On Mon, Dec 28, 2020 at 09:56:15AM +0800, yukuai (C) wrote: > 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 Looks the tagset depth is a bit low. > number of disks: 13 (network drive, and they form a dm-multipath) > default queue_depth: 32 Another candidate solution may be to always return true from hctx_may_queue() for this kind of queue because queue_depth has provided fair allocation for each LUN, and looks not necessary to do that again. > 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. BTW, I just observe sg_io on rhel8 & rhel7 on mpath over scsi_debug, looks not see any such activity. > > By the way, I'm not sure this is a common scenario, however, sq don't > have such problem, If it is done by mpath at default setting, I think it can be thought as one common case. Thanks, Ming
