On 10/24/23 18:33, Ming Lei wrote:
Yeah, performance does drop when queue depth is cut to half if queue
depth is low enough.
However, it isn't enough to just test perf over one LUN, what is the
perf effect when running IOs over the 2 or 5 data LUNs
concurrently?
I think that the results I shared are sufficient because these show the
worst possible performance impact of fair tag sharing (two active
logical units and much more activity on one logical unit than on the
other).
SATA should have similar issue too, and I think the improvement may
be more generic to bypass fair tag sharing in case of low queue depth
(such as < 32) if turns out the fair tag sharing doesn't work well in
case low queue depth.
Also the 'fairness' could be enhanced dynamically by scsi LUN's
queue depth, which can be adjusted dynamically.
Most SATA devices are hard disks. Hard disk IOPS are constrained by the
speed with which the head of a hard disk can move. That speed hasn't
changed much during the past 40 years. I'm not sure that hard disks are
impacted as much as SSD devices by fair tag sharing.
Any algorithm that is more complicated than what I posted probably would
have a negative performance impact on storage devices that use NAND
technology, e.g. UFS devices. So I prefer to proceed with this patch
series and solve any issues with ATA devices separately. Once this patch
series has been merged, it could be used as a basis for a solution for
ATA devices. A solution for ATA devices does not have to be implemented
in the block layer core - it could e.g. be implemented in the ATA subsystem.
Thanks,
Bart.
