From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757084Ab3KYSAi (ORCPT ); Mon, 25 Nov 2013 13:00:38 -0500 Received: from mail-oa0-f51.google.com ([209.85.219.51]:39319 "EHLO mail-oa0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756729Ab3KYSAg (ORCPT ); Mon, 25 Nov 2013 13:00:36 -0500 Date: Mon, 25 Nov 2013 12:00:30 -0600 From: Seth Jennings To: Dan Streetman Cc: linux-mm@kvack.org, linux-kernel , Bob Liu , Minchan Kim , Weijie Yang Subject: Re: [PATCH v2] mm/zswap: change zswap to writethrough cache Message-ID: <20131125180030.GA23396@cerebellum.variantweb.net> References: <1384976973-32722-1-git-send-email-ddstreet@ieee.org> <20131122172916.GB6477@cerebellum.variantweb.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131122172916.GB6477@cerebellum.variantweb.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 22, 2013 at 11:29:16AM -0600, Seth Jennings wrote: > On Wed, Nov 20, 2013 at 02:49:33PM -0500, Dan Streetman wrote: > > Currently, zswap is writeback cache; stored pages are not sent > > to swap disk, and when zswap wants to evict old pages it must > > first write them back to swap cache/disk manually. This avoids > > swap out disk I/O up front, but only moves that disk I/O to > > the writeback case (for pages that are evicted), and adds the > > overhead of having to uncompress the evicted pages, and adds the > > need for an additional free page (to store the uncompressed page) > > at a time of likely high memory pressure. Additionally, being > > writeback adds complexity to zswap by having to perform the > > writeback on page eviction. > > > > This changes zswap to writethrough cache by enabling > > frontswap_writethrough() before registering, so that any > > successful page store will also be written to swap disk. All the > > writeback code is removed since it is no longer needed, and the > > only operation during a page eviction is now to remove the entry > > from the tree and free it. > > I like it. It gets rid of a lot of nasty writeback code in zswap. > > I'll have to test before I ack, hopefully by the end of the day. > > Yes, this will increase writes to the swap device over the delayed > writeback approach. I think it is a good thing though. I think it > makes the difference between zswap and zram, both in operation and in > application, more apparent. Zram is the better choice for embedded where > write wear is a concern, and zswap being better if you need more > flexibility to dynamically manage the compressed pool. One thing I realized while doing my testing was that making zswap writethrough also impacts synchronous reclaim. Zswap, as it is now, makes the swapcache page clean during swap_writepage() which allows shrink_page_list() to immediately reclaim it. Making zswap writethrough eliminates this advantage and swapcache pages must be scanned again before they can be reclaimed, as is the case with normal swapping. Just something I am thinking about. Seth