Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754354Ab3HWOKU (ORCPT ); Fri, 23 Aug 2013 10:10:20 -0400 Received: from mail-vb0-f54.google.com ([209.85.212.54]:64471 "EHLO mail-vb0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753361Ab3HWOKS (ORCPT ); Fri, 23 Aug 2013 10:10:18 -0400 MIME-Version: 1.0 In-Reply-To: <20130822004250.GB4665@bbox> References: <1377065791-2959-1-git-send-email-minchan@kernel.org> <52148730.4000709@oracle.com> <20130822004250.GB4665@bbox> Date: Fri, 23 Aug 2013 22:10:17 +0800 Message-ID: Subject: Re: [PATCH v7 0/5] zram/zsmalloc promotion From: Bob Liu To: Minchan Kim Cc: Bob Liu , Greg Kroah-Hartman , Andrew Morton , Jens Axboe , Seth Jennings , Nitin Gupta , Konrad Rzeszutek Wilk , Luigi Semenzato , Linux-Kernel , Linux-MM , Pekka Enberg , Mel Gorman Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4006 Lines: 93 Hi Minchan, On Thu, Aug 22, 2013 at 8:42 AM, Minchan Kim wrote: > Hi Bob, > > On Wed, Aug 21, 2013 at 05:24:00PM +0800, Bob Liu wrote: >> Hi Minchan, >> >> On 08/21/2013 02:16 PM, Minchan Kim wrote: >> > It's 7th trial of zram/zsmalloc promotion. >> > I rewrote cover-letter totally based on previous discussion. >> > >> > The main reason to prevent zram promotion was no review of >> > zsmalloc part while Jens, block maintainer, already acked >> > zram part. >> > >> > At that time, zsmalloc was used for zram, zcache and zswap so >> > everybody wanted to make it general and at last, Mel reviewed it >> > when zswap was submitted to merge mainline a few month ago. >> > Most of review was related to zswap writeback mechanism which >> > can pageout compressed page in memory into real swap storage >> > in runtime and the conclusion was that zsmalloc isn't good for >> > zswap writeback so zswap borrowed zbud allocator from zcache to >> > replace zsmalloc. The zbud is bad for memory compression ratio(2) >> > but it's very predictable behavior because we can expect a zpage >> > includes just two pages as maximum. Other reviews were not major. >> > http://lkml.indiana.edu/hypermail/linux/kernel/1304.1/04334.html >> > >> > Zcache doesn't use zsmalloc either so zsmalloc's user is only >> > zram now so this patchset moves it into zsmalloc directory. >> > Recently, Bob tried to move zsmalloc under mm directory to unify >> > zram and zswap with adding pseudo block device in zswap(It's >> > very weired to me) but he was simple ignoring zram's block device >> > (a.k.a zram-blk) feature and considered only swap usecase of zram, >> > in turn, it lose zram's good concept. >> > >> >> Yes, I didn't notice the feature that zram can be used as a normal block >> device. >> >> >> > Mel raised an another issue in v6, "maintainance headache". >> > He claimed zswap and zram has a similar goal that is to compresss >> > swap pages so if we promote zram, maintainance headache happens >> > sometime by diverging implementaion between zswap and zram >> > so that he want to unify zram and zswap. For it, he want zswap >> > to implement pseudo block device like Bob did to emulate zram so >> > zswap can have an advantage of writeback as well as zram's benefit. >> >> If consider zram as a swap device only, I still think it's better to add >> a pseudo block device to zswap and just disable the writeback of zswap. > > Why do you think zswap is better? > In my opinion: 1. It's easy for zswap to do the same thing by adding a few small changes. 2. zswap won't get to the block layer which can reduce a lot of overheads. 3. zswap is transparent to current users who are using normal block device as the swap device. > I don't know but when I read http://lwn.net/Articles/334649/, it aimed > for compressing page caches as well as swap pages but it made widespread > hooks in core (I guess that's why zcache had a birth later by Nitin and Dan) > so reviewers guided him to support anon pages only to merge it. > And at that time, it was a specific virtual block device for only supporting > swap. AFAIRC, akpm suggested to make it general block device so other party > can have a benefit. > > You can type "zram tmp" in google and will find many article related > to use zram as tmp and I have been received some questions/reports I see. But i think if using shmem as tmp, the pages can be reclaimed during memory pressure, get to zswap and compressed as well. Mel also pointed a situation using zram as tmpfs may make things worse. > from anonymous guys by private mail. And Jorome, Redhat guy, has > contributed that part like partial I/O. > I am not going to block zram being promoted, just some different voice. -- Regards, --Bob -- 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/