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From: Sagi Grimberg <sagi@grimberg.me>
To: Ming Lei <ming.lei@redhat.com>
Cc: Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
	Long Li <longli@microsoft.com>, Christoph Hellwig <hch@lst.de>,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-mapping queue
Date: Tue, 12 Nov 2019 09:35:08 -0800
Message-ID: <4664ca6f-2ebb-c69c-5b7f-226a86394adf@grimberg.me> (raw)
In-Reply-To: <20191112095649.GE15079@ming.t460p>


>>> f9dde187fa92("nvme-pci: remove cq check after submission") removes
>>> cq check after submission, this change actually causes performance
>>> regression on some NVMe drive in which single nvmeq handles requests
>>> originated from more than one blk-mq sw queues(call it multi-mapping
>>> queue).
>>>
>>> Actually polling IO after submission can handle IO more efficiently,
>>> especially for multi-mapping queue:
>>>
>>> 1) the poll itself is very cheap, and lockless check on cq is enough,
>>> see nvme_cqe_pending(). Especially the check can be done after batch
>>> submission is done.
>>>
>>> 2) when IO completion is observed via the poll in submission, the requst
>>> may be completed without interrupt involved, or the interrupt handler
>>> overload can be decreased.
>>>
>>> 3) when single sw queue is submiting IOs to this hw queue, if IOs completion
>>> is observed via this poll, the IO can be completed locally, which is
>>> cheaper than remote completion.
>>>
>>> Follows test result done on Azure L80sv2 guest with NVMe drive(
>>> Microsoft Corporation Device b111). This guest has 80 CPUs and 10
>>> numa nodes, and each NVMe drive supports 8 hw queues.
>>
>> I think that the cpu lockup is a different problem, and we should
>> separate this patch from that problem..
> 
> Why?
> 
> Most of CPU lockup is a performance issue in essence. In theory,
> improvement in IO path could alleviate the soft lockup.

I don't think its a performance issue, being exposed to stall in hard
irq is a fundamental issue. I don't see how this patch solves it.

>>> 1) test script:
>>> fio --bs=4k --ioengine=libaio --iodepth=64 --filename=/dev/nvme0n1 \
>>> 	--iodepth_batch_submit=16 --iodepth_batch_complete_min=16 \
>>> 	--direct=1 --runtime=30 --numjobs=1 --rw=randread \
>>> 	--name=test --group_reporting --gtod_reduce=1
>>>
>>> 2) test result:
>>>        | v5.3       | v5.3 with this patchset
>>> -------------------------------------------
>>> IOPS | 130K       | 424K
>>>
>>> Given IO is handled more efficiently in this way, the original report
>>> of CPU lockup[1] on Hyper-V can't be observed any more after this patch
>>> is applied. This issue is usually triggered when running IO from all
>>> CPUs concurrently.
>>>
>>
>> This is just adding code that we already removed but in a more
>> convoluted way...
> 
> That commit removing the code actually causes regression for Azure
> NVMe.

This issue has been observed long before we removed the polling from
the submission path and the cq_lock split.

>> The correct place that should optimize the polling is aio/io_uring and
>> not the driver locally IMO. Adding blk_poll to aio_getevents like
>> io_uring would be a lot better I think..
> 
> This poll is actually one-shot poll, and I shouldn't call it poll, and
> it should have been called as 'check cq'.
> 
> I believe it has been tried for supporting aio poll before, seems not
> successful.

Is there a fundamental reason why it can work for io_uring and cannot
work for aio?

>>> I also run test on Optane(32 hw queues) in big machine(96 cores, 2 numa
>>> nodes), small improvement is observed on running the above fio over two
>>> NVMe drive with batch 1.
>>
>> Given that you add shared lock and atomic ops in the data path you are
>> bound to hurt some latency oriented workloads in some way..
> 
> The spin_trylock_irqsave() is just called in case that nvme_cqe_pending() is
> true. My test on Optane doesn't show that latency is hurt.

It also condition on the multi-mapping bit..

Can we know for a fact that this doesn't hurt what-so-ever? If so, we
should always do it, not conditionally do it. I would test this for
io_uring test applications that are doing heavy polling. I think
Jens had some benchmarks he used for how fast io_uring can go in
a single cpu core...

> However, I just found the Azure's NVMe is a bit special, in which
> the 'Interrupt Coalescing' Feature register shows zero. But IO interrupt is
> often triggered when there are many commands completed by drive.
> 
> For example in fio test(4k, randread aio, single job), when IOPS is
> 110K, interrupts per second is just 13~14K. When running heavy IO, the
> interrupts per second can just reach 40~50K at most. And for normal nvme
> drive, if 'Interrupt Coalescing' isn't used, most of times one interrupt
> just complete one request in the rand IO test.
> 
> That said Azure's implementation must apply aggressive interrupt coalescing
> even though the register doesn't claim it.

Did you check how many completions a reaped per interrupt?

> That seems the root cause of soft lockup for Azure since lots of requests
> may be handled in one interrupt event, especially when interrupt event is
> delay-handled by CPU. Then it can explain why this patch improves Azure
> NVNe so much in single job fio.
> 
> But for other drives with N:1 mapping, the soft lockup risk still exists.

As I said, we can discuss this as an optimization, but we should not
consider this as a solution to the irq-stall issue reported on Azure as
we agree that it doesn't solve the fundamental problem.

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  reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-08  3:55 [PATCH 0/2] nvme-pci: improve IO performance via poll after batch submission Ming Lei
2019-11-08  3:55 ` [PATCH 1/2] nvme-pci: move sq/cq_poll lock initialization into nvme_init_queue Ming Lei
2019-11-08  4:12   ` Keith Busch
2019-11-08  7:09     ` Ming Lei
2019-11-08  3:55 ` [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-mapping queue Ming Lei
2019-11-11 20:44   ` Christoph Hellwig
2019-11-12  0:33     ` Long Li
2019-11-12  1:35       ` Sagi Grimberg
2019-11-12  2:39       ` Ming Lei
2019-11-12 16:25         ` Hannes Reinecke
2019-11-12 16:49           ` Keith Busch
2019-11-12 17:29             ` Hannes Reinecke
2019-11-13  3:05               ` Ming Lei
2019-11-13  3:17                 ` Keith Busch
2019-11-13  3:57                   ` Ming Lei
2019-11-12 21:20         ` Long Li
2019-11-12 21:36           ` Keith Busch
2019-11-13  0:50             ` Long Li
2019-11-13  2:24           ` Ming Lei
2019-11-12  2:07     ` Ming Lei
2019-11-12  1:44   ` Sagi Grimberg
2019-11-12  9:56     ` Ming Lei
2019-11-12 17:35       ` Sagi Grimberg [this message]
2019-11-12 21:17         ` Long Li
2019-11-12 23:44         ` Jens Axboe
2019-11-13  2:47         ` Ming Lei
2019-11-12 18:11   ` Nadolski, Edmund
2019-11-13 13:46     ` Ming Lei

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