netdev - Re: [RFC] [PATCH] Optimize TCP sendmsg in favour of fast devices?

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Date:	Fri, 29 Jan 2010 17:20:20 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, eric.dumazet@...il.com,
	ilpo.jarvinen@...sinki.fi, netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Optimize TCP sendmsg in favour of fast devices?

Herbert Xu <herbert@...dor.apana.org.au> wrote on 01/29/2010 05:03:46 PM:
>
> > Same 5 runs of single netperf's:
> >
> > 0. Driver unsets F_SG but sets F_GSO:
> >         Org (16K):      BW: 18180.71    SD: 13.485
> >         New (16K):      BW: 18113.15    SD: 13.551
> >         Org (64K):      BW: 21980.28    SD: 10.306
> >         New (64K):      BW: 21386.59    SD: 10.447
> >
> > 1. Driver unsets F_SG, and with GSO off
> >         Org (16K):      BW: 10894.62    SD: 26.591
> >         New (16K):      BW: 7262.10     SD: 35.340
> >         Org (64K):      BW: 12396.41    SD: 23.357
> >         New (64K):      BW: 7853.02     SD: 32.405
> >
> >
> > 2. Driver unsets F_SG and uses ethtool to set GSO:
> >         Org (16K):      BW: 18094.11    SD: 13.603
> >         New (16K):      BW: 17952.38    SD: 13.743
> >         Org (64K):      BW: 21540.78    SD: 10.771
> >         New (64K):      BW: 21818.35    SD: 10.598
>
> Hmm, any idea what is causing case 0 to be different from case 2?
> In particular, the 64K performance in case 0 appears to be a
> regression but in case 2 it's showing up as an improvement.
>
> AFAICS these two cases should produce identical results, or is
> this just jitter across tests?

You are right about the jitter. I have run this many times, most
of the times #0 and #2 are almost identical, but sometimes varies
a bit.

Also about my earlier ethtool comment:

> Yes, an ethtool bug (version 6). The test case #1 above, I
> have written that GSO is off but ethtool "thinks" it is on
> (after a modprobe -r cxgb3; modprobe cxgb3). So for test #2,
> I simply run "ethtool ... gso on", and GSO is now really on
> in the kernel, explaining the better results.

Hmmm, I had a bad ethtool it seems. I built the latest one to
debug this problem but this shows settings correctly.

thanks,

- KK

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