RE: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-mapping queue

[Long Li <longli@microsoft.com>]

>Subject: Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-
>mapping queue
>
>On Tue, Nov 12, 2019 at 12:33:50AM +0000, Long Li wrote:
>> >From: Christoph Hellwig <hch@lst.de>
>> >Sent: Monday, November 11, 2019 12:45 PM
>> >To: Ming Lei <ming.lei@redhat.com>
>> >Cc: linux-nvme@lists.infradead.org; Keith Busch <kbusch@kernel.org>;
>> >Jens Axboe <axboe@fb.com>; Christoph Hellwig <hch@lst.de>; Sagi
>> >Grimberg <sagi@grimberg.me>; Long Li <longli@microsoft.com>
>> >Subject: Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for
>> >multi- mapping queue
>> >
>> >On Fri, Nov 08, 2019 at 11:55:08AM +0800, Ming Lei wrote:
>> >> 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).
>> >
>> >> 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.
>> >
>> >Have you actually seen this on a real nvme drive as well?
>> >
>> >Note that it is kinda silly to limit queues like that in VMs, so I
>> >really don't think we should optimize the driver for this particular case.
>>
>> I tested on an Azure L80s_v2 VM with newer Samsung P983 NVMe SSD
>(with 32 hardware queues). Tests also showed soft lockup when 32 queues
>are shared by 80 CPUs.
>>
>
>BTW, do you see if this simple change makes a difference?

Yes, I can confirm the patch fixed lockup on this VM configuration.  There is also no performance regression.

>
>> The issue will likely show up if the number of NVMe hardware queues is
>> less than the number of CPUs. I think this may be a likely
>> configuration on a very large system. (e.g. the largest VM on Azure
>> has 416 cores)
>>
>
>'the number of NVMe hardware queues' above should be the number of
>single NVMe drive.
>I believe 32 hw queues is common, also poll queues may take several from
>the total 32.
>When interrupt handling on single CPU core can't catch up with NVMe's IO
>handling, soft lockup could be triggered. Of course, there are lot kinds of
>supported processors by Linux.
>
>Also when (nr_nvme_drives * nr_nvme_hw_queues) > nr_cpu_cores, one
>same CPU can be assigned to handle more than 1 nvme IO queue interrupt
>from different NVMe drive, the situation becomes worse.
>
>
>Thanks,
>Ming


_______________________________________________
Linux-nvme mailing list
Linux-nvme@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-nvme