netdev - Re: [PATCH net-next 0/6] kcm: Kernel Connection Multiplexor (KCM)

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Date:	Tue, 24 Nov 2015 10:09:34 -0800
From:	Rick Jones <rick.jones2@....com>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Florian Westphal <fw@...len.de>
Cc:	David Miller <davem@...emloft.net>, tom@...bertland.com,
	hannes@...essinduktion.org, netdev@...r.kernel.org,
	kernel-team@...com, davewatson@...com, alexei.starovoitov@...il.com
Subject: Re: [PATCH net-next 0/6] kcm: Kernel Connection Multiplexor (KCM)

On 11/24/2015 07:49 AM, Eric Dumazet wrote:
> But in the end, latencies were bigger, because the application had to
> copy from kernel to user (read()) the full message in one go. While if
> you wake up application for every incoming GRO message, we prefill cpu
> caches, and the last read() only has to copy the remaining part and
> benefit from hot caches (RFS up2date state, TCP socket structure, but
> also data in the application)

You can see something similar (at least in terms of latency) when 
messing about with MTU sizes.  For some message sizes - 8KB being a 
popular one - you will see higher latency on the likes of netperf TCP_RR 
with JumboFrames than you would with the standard 1500 byte MTU. 
Something I saw on GbE links years back anyway.  I chalked it up to 
getting better parallelism between the NIC and the host.

Of course the service demands were lower with JumboFrames...

rick jones
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