> -----Original Message----- > From: Christoph Hellwig [mailto:h...@lst.de] > Sent: Thursday, July 20, 2017 1:18 PM > To: Shivasharan Srikanteshwara > Cc: Christoph Hellwig; linux-scsi@vger.kernel.org; martin.peter...@oracle.com; > the...@redhat.com; j...@linux.vnet.ibm.com; Sumit Saxena; h...@suse.com; > Kashyap Desai > Subject: Re: [PATCH v2 11/15] megaraid_sas: Set device queue_depth same as > HBA can_queue value in scsi-mq mode > > I still don't understand why you don't want to do the same for the non-mq path.
Hi Christoph, Sorry for delay in response. MQ case - If there is any block layer requeue happens, we see performance drop. So we avoid re-queue increasing Device QD = HBA QD. Performance drop due to block layer re-queue is more in case of HDD (sequential IO converted into random IO). Non-MQ case. If we increase Device QD = HBA QD for no-mq case, we see performance drop for certain profiles. For example SATA SSD, previous driver in non-mq set Device QD=32. In this case, if we have more outstanding IO per device (more than 32), block layer attempts soft merge and eventually end user experience higher performance due to block layer attempting soft merge. Same is not correct in MQ case, as IO scheduler in MQ adds overhead if at all there is any throttling or staging due to device QD. Below is example of single SATA SSD, Sequential Read, BS=4K, IO depth = 256 MQ enable, Device QD = 32 achieves 137K IOPS MQ enable, Device QD = 916 (HBA QD) achieves 145K IOPS MQ disable, Device QD = 32 achieves 237K IOPS MQ disable, Device QD = 916 (HBA QD) achieves 145K IOPS Ideally we want to keep same QD settings in non-MQ mode, but trying to avoid as we may face some regression from end user as explained. Thanks, Shivasharan
