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From: Long Li <longli@microsoft.com>
To: Christoph Hellwig <hch@lst.de>, Ming Lei <ming.lei@redhat.com>
Cc: Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
	Sagi Grimberg <sagi@grimberg.me>,
	"linux-nvme@lists.infradead.org" <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 00:33:50 +0000
Message-ID: <CY4PR21MB0741004E62F9C50B8EF7DA9ECE770@CY4PR21MB0741.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20191111204446.GA26028@lst.de>

>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. 

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)

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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 [this message]
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
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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