From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rafal Mielniczuk Subject: Re: [PATCH RFC v2 0/5] Multi-queue support for xen-blkfront and xen-blkback Date: Tue, 11 Aug 2015 09:45:42 +0000 Message-ID: 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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta3.messagelabs.com ([195.245.230.39]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1ZP6B5-0000j1-Ia for xen-devel@lists.xenproject.org; Tue, 11 Aug 2015 09:49:03 +0000 Content-Language: en-US List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Bob Liu , Jens Axboe Cc: Jonathan Davies , Felipe Franciosi , "linux-kernel@vger.kernel.org" , Marcus Granado , Christoph Hellwig , David Vrabel , Arianna Avanzini , "xen-devel@lists.xenproject.org" , "boris.ostrovsky@oracle.com" List-Id: xen-devel@lists.xenproject.org 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: >>> On 01/07/15 04:03, Jens Axboe wrote: >>>> On 06/30/2015 08:21 AM, Marcus Granado wrote: >>>>> Hi, >>>>> >>>>> Our measurements for the multiqueue patch indicate a clear improvement >>>>> in iops when more queues are used. >>>>> >>>>> The measurements were obtained under the following conditions: >>>>> >>>>> - using blkback as the dom0 backend with the multiqueue patch applied to >>>>> a dom0 kernel 4.0 on 8 vcpus. >>>>> >>>>> - using a recent Ubuntu 15.04 kernel 3.19 with multiqueue frontend >>>>> applied to be used as a guest on 4 vcpus >>>>> >>>>> - using a micron RealSSD P320h as the underlying local storage on a Dell >>>>> PowerEdge R720 with 2 Xeon E5-2643 v2 cpus. >>>>> >>>>> - fio 2.2.7-22-g36870 as the generator of synthetic loads in the guest. >>>>> We used direct_io to skip caching in the guest and ran fio for 60s >>>>> reading a number of block sizes ranging from 512 bytes to 4MiB. Queue >>>>> depth of 32 for each queue was used to saturate individual vcpus in the >>>>> guest. >>>>> >>>>> We were interested in observing storage iops for different values of >>>>> block sizes. Our expectation was that iops would improve when increasing >>>>> the number of queues, because both the guest and dom0 would be able to >>>>> make use of more vcpus to handle these requests. >>>>> >>>>> These are the results (as aggregate iops for all the fio threads) that >>>>> we got for the conditions above with sequential reads: >>>>> >>>>> fio_threads io_depth block_size 1-queue_iops 8-queue_iops >>>>> 8 32 512 158K 264K >>>>> 8 32 1K 157K 260K >>>>> 8 32 2K 157K 258K >>>>> 8 32 4K 148K 257K >>>>> 8 32 8K 124K 207K >>>>> 8 32 16K 84K 105K >>>>> 8 32 32K 50K 54K >>>>> 8 32 64K 24K 27K >>>>> 8 32 128K 11K 13K >>>>> >>>>> 8-queue iops was better than single queue iops for all the block sizes. >>>>> There were very good improvements as well for sequential writes with >>>>> block size 4K (from 80K iops with single queue to 230K iops with 8 >>>>> queues), and no regressions were visible in any measurement performed. >>>> Great results! And I don't know why this code has lingered for so long, >>>> so thanks for helping get some attention to this again. >>>> >>>> Personally I'd be really interested in the results for the same set of >>>> tests, but without the blk-mq patches. Do you have them, or could you >>>> potentially run them? >>>> >>> 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"