On Wed, Feb 19, 2020 at 01:53:53AM +0000, Damien Le Moal wrote: > 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. That is why I asked if NVMe HDD will attempt to sort/merge IO among SQs from the beginning, but Tim said no, see: https://lore.kernel.org/linux-block/20200212215251.GA25314@ming.t460p/T/#m2d0eff5ef8fcaced0f304180e571bb8fefc72e84 It could be cheap for NVMe HDD to do that, given all queues/requests just stay in system's RAM. Also I guess internal IO sort/merge may not be good enough compared with SW's implementation: 1) device internal queue depth is often low, and the participated requests won't be enough many, but SW's scheduler queue depth is often 2 times of device queue depth. 2) HDD drive doesn't have context info, so when concurrent IOs are run from multiple contexts, HDD internal reorder/merge can't work well enough. blk-mq doesn't address this case too, however the legacy IO path does consider that via IOC batch. Thanks, Ming
