netdev - Re: TCP performance regression

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Message-ID: <1384265785.28458.4.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 12 Nov 2013 06:16:25 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willy Tarreau <w@....eu>
Cc:	David Laight <David.Laight@...LAB.COM>,
	Sujith Manoharan <sujith@...jith.org>, netdev@...r.kernel.org,
	Dave Taht <dave.taht@...il.com>
Subject: Re: TCP performance regression

On Tue, 2013-11-12 at 08:42 +0100, Willy Tarreau wrote:

> Well, it's not *that* large, 532 descriptors is 800 kB or 6.4 ms with
> 1500-bytes packets, and 273 microseconds for 64-byte packets. In fact
> it's not a slow interface, it's the systems it runs on which are
> generally not that fast. For example it is possible to saturate two
> gig ports at once on a single-core Armada370. But you need buffers
> large enough to compensate for the context switch time if you use
> multiple threads to send.

With GSO, each 1500-bytes packet might need 2 descriptors anyway (one
for the headers, one for the payload), so 532 descriptors only hold 400
kB or 3.2 ms ;)

If the NIC was supporting TSO, this would be another story, as a 64KB
packet could use only 3 descriptors.



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