netdev - Re: [PATCH v6 12/12] net/mlx4_en: add prefetch in xdp rx path

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Date:	Fri, 08 Jul 2016 05:56:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Brenden Blanco <bblanco@...mgrid.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Martin KaFai Lau <kafai@...com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Ari Saha <as754m@....com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Or Gerlitz <gerlitz.or@...il.com>, john.fastabend@...il.com,
	hannes@...essinduktion.org, Thomas Graf <tgraf@...g.ch>,
	Tom Herbert <tom@...bertland.com>,
	Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v6 12/12] net/mlx4_en: add prefetch in xdp rx path

On Thu, 2016-07-07 at 19:15 -0700, Brenden Blanco wrote:
> XDP programs read and/or write packet data very early, and cache miss is
> seen to be a bottleneck.
> 
> Add prefetch logic in the xdp case 3 packets in the future. Throughput
> improved from 10Mpps to 12.5Mpps.  LLC misses as reported by perf stat
> reduced from ~14% to ~7%.  Prefetch values of 0 through 5 were compared
> with >3 showing dimishing returns.

This is what I feared with XDP.

Instead of making generic changes in the driver(s), we now have 'patches
that improve XDP numbers'

Careful prefetches make sense in NIC drivers, regardless of XDP being
used or not.

On mlx4, prefetching next cqe could probably help as well.