linux-kernel - Re: [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory

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Date:   Thu, 1 Mar 2018 16:29:19 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Stephen Bates <sbates@...thlin.com>
Cc:     Sagi Grimberg <sagi@...mberg.me>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Steve Wise <swise@...ngridcomputing.com>
Subject: Re: [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory



On 01/03/18 04:20 PM, Jason Gunthorpe wrote:
> On Thu, Mar 01, 2018 at 11:00:51PM +0000, Stephen  Bates wrote:

> No, locality matters. If you have a bunch of NICs and bunch of drives
> and the allocator chooses to put all P2P memory on a single drive your
> performance will suck horribly even if all the traffic is offloaded.
> 
> Performance will suck if you have speed differences between the PCI-E
> devices. Eg a bunch of Gen 3 x8 NVMe cards paired with a Gen 4 x16 NIC
> will not reach full performance unless the p2p buffers are properly
> balanced between all cards.

This would be solved better by choosing the closest devices in the 
hierarchy in the p2pmem_find function (ie. preferring devices involved 
in the transaction). Also, based on the current code, it's a bit of a 
moot point seeing it requires all devices to be on the same switch. So 
unless you have a giant switch hidden somewhere you're not going to get 
too many NIC/NVME pairs.

In any case, we are looking at changing both those things so distance in 
the topology is preferred and the ability to span multiple switches is 
allowed. We're also discussing changing it to "user picks" which 
simplifies all of this.

Logan