Re: Early SPECWeb99 results on 2.5.33 with TSO on e1000

Todd Underwood
2002-09-12T07:26:25Z

folx,

sorry for the late reply. catching up on kernel mail.

so all this TSO stuff looks v. v. similar to the IP-only fragmentation
that patricia gilfeather and i implemented on alteon acenics a couple of
years ago (see http://www.cs.unm.edu/~maccabe/SSL/frag/FragPaper1/ for a
general overview). it's exciting to see someone else take a stab on
different hardware and approaching some of the tcp-specific issues.

the main different, though, is that general purpose kernel development
still focussed on the improvements in *sending* speed. for real high
performance networking, the improvements are necessary in *receiving* cpu
utilization, in our estimation. (see our analysis of interrupt overhead
and the effect on receivers at gigabit speeds--i hope that this has become
common understanding by now)

i guess i can't disagree with david miller that the improvments in TSO are
due entirely to header retransmission for sending, but that's only because
sending wasn't CPU-intensive in the first place. we were able to get a
significant reduction in receiver cpu-utilization by reassembling IP
fragments on the receiver side (sort of a standards-based interrupt
mitigation strategy that has the benefit of not increasing latency the way
interrupt coalescing does).

anyway, nice work,

t.

On Thu, 5 Sep 2002, David S. Miller wrote:

> It's the DMA bandwidth saved, most of the specweb runs on x86 hardware
> is limited by the DMA throughput of the PCI host controller. In
> particular some controllers are limited to smaller DMA bursts to
> work around hardware bugs.
>
> Ie. the headers that don't need to go across the bus are the critical
> resource saved by TSO.
>
> I think I've said this a million times, perhaps the next person who
> tries to figure out where the gains come from can just reply with
> a pointer to a URL of this email I'm typing right now :-)

--
todd underwood, vp & cto
oso grande technologies, inc.
todd@osogrande.com

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