On 2020/02/19 10:32, Ming Lei wrote: > On Wed, Feb 19, 2020 at 02:41:14AM +0900, Keith Busch wrote: >> On Tue, Feb 18, 2020 at 10:54:54AM -0500, Tim Walker wrote: >>> With regards to our discussion on queue depths, it's common knowledge >>> that an HDD choses commands from its internal command queue to >>> optimize performance. The HDD looks at things like the current >>> actuator position, current media rotational position, power >>> constraints, command age, etc to choose the best next command to >>> service. A large number of commands in the queue gives the HDD a >>> better selection of commands from which to choose to maximize >>> throughput/IOPS/etc but at the expense of the added latency due to >>> commands sitting in the queue. >>> >>> NVMe doesn't allow us to pull commands randomly from the SQ, so the >>> HDD should attempt to fill its internal queue from the various SQs, >>> according to the SQ servicing policy, so it can have a large number of >>> commands to choose from for its internal command processing >>> optimization. >> >> You don't need multiple queues for that. While the device has to fifo >> fetch commands from a host's submission queue, it may reorder their >> executuion and completion however it wants, which you can do with a >> single queue. >> >>> It seems to me that the host would want to limit the total number of >>> outstanding commands to an NVMe HDD >> >> The host shouldn't have to decide on limits. NVMe lets the device report >> it's queue count and depth. It should the device's responsibility to > > Will NVMe HDD support multiple NS? If yes, this queue depth isn't > enough, given all NSs share this single host queue depth. > >> report appropriate values that maximize iops within your latency limits, >> and the host will react accordingly. > > Suppose NVMe HDD just wants to support single NS and there is single queue, > if the device just reports one host queue depth, block layer IO sort/merge > can only be done when there is device saturation feedback provided. > > So, looks either NS queue depth or per-NS device saturation feedback > mechanism is needed, otherwise NVMe HDD may have to do internal IO > sort/merge. SAS and SATA HDDs today already do internal IO reordering and merging, a lot. That is partly why even with "none" set as the scheduler, you can see iops increasing with QD used. But yes, I think you do have a point with the saturation feedback. This may be necessary for better scheduling host-side. > > > Thanks, > Ming > > -- Damien Le Moal Western Digital Research
