netdev - Re: [PATCH 0/9 Rev3] Implement batching skb API and support in IPoIB

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Date:	Thu, 9 Aug 2007 08:49:40 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, gaagaan@...il.com,
	general@...ts.openfabrics.org, hadi@...erus.ca, jagana@...ibm.com,
	jeff@...zik.org, johnpol@....mipt.ru, kaber@...sh.net,
	kumarkr@...ux.ibm.com, mcarlson@...adcom.com, mchan@...adcom.com,
	netdev@...r.kernel.org, peter.p.waskiewicz.jr@...el.com,
	rdreier@...co.com, rick.jones2@...com, Robert.Olsson@...a.slu.se,
	shemminger@...ux-foundation.org, sri@...ibm.com, tgraf@...g.ch,
	xma@...ibm.com
Subject: Re: [PATCH 0/9 Rev3] Implement batching skb API and support in IPoIB

Herbert Xu <herbert@...dor.apana.org.au> wrote on 08/08/2007 07:12:47 PM:

> On Wed, Aug 08, 2007 at 03:49:00AM -0700, David Miller wrote:
> >
> > Not because I think it obviates your work, but rather because I'm
> > curious, could you test a TSO-in-hardware driver converted to
> > batching and see how TSO alone compares to batching for a pure
> > TCP workload?
>
> You could even lower the bar by disabling TSO and enabling
> software GSO.

I will try with E1000 (though I didn't see improvement when I tested a long
time back). The difference I expect is that TSO would help with large
packets and not necessarily small/medium packets and not definitely in
the case of multiple different skbs (as opposed to single large skb)
getting
queue'd. I think these are two different workloads.

> > I personally don't think it will help for that case at all as
> > TSO likely does better job of coalescing the work _and_ reducing
> > bus traffic as well as work in the TCP stack.
>
> I agree.  I suspect the bulk of the effort is in getting
> these skb's created and processed by the stack so that by
> the time that they're exiting the qdisc there's not much
> to be saved anymore.

However, I am getting a large improvement for IPoIB specifically for this
same case. The reason - batching will help only when queue gets full and
stopped (and to a lesser extent if tx lock was not got, which results
in fewer amount of batching that can be done).

thanks,

- KK

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