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Date: Mon, 28 Dec 2020 09:56:15 +0800 From: "yukuai (C)" <yukuai3@...wei.com> To: Ming Lei <ming.lei@...hat.com> CC: <axboe@...nel.dk>, <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>, <zhangxiaoxu5@...wei.com> Subject: Re: [PATCH 1/3] blk-mq: allow hardware queue to get more tag while sharing a tag set Hi, On 2020/12/27 19:58, Ming Lei wrote: > Hi Yu Kuai, > > On Sat, Dec 26, 2020 at 06:28:06PM +0800, Yu Kuai wrote: >> When sharing a tag set, if most disks are issuing small amount of IO, and >> only a few is issuing a large amount of IO. Current approach is to limit >> the max amount of tags a disk can get equally to the average of total >> tags. Thus the few heavy load disk can't get enough tags while many tags >> are still free in the tag set. > > Yeah, current approach just allocates same share for each active queue > which is evaluated in each timeout period. > > That said you are trying to improve the following case: > - heavy IO on one or several disks, and the average share for these > disks become bottleneck of IO performance > - small amount IO on other disks attached to the same host, and all IOs are > submitted to disk in <30 second period. > > Just wondering if you may share the workload you are trying to optimize, > or it is just one improvement in theory? And what is the disk(hdd, ssd > or nvme) and host? And how many disks in your setting? And how deep the tagset > depth is? The details of the environment that we found the problem are as follows: total driver tags: 128 number of disks: 13 (network drive, and they form a dm-multipath) default queue_depth: 32 disk performance: when test with 4k randread and single thread, iops is 300. And can up to 4000 with 32 thread. test cmd: fio -ioengine=psync -numjobs=32 ... We found that mpath will issue sg_io periodically(about 15s),which lead to active_queues setting to 13 for about 5s in every 15s. By the way, I'm not sure this is a common scenario, however, sq don't have such problem, Thanks Yu Kuai
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