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

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2020/02/19 11:16, Ming Lei wrote:
> 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.

Yes. Keith also commented on that. SQEs have to be removed in order from
the SQ, but that does not mean that the disk has to execute them in order.
So I do not think this is an issue.

> 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.

Drive internal QD can actually be quite large to accommodate for internal
house-keeping commands (e.g. ATI/FTI refreshes, media cache flushes, etc)
while simultaneously executing incoming user commands. These internal task
are often one of the reason for SAS drives to return QF at different
host-seen QD, and why in the end NVMe may need a mechanism similar to task
set full notifications in SAS.

> 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
> 
> 


-- 
Damien Le Moal
Western Digital Research