Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752081AbdHDSYw (ORCPT ); Fri, 4 Aug 2017 14:24:52 -0400 Received: from mail-vk0-f45.google.com ([209.85.213.45]:36571 "EHLO mail-vk0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751335AbdHDSYv (ORCPT ); Fri, 4 Aug 2017 14:24:51 -0400 MIME-Version: 1.0 In-Reply-To: <20170804182109.GA16128@linux.intel.com> References: <20170728165604.10455-1-ross.zwisler@linux.intel.com> <20170728173143.GE15980@bombadil.infradead.org> <20170802221359.GA20666@linux.intel.com> <20170803001315.GF32020@bbox> <20170803211335.GA1260@linux.intel.com> <20170804035441.GA305@bbox> <20170804081740.GA2083@bbox> <20170804182109.GA16128@linux.intel.com> From: Dan Williams Date: Fri, 4 Aug 2017 11:24:49 -0700 Message-ID: Subject: Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt To: Ross Zwisler Cc: Minchan Kim , Matthew Wilcox , Andrew Morton , "linux-kernel@vger.kernel.org" , "karam . lee" , Jerome Marchand , Nitin Gupta , seungho1.park@lge.com, Christoph Hellwig , Dave Chinner , Jan Kara , Jens Axboe , Vishal Verma , "linux-nvdimm@lists.01.org" , Dave Jiang 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: 2920 Lines: 62 On Fri, Aug 4, 2017 at 11:21 AM, Ross Zwisler wrote: > On Fri, Aug 04, 2017 at 11:01:08AM -0700, Dan Williams wrote: >> [ adding Dave who is working on a blk-mq + dma offload version of the >> pmem driver ] >> >> On Fri, Aug 4, 2017 at 1:17 AM, Minchan Kim wrote: >> > On Fri, Aug 04, 2017 at 12:54:41PM +0900, Minchan Kim wrote: >> [..] >> >> Thanks for the testing. Your testing number is within noise level? >> >> >> >> I cannot understand why PMEM doesn't have enough gain while BTT is significant >> >> win(8%). I guess no rw_page with BTT testing had more chances to wait bio dynamic >> >> allocation and mine and rw_page testing reduced it significantly. However, >> >> in no rw_page with pmem, there wasn't many cases to wait bio allocations due >> >> to the device is so fast so the number comes from purely the number of >> >> instructions has done. At a quick glance of bio init/submit, it's not trivial >> >> so indeed, i understand where the 12% enhancement comes from but I'm not sure >> >> it's really big difference in real practice at the cost of maintaince burden. >> > >> > I tested pmbench 10 times in my local machine(4 core) with zram-swap. >> > In my machine, even, on-stack bio is faster than rw_page. Unbelievable. >> > >> > I guess it's really hard to get stable result in severe memory pressure. >> > It would be a result within noise level(see below stddev). >> > So, I think it's hard to conclude rw_page is far faster than onstack-bio. >> > >> > rw_page >> > avg 5.54us >> > stddev 8.89% >> > max 6.02us >> > min 4.20us >> > >> > onstack bio >> > avg 5.27us >> > stddev 13.03% >> > max 5.96us >> > min 3.55us >> >> The maintenance burden of having alternative submission paths is >> significant especially as we consider the pmem driver ising more >> services of the core block layer. Ideally, I'd want to complete the >> rw_page removal work before we look at the blk-mq + dma offload >> reworks. >> >> The change to introduce BDI_CAP_SYNC is interesting because we might >> have use for switching between dma offload and cpu copy based on >> whether the I/O is synchronous or otherwise hinted to be a low latency >> request. Right now the dma offload patches are using "bio_segments() > >> 1" as the gate for selecting offload vs cpu copy which seem >> inadequate. > > Okay, so based on the feedback above and from Jens[1], it sounds like we want > to go forward with removing the rw_page() interface, and instead optimize the > regular I/O path via on-stack BIOS and dma offload, correct? > > If so, I'll prepare patches that fully remove the rw_page() code, and let > Minchan and Dave work on their optimizations. I think the conversion to on-stack-bio should be done in the same patchset that removes rw_page. We don't want to leave a known performance regression while the on-stack-bio work is in-flight.