netdev - Re: tcp: Do not apply TSO segment limit to non-TSO packets

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Message-ID: <20150102220107.GA28599@gondor.apana.org.au>
Date:	Sat, 3 Jan 2015 09:01:07 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, thomas.jarosch@...ra2net.com,
	netdev@...r.kernel.org, edumazet@...gle.com,
	steffen.klassert@...unet.com, bhutchings@...arflare.com
Subject: Re: tcp: Do not apply TSO segment limit to non-TSO packets

On Fri, Jan 02, 2015 at 03:36:55PM -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Fri, 02 Jan 2015 10:24:00 -0800
>
> > Non TSO/GSO path is known to be better for devices unable to perform TX
> > checksumming, as we compute the checksum at the time we copy data from
> > user to kernel (csum_and_copy_from_user() from tcp_sendmsg())).
> 
> Non-SG capable devices suffer in this scenerio as well.

Yes I was referring to using GSO on non-SG/non-checksumming devices.
Anything that supports checksum/SG should obviously be using GSO.

IIRC when I first tested this GSO is basically on par for the non-SG
case as the overhead of the extra copying was offset by the benefit
of a larger MTU through the stack.

So has anyone actually observed worse performance with GSO on these
devices (you could even test on a GSO-capable device simply by
disabling checksumming)?

Also the fact that this bug took two years to surface means that
very few people are actually using non-GSO in the real world as
this bug is easily triggered by a PMTU event.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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