netdev - Re: [PATCH net-next] net-gro: restore frag0 optimization

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Date:	Tue, 01 Apr 2014 10:16:44 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Schmidt <mschmidt@...hat.com>
Cc:	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jerry Chu <hkchu@...gle.com>
Subject: Re: [PATCH net-next] net-gro: restore frag0 optimization

On Tue, 2014-04-01 at 18:40 +0200, Michal Schmidt wrote:

> Eric,
> thank you. The patch improves the performance. Though it's still not as
> fast as it was before the commit "net-gro: Prepare GRO stack for the
> upcoming tunneling support". In repeated netperf runs my reporter now
> sees occasional results above 9 Gb/s, but on average it's only 7 Gb/s.
> 
> With your patch only Ethernet headers (and not other headers) are copied
> into skbs' heads, but this is done for all skbs. Previously (before
> Jerry's patch) no copying was needed for skbs that were GRO_MERGED.
> Is this correct?

Note that TCP performance of a single flow is very dependent from
various factors, especially on a NUMA platform.

For example, consuming less cpu can have a side effect because it allows
it to enter deeper wait states on idle periods.

If you see a problem doing the memcpy(xxx, yyy, 14) then it means
that we would have had the issue later anyway.

Thats why some drivers are doing a prefetch(frame) way before calling
napi_gro_frags()

Sure, GRO stack is a bit more complex because it now handles
encapsulation.

So is the GSO stack, and we are OK with that because
it improves general use cases in DC/virtualization world.

Make sure everything is setup properly (irq smp_affinity),
and try to use multiple TCP flows, or make sure this is not a sender
issue.

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