Re: [LSF/MM/BPF TOPIC] NVMe HDD

[Keith Busch <kbusch@kernel.org>]

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
report appropriate values that maximize iops within your latency limits,
and the host will react accordingly.