Hi John, On Mon, Oct 09, 2017 at 01:09:22PM +0100, John Garry wrote: > On 30/09/2017 11:27, Ming Lei wrote: > > Hi Jens, > > > > In Red Hat internal storage test wrt. blk-mq scheduler, we > > found that I/O performance is much bad with mq-deadline, especially > > about sequential I/O on some multi-queue SCSI devcies(lpfc, qla2xxx, > > SRP...) > > > > Turns out one big issue causes the performance regression: requests > > are still dequeued from sw queue/scheduler queue even when ldd's > > queue is busy, so I/O merge becomes quite difficult to make, then > > sequential IO degrades a lot. > > > > This issue becomes one of mains reasons for reverting default SCSI_MQ > > in V4.13. > > > > The 1st patch takes direct issue in blk_mq_request_bypass_insert(), > > then we can improve dm-mpath's performance in part 2, which will > > be posted out soon. > > > > The 2nd six patches improve this situation, and brings back > > some performance loss. > > > > With this change, SCSI-MQ sequential I/O performance is > > improved much, Paolo reported that mq-deadline performance > > improved much[2] in his dbench test wrt V2. Also performanc > > improvement on lpfc/qla2xx was observed with V1.[1] > > > > Please consider it for V4.15. > > > > [1] http://marc.info/?l=linux-block&m=150151989915776&w=2 > > [2] https://marc.info/?l=linux-block&m=150217980602843&w=2 > > > > I tested this series for the SAS controller on HiSilicon hip07 platform as I > am interested in enabling MQ for this driver. Driver is > ./drivers/scsi/hisi_sas/. > > So I found that that performance is improved when enabling default SCSI_MQ > with this series vs baseline. However, it is still not as a good as when > default SCSI_MQ is disabled. > > Here are some figures I got with fio: > 4.14-rc2 without default SCSI_MQ > read, rw, write IOPS > 952K, 133K/133K, 800K > > 4.14-rc2 with default SCSI_MQ > read, rw, write IOPS > 311K, 117K/117K, 320K > > This series* without default SCSI_MQ > read, rw, write IOPS > 975K, 132K/132K, 790K > > This series* with default SCSI_MQ > read, rw, write IOPS > 770K, 164K/164K, 594K Thanks for testing this patchset! Looks there is big improvement, but the gap compared with block legacy is not small too. > > Please note that hisi_sas driver does not enable mq by exposing multiple > queues to upper layer (even though it has multiple queues). I have been > playing with enabling it, but my performance is always worse... > > * I'm using > https://github.com/ming1/linux/commits/blk_mq_improve_scsi_mpath_perf_V5.1, > as advised by Ming Lei. Could you test on the following branch and see if it makes a difference? https://github.com/ming1/linux/commits/blk_mq_improve_scsi_mpath_perf_V6.1_test BTW, one big change is in the following commit, which just takes block legacy's policy to dequeue request, and I can observe some improvement on virtio-scsi too, and this commit is just for verification/debug purpose, which is never posted out before. https://github.com/ming1/linux/commit/94a117fdd9cfc1291445e5a35f04464c89c9ce70 Thanks, Ming
