Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934077AbbHLKRW (ORCPT ); Wed, 12 Aug 2015 06:17:22 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:23911 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755352AbbHLKRT (ORCPT ); Wed, 12 Aug 2015 06:17:19 -0400 Message-ID: <55CB1CF7.20102@oracle.com> Date: Wed, 12 Aug 2015 18:16:23 +0800 From: Bob Liu User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130308 Thunderbird/17.0.4 MIME-Version: 1.0 To: Jens Axboe CC: Rafal Mielniczuk , Marcus Granado , Arianna Avanzini , Felipe Franciosi , "linux-kernel@vger.kernel.org" , Christoph Hellwig , David Vrabel , "xen-devel@lists.xenproject.org" , "boris.ostrovsky@oracle.com" , Jonathan Davies Subject: Re: [Xen-devel] [PATCH RFC v2 0/5] Multi-queue support for xen-blkfront and xen-blkback References: <1410479844-2864-1-git-send-email-avanzini.arianna@gmail.com> <20141001202721.GF12581@laptop.dumpdata.com> <20150428073646.GA16022@infradead.org> <553F3ADF.3000301@gmail.com> <555327A5.1060200@oracle.com> <5592A5EF.2050005@citrix.com> <55935848.7080909@fb.com> <55C8C8CE.7020301@fb.com> <55C99130.3020501@oracle.com> <55CA31AB.3030308@fb.com> In-Reply-To: <55CA31AB.3030308@fb.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Source-IP: userv0021.oracle.com [156.151.31.71] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4602 Lines: 96 On 08/12/2015 01:32 AM, Jens Axboe wrote: > On 08/11/2015 03:45 AM, Rafal Mielniczuk wrote: >> On 11/08/15 07:08, Bob Liu wrote: >>> On 08/10/2015 11:52 PM, Jens Axboe wrote: >>>> On 08/10/2015 05:03 AM, Rafal Mielniczuk wrote: ... >>>>> Hello, >>>>> >>>>> We rerun the tests for sequential reads with the identical settings but with Bob Liu's multiqueue patches reverted from dom0 and guest kernels. >>>>> The results we obtained were *better* than the results we got with multiqueue patches applied: >>>>> >>>>> fio_threads io_depth block_size 1-queue_iops 8-queue_iops *no-mq-patches_iops* >>>>> 8 32 512 158K 264K 321K >>>>> 8 32 1K 157K 260K 328K >>>>> 8 32 2K 157K 258K 336K >>>>> 8 32 4K 148K 257K 308K >>>>> 8 32 8K 124K 207K 188K >>>>> 8 32 16K 84K 105K 82K >>>>> 8 32 32K 50K 54K 36K >>>>> 8 32 64K 24K 27K 16K >>>>> 8 32 128K 11K 13K 11K >>>>> >>>>> We noticed that the requests are not merged by the guest when the multiqueue patches are applied, >>>>> which results in a regression for small block sizes (RealSSD P320h's optimal block size is around 32-64KB). >>>>> >>>>> We observed similar regression for the Dell MZ-5EA1000-0D3 100 GB 2.5" Internal SSD >>>>> >>>>> As I understand blk-mq layer bypasses I/O scheduler which also effectively disables merges. >>>>> Could you explain why it is difficult to enable merging in the blk-mq layer? >>>>> That could help closing the performance gap we observed. >>>>> >>>>> Otherwise, the tests shows that the multiqueue patches does not improve the performance, >>>>> at least when it comes to sequential read/writes operations. >>>> blk-mq still provides merging, there should be no difference there. Does the xen patches set BLK_MQ_F_SHOULD_MERGE? >>>> >>> Yes. >>> Is it possible that xen-blkfront driver dequeue requests too fast after we have multiple hardware queues? >>> Because new requests don't have the chance merging with old requests which were already dequeued and issued. >>> >> >> For some reason we don't see merges even when we set multiqueue to 1. >> Below are some stats from the guest system when doing sequential 4KB reads: >> >> $ fio --name=test --ioengine=libaio --direct=1 --rw=read --numjobs=8 >> --iodepth=32 --time_based=1 --runtime=300 --bs=4KB >> --filename=/dev/xvdb >> >> $ iostat -xt 5 /dev/xvdb >> avg-cpu: %user %nice %system %iowait %steal %idle >> 0.50 0.00 2.73 85.14 2.00 9.63 >> >> Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s >> avgrq-sz avgqu-sz await r_await w_await svctm %util >> xvdb 0.00 0.00 156926.00 0.00 627704.00 0.00 >> 8.00 30.06 0.19 0.19 0.00 0.01 100.48 >> >> $ cat /sys/block/xvdb/queue/scheduler >> none >> >> $ cat /sys/block/xvdb/queue/nomerges >> 0 >> >> Relevant bits from the xenstore configuration on the dom0: >> >> /local/domain/0/backend/vbd/2/51728/dev = "xvdb" >> /local/domain/0/backend/vbd/2/51728/backend-kind = "vbd" >> /local/domain/0/backend/vbd/2/51728/type = "phy" >> /local/domain/0/backend/vbd/2/51728/multi-queue-max-queues = "1" >> >> /local/domain/2/device/vbd/51728/multi-queue-num-queues = "1" >> /local/domain/2/device/vbd/51728/ring-ref = "9" >> /local/domain/2/device/vbd/51728/event-channel = "60" > > If you add --iodepth-batch=16 to that fio command line? Both mq and non-mq relies on plugging to get > batching in the use case above, otherwise IO is dispatched immediately. O_DIRECT is immediate. > I'd be more interested in seeing a test case with buffered IO of a file system on top of the xvdb device, > if we're missing merging for that case, then that's a much bigger issue. > I was using the null block driver for xen blk-mq test. There were not merges happen any more even after patch: https://lkml.org/lkml/2015/7/13/185 (Which just converted xen block driver to use blk-mq apis) Will try a file system soon. -- 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/