gain NVMe SQ in-flight entries #

[Huaicheng Li (coperd)]

Hi Keith,


> Assuming you mean the SQ tail, you can't view that without writing some
> additional code to expose this to a user. But why do you even want to
> know this?

I’m doing some simple experiments to test the effect of queue size on performance.
In the meanwhile, I’m curious to know the utilization of NVMe queues under different 
workloads.


> You can see how many are active on any given queue by reading the "tags"
> file under /sys/block/nvmeXnY/mq/<queue>/tags


Running with QEMU NVMe emulation, cat /sys/block/nvmeXn1/mq/0/tags always gives me 
(VM has one core, thus one IO queue, /sys only expose IO queues, no admin queue?)

"nr_tags=1023, reserved_tags=0, bits_per_word=6
nr_free=1023, nr_reserved=0
active_queues=0”

Is this a problem of QEMU NVMe? I don’t see /sys/xx/tags changes when running fio. 

Thanks!

Best,
Huaicheng



> On Jul 8, 2016, at 6:22 PM, Keith Busch <keith.busch at intel.com> wrote:
> 
> On Fri, Jul 08, 2016 at 06:03:33PM -0500, Huaicheng Li wrote:
>> Hi all,
>> 
>> (1). When nvme_queue_rq() queues a new I/O to the SQ, why doesn't it
>> check whether the SQ is full or not?
> 
> It can never be full. We don't preallocate enough tags to fill it.
> 
>> Similarly, when nvme_process_rq() handles a CQE, it seems not checking
>> whether CQ is empty? Why is that?
> 
> Huh? We check the phase bits to know if there's a new entry.
> 
>> (2). Why does nvme_queue_rq() need to call __nvme_process_cq()?? That
>> should be the job of interrupt handler or poll().
>> Anything missing here?
> 
> It's a micro optimization. We already hold the queue lock, might as well
> opprotunistically see if there's any work completed. Worst case scenario
> is nothing happens.
> 
>> (3). Is there an easy way to figure out how many entries are in the
>> SQ? 
> 
> You can see how many are active on any given queue by reading the "tags"
> file under /sys/block/nvmeXnY/mq/<queue>/tags
> 
> The tags are shared among multiple namespaces, so you only need to
> query one's mq tags file see how many are active.
> 
>> We can know where SQ_tail is in nvme_queue structure, how can the
>> host know where the queue's head is **immediately**?
> 
> The host doesn't really know where the SQ head is. Only the controller
> knows that, though it does provide a hint on each completion queue entry
> where it was at the time the CQE was posted.
> 
> Assuming you mean the SQ tail, you can't view that without writing some
> additional code to expose this to a user. But why do you even want to
> know this?