linux-kernel - Re: [PATCH v2] RDMA/cma: Make CM response timeout and # CM retries configurable

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Date:   Thu, 13 Jun 2019 14:23:55 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Håkon Bugge <haakon.bugge@...cle.com>
Cc:     Doug Ledford <dledford@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Parav Pandit <parav@...lanox.com>,
        Steve Wise <swise@...ngridcomputing.com>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] RDMA/cma: Make CM response timeout and # CM retries
 configurable

On Thu, Jun 13, 2019 at 06:58:30PM +0200, Håkon Bugge wrote:

> If you refer to the backlog parameter in rdma_listen(), I cannot see
> it being used at all for IB.
> 
> For CX-3, which is paravirtualized wrt. MAD packets, it is the proxy
> UD receive queue length for the PF driver that can be construed as a
> backlog. 

No, in IB you can drop UD packets if your RQ is full - so the proxy RQ
is really part of the overall RQ on QP1.

The backlog starts once packets are taken off the RQ and begin the
connection accept processing.

> Customer configures #VMs and different workload may lead to way
> different number of CM connections. The proxying of MAD packet
> through the PF driver has a finite packet rate. With 64 VMs, 10.000
> QPs on each, all going down due to a switch failing or similar, you
> have 640.000 DREQs to be sent, and with the finite packet rate of MA
> packets through the PF, this takes more than the current CM
> timeout. And then you re-transmit and increase the burden of the PF
> proxying.

I feel like the performance of all this proxying is too low to support
such a large work load :(

Can it be improved?

Jason