netdev - Re: A second case of XPS considerably reducing single-stream performance

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Date:   Wed, 24 Aug 2016 16:46:22 -0700
From:   Rick Jones <rick.jones2@....com>
To:     netdev@...r.kernel.org
Cc:     sathya.perla@...adcom.com, ajit.khaparde@...adcom.com,
        sriharsha.basavapatna@...adcom.com, somnath.kotur@...adcom.com
Subject: Re: A second case of XPS considerably reducing single-stream
 performance

Also, while it doesn't seem to have the same massive effect on 
throughput, I can also see out of order behaviour happening when the 
sending VM is on a node with a ConnectX-3 Pro NIC.  Its driver is also 
enabling XPS it would seem.  I'm not *certain* but looking at the traces 
it appears that with the ConnectX-3 Pro there is more interleaving of 
the out-of-order traffic than there is with the Skyhawk.  The ConnectX-3 
Pro happens to be in a newer generation server with a newer processor 
than the other systems where I've seen this.

I do not see the out-of-order behaviour when the NIC at the sending end 
is a BCM57840.  It does not appear that the bnx2x driver in the 4.4 
kernel is enabling XPS.

So, it would seem that there are three cases of enabling XPS resulting 
in out-of-order traffic, two of which result in a non-trivial loss of 
performance.

happy benchmarking,

rick jones