From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 469DAC433F5 for ; Mon, 25 Apr 2022 06:47:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230078AbiDYGuf (ORCPT ); Mon, 25 Apr 2022 02:50:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57312 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229568AbiDYGud (ORCPT ); Mon, 25 Apr 2022 02:50:33 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 66A2A69483; Sun, 24 Apr 2022 23:47:28 -0700 (PDT) Received: from kwepemi500017.china.huawei.com (unknown [172.30.72.53]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4KmwXW0KbYzfb6Z; Mon, 25 Apr 2022 14:46:35 +0800 (CST) Received: from kwepemm600009.china.huawei.com (7.193.23.164) by kwepemi500017.china.huawei.com (7.221.188.110) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Mon, 25 Apr 2022 14:47:26 +0800 Received: from [10.174.176.73] (10.174.176.73) by kwepemm600009.china.huawei.com (7.193.23.164) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Mon, 25 Apr 2022 14:47:25 +0800 Subject: Re: [PATCH -next RFC v3 0/8] improve tag allocation under heavy load To: Damien Le Moal , , , , , , CC: , , References: <20220415101053.554495-1-yukuai3@huawei.com> <3fbadd9f-11dd-9043-11cf-f0839dcf30e1@opensource.wdc.com> <63e84f2a-2487-a0c3-cab2-7d2011bc2db4@huawei.com> <55e8b04f-0d2f-2ce1-6514-5abd0b67fd48@opensource.wdc.com> From: "yukuai (C)" Message-ID: <6957af40-8720-d74b-5be7-6bcdd9aa1089@huawei.com> Date: Mon, 25 Apr 2022 14:47:24 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <55e8b04f-0d2f-2ce1-6514-5abd0b67fd48@opensource.wdc.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.176.73] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To kwepemm600009.china.huawei.com (7.193.23.164) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 在 2022/04/25 14:23, Damien Le Moal 写道: > On 4/25/22 15:14, yukuai (C) wrote: >> 在 2022/04/25 11:24, Damien Le Moal 写道: >>> On 4/24/22 11:43, yukuai (C) wrote: >>>> friendly ping ... >>>> >>>> 在 2022/04/15 18:10, Yu Kuai 写道: >>>>> Changes in v3: >>>>> - update 'waiters_cnt' before 'ws_active' in sbitmap_prepare_to_wait() >>>>> in patch 1, in case __sbq_wake_up() see 'ws_active > 0' while >>>>> 'waiters_cnt' are all 0, which will cause deap loop. >>>>> - don't add 'wait_index' during each loop in patch 2 >>>>> - fix that 'wake_index' might mismatch in the first wake up in patch 3, >>>>> also improving coding for the patch. >>>>> - add a detection in patch 4 in case io hung is triggered in corner >>>>> cases. >>>>> - make the detection, free tags are sufficient, more flexible. >>>>> - fix a race in patch 8. >>>>> - fix some words and add some comments. >>>>> >>>>> Changes in v2: >>>>> - use a new title >>>>> - add patches to fix waitqueues' unfairness - path 1-3 >>>>> - delete patch to add queue flag >>>>> - delete patch to split big io thoroughly >>>>> >>>>> In this patchset: >>>>> - patch 1-3 fix waitqueues' unfairness. >>>>> - patch 4,5 disable tag preemption on heavy load. >>>>> - patch 6 forces tag preemption for split bios. >>>>> - patch 7,8 improve large random io for HDD. We do meet the problem and >>>>> I'm trying to fix it at very low cost. However, if anyone still thinks >>>>> this is not a common case and not worth to optimize, I'll drop them. >>>>> >>>>> There is a defect for blk-mq compare to blk-sq, specifically split io >>>>> will end up discontinuous if the device is under high io pressure, while >>>>> split io will still be continuous in sq, this is because: >>>>> >>>>> 1) new io can preempt tag even if there are lots of threads waiting. >>>>> 2) split bio is issued one by one, if one bio can't get tag, it will go >>>>> to wail. >>>>> 3) each time 8(or wake batch) requests is done, 8 waiters will be woken up. >>>>> Thus if a thread is woken up, it will unlikey to get multiple tags. >>>>> >>>>> The problem was first found by upgrading kernel from v3.10 to v4.18, >>>>> test device is HDD with 256 'max_sectors_kb', and test case is issuing 1m >>>>> ios with high concurrency. >>>>> >>>>> Noted that there is a precondition for such performance problem: >>>>> There is a certain gap between bandwidth for single io with >>>>> bs=max_sectors_kb and disk upper limit. >>>>> >>>>> During the test, I found that waitqueues can be extremly unbalanced on >>>>> heavy load. This is because 'wake_index' is not set properly in >>>>> __sbq_wake_up(), see details in patch 3. >>>>> >>>>> Test environment: >>>>> arm64, 96 core with 200 BogoMIPS, test device is HDD. The default >>>>> 'max_sectors_kb' is 1280(Sorry that I was unable to test on the machine >>>>> where 'max_sectors_kb' is 256).>> >>>>> The single io performance(randwrite): >>>>> >>>>> | bs | 128k | 256k | 512k | 1m | 1280k | 2m | 4m | >>>>> | -------- | ---- | ---- | ---- | ---- | ----- | ---- | ---- | >>>>> | bw MiB/s | 20.1 | 33.4 | 51.8 | 67.1 | 74.7 | 82.9 | 82.9 | >>> >>> These results are extremely strange, unless you are running with the >>> device write cache disabled ? If you have the device write cache enabled, >>> the problem you mention above would be most likely completely invisible, >>> which I guess is why nobody really noticed any issue until now. >>> >>> Similarly, with reads, the device side read-ahead may hide the problem, >>> albeit that depends on how "intelligent" the drive is at identifying >>> sequential accesses. >>> >>>>> >>>>> It can be seen that 1280k io is already close to upper limit, and it'll >>>>> be hard to see differences with the default value, thus I set >>>>> 'max_sectors_kb' to 128 in the following test. >>>>> >>>>> Test cmd: >>>>> fio \ >>>>> -filename=/dev/$dev \ >>>>> -name=test \ >>>>> -ioengine=psync \ >>>>> -allow_mounted_write=0 \ >>>>> -group_reporting \ >>>>> -direct=1 \ >>>>> -offset_increment=1g \ >>>>> -rw=randwrite \ >>>>> -bs=1024k \ >>>>> -numjobs={1,2,4,8,16,32,64,128,256,512} \ >>>>> -runtime=110 \ >>>>> -ramp_time=10 >>>>> >>>>> Test result: MiB/s >>>>> >>>>> | numjobs | v5.18-rc1 | v5.18-rc1-patched | >>>>> | ------- | --------- | ----------------- | >>>>> | 1 | 67.7 | 67.7 | >>>>> | 2 | 67.7 | 67.7 | >>>>> | 4 | 67.7 | 67.7 | >>>>> | 8 | 67.7 | 67.7 | >>>>> | 16 | 64.8 | 65.6 | >>>>> | 32 | 59.8 | 63.8 | >>>>> | 64 | 54.9 | 59.4 | >>>>> | 128 | 49 | 56.9 | >>>>> | 256 | 37.7 | 58.3 | >>>>> | 512 | 31.8 | 57.9 | >>> >>> Device write cache disabled ? >>> >>> Also, what is the max QD of this disk ? >>> >>> E.g., if it is SATA, it is 32, so you will only get at most 64 scheduler >>> tags. So for any of your tests with more than 64 threads, many of the >>> threads will be waiting for a scheduler tag for the BIO before the >>> bio_split problem you explain triggers. Given that the numbers you show >>> are the same for before-after patch with a number of threads <= 64, I am >>> tempted to think that the problem is not really BIO splitting... >>> >>> What about random read workloads ? What kind of results do you see ? >> >> Hi, >> >> Sorry about the misleading of this test case. >> >> This testcase is high concurrency huge randwrite, it's just for the >> problem that split bios won't be issued continuously, which is the >> root cause of the performance degradation as the numjobs increases. >> >> queue_depth is 32, and numjobs is 64, thus when numjobs is not greater >> than 8, performance is fine, because the ratio of sequential io should >> be 7/8. However, as numjobs increases, performance is worse because >> the ratio is lower. For example, when numjobs is 512, the ratio of >> sequential io is about 20%. > > But with 512 jobs, you will get only 64 jobs only with IOs in the queue. > All other jobs will be waiting for a scheduler tag before being able to > issue their large BIO. No ? Hi, It's right. In fact, after this patchset, since each large io will need total 8 tags, only 8 jobs can be in the queue while others are waiting for scheduler tag. > > It sounds like the set of scheduler tags should be a bit more elastic: > always allow BIOs from a split of a large BIO to be submitted (that is to > get a scheduler tag) even if that causes a temporary excess of the number > of requests beyond the default number of scheduler tags. Doing so, all > fragments of a large BIOs can be queued immediately. From there, if the > scheduler operates correctly, all the requests from the large BIOs split > would be issued in sequence to the device. This solution sounds feasible in theory, however, I'm not sure yet how to implement that 'temporary excess'. Thanks, Kuai > > >> >> patch 6-8 will let split bios still be issued continuously under high >> pressure. >> >> Thanks, >> Kuai >> > >