to
>> (much) slower backing disks. Under memory pressure, these pages are freed and
>> when needed again, they have to be read from disks again. When combined working
>> set of all running application exceeds amount of physical RAM, we get extereme
>> slowdown as reading a page from disk can take time in order of milliseconds.
>>
>> Memory compression increases effective memory size and allows more pages to
>> stay in RAM. Since de/compressing memory pages is several orders of magnitude
>> faster than disk I/O, this can provide signifant performance gains for many
>> workloads. Also, with multi-cores becoming common, benefits of reduced disk I/O
>> should easily outweigh the problem of increased CPU usage.
>>
>> It is implemented as a "backend" for cleancache_ops [1] which provides
>> callbacks for events such as when a page is to be removed from the page cache
>> and when it is required again. We use them to implement a 'second chance' cache
>> for these evicted page cache pages by compressing and storing them in memory
>> itself.
>>
<snip>

> 
> So why would someone want to use zram if they have transparent page cache compression with zcache? That is, why is this not a replacement for zram?
> 

zcache complements zram; it's not a replacement:

 - zram compresses anonymous pages while zcache is for page cache compression.
So, workload which depends heavily on "heap memory" usage will tend to prefer
zram and those which are I/O intensive will prefer zcache. Though I have not
yet experimented much, most workloads may want to have a mix of them.

 - zram is not just for swap. /dev/zram<id> are generic in-memory compressed
block devices which can be used for, say, /tmp, /var/... etc. temporary storage.

 - /dev/zram<id> being a generic block devices, can be used as raw disk in other
OSes also (using virtualization): For example:
http://www.vflare.org/2010/05/compressed-ram-disk-for-windows-virtual.html

Thanks,
Nitin
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