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From: Sagi Grimberg <sagi@grimberg.me>
To: Ming Lei <ming.lei@redhat.com>, linux-nvme@lists.infradead.org
Cc: Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
	Long Li <longli@microsoft.com>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-mapping queue
Date: Mon, 11 Nov 2019 17:44:10 -0800
Message-ID: <82fb330e-a507-999a-69f3-947f13bbaae1@grimberg.me> (raw)
In-Reply-To: <20191108035508.26395-3-ming.lei@redhat.com>


> 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..

> 
> 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...

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..

> 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..

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  parent 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 [this message]
2019-11-12  9:56     ` Ming Lei
2019-11-12 17:35       ` Sagi Grimberg
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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