Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760706AbbLCPtd (ORCPT ); Thu, 3 Dec 2015 10:49:33 -0500 Received: from mx1.redhat.com ([209.132.183.28]:42074 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758101AbbLCPtb (ORCPT ); Thu, 3 Dec 2015 10:49:31 -0500 Date: Thu, 3 Dec 2015 10:49:30 -0500 (EST) From: Mikulas Patocka X-X-Sender: mpatocka@file01.intranet.prod.int.rdu2.redhat.com To: Baolin Wang cc: Mark Brown , Jens Axboe , keith.busch@intel.com, Jan Kara , Arnd Bergmann , Mike Snitzer , neilb@suse.com, LKML , linux-raid@vger.kernel.org, dm-devel@redhat.com, "Garg, Dinesh" , tj@kernel.org, bart.vanassche@sandisk.com, jmoyer@redhat.com, Alasdair G Kergon Subject: Re: [dm-devel] [PATCH 0/2] Introduce the request handling for dm-crypt In-Reply-To: Message-ID: References: <20151111181813.GD12236@redhat.com> <20151112100422.GM12392@sirena.org.uk> <5644AFA2.6040201@kernel.dk> <20151113115144.GR12392@sirena.org.uk> <20151202195657.GB11127@agk-dp.fab.redhat.com> User-Agent: Alpine 2.02 (LRH 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3309 Lines: 71 On Thu, 3 Dec 2015, Baolin Wang wrote: > On 3 December 2015 at 10:56, Baolin Wang wrote: > > On 3 December 2015 at 03:56, Alasdair G Kergon wrote: > >> On Wed, Dec 02, 2015 at 08:46:54PM +0800, Baolin Wang wrote: > >>> These are the benchmarks for request based dm-crypt. Please check it. > >> > >> Now please put request-based dm-crypt completely to one side and focus > >> just on the existing bio-based code. Why is it slower and what can be > >> adjusted to improve this? > >> > > > > OK. I think I find something need to be point out. > > 1. From the IO block size test in the performance report, for the > > request based, we can find it can not get the corresponding > > performance if we just expand the IO size. Because In dm crypt, it > > will map the data buffer of one request with scatterlists, and send > > all scatterlists of one request to the encryption engine to encrypt or > > decrypt. I found if the scatterlist list number is small and each > > scatterlist length is bigger, it will improve the encryption speed, > > that helps the engine palys best performance. But a big IO size does > > not mean bigger scatterlists (maybe many scatterlists with small > > length), that's why we can not get the corresponding performance if we > > just expand the IO size I think. > > > > 2. Why bio based is slower? > > If you understand 1, you can obviously understand the crypto engine > > likes bigger scatterlists to improve the performance. But for bio > > based, it only send one scatterlist (the scatterlist's length is > > always '1 << SECTOR_SHIFT' = 512) to the crypto engine at one time. It > > means if the bio size is 1M, the bio based will send 2048 times (evey > > time the only one scatterlist length is 512 bytes) to crypto engine to > > handle, which is more time-consuming and ineffective for the crypto > > engine. But for request based, it can map the whole request with many > > scatterlists (not just one scatterlist), and send all the scatterlists > > to the crypto engine which can improve the performance, is it right? > > > > Another optimization solution I think is we can expand the scatterlist > > entry number for bio based. > > > > I did some testing about my assumption of expanding the scatterlist > entry number for bio based. I did some modification for the bio based > to support multiple scatterlists, then it will get the same > performance as the request based things. > > 1. bio based with expanding the scatterlist entry > time dd if=/dev/dm-0 of=/dev/null bs=64K count=16384 iflag=direct > 1073741824 bytes (1.1 GB) copied, 94.5458 s, 11.4 MB/s > real 1m34.562s > user 0m0.030s > sys 0m3.850s > > 2. Sequential read 1G with requset based: > time dd if=/dev/dm-0 of=/dev/null bs=64K count=16384 iflag=direct > 1073741824 bytes (1.1 GB) copied, 94.8922 s, 11.3 MB/s > real 1m34.908s > user 0m0.030s > sys 0m4.000s Measuring the system time this way is completely wrong because it doesn't account for the time spent in kernel threads. Mikulas -- 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/