Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752451Ab0GRIM3 (ORCPT <rfc822;w@1wt.eu>);
	Sun, 18 Jul 2010 04:12:29 -0400
Received: from mail-pv0-f174.google.com ([74.125.83.174]:64110 "EHLO
	mail-pv0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751719Ab0GRIMX (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Sun, 18 Jul 2010 04:12:23 -0400
Message-ID: <4C42B782.7060106@vflare.org>
Date: Sun, 18 Jul 2010 13:42:50 +0530
From: Nitin Gupta <ngupta@vflare.org>
Reply-To: ngupta@vflare.org
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.4) Gecko/20100624 Fedora/3.1-1.fc13 Thunderbird/3.1
MIME-Version: 1.0
To: Pekka Enberg <penberg@cs.helsinki.fi>
CC: Hugh Dickins <hugh.dickins@tiscali.co.uk>,
        Andrew Morton <akpm@linux-foundation.org>, Greg KH <greg@kroah.com>,
        Dan Magenheimer <dan.magenheimer@oracle.com>,
        Rik van Riel <riel@redhat.com>, Avi Kivity <avi@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Minchan Kim <minchan.kim@gmail.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        linux-mm <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/8] zcache: page cache compression support
References: <1279283870-18549-1-git-send-email-ngupta@vflare.org> <4C42B228.9040401@cs.helsinki.fi>
In-Reply-To: <4C42B228.9040401@cs.helsinki.fi>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2334
Lines: 47

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/