netdev - Re: [PATCH 00/10] Implement batching skb API

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Date:	Fri, 20 Jul 2007 08:57:37 +0100
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	davem@...emloft.net, gaagaan@...il.com,
	general@...ts.openfabrics.org, hadi@...erus.ca,
	herbert@...dor.apana.org.au, 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, sri@...ibm.com,
	tgraf@...g.ch, xma@...ibm.com
Subject: Re: [PATCH 00/10] Implement batching skb API

On Fri, 20 Jul 2007 13:00:25 +0530
Krishna Kumar2 <krkumar2@...ibm.com> wrote:

> Stephen Hemminger <shemminger@...ux-foundation.org> wrote on 07/20/2007
> 12:48:48 PM:
> 
> > You may see worse performance with batching in the real world when
> > running over WAN's.  Like TSO, batching will generate back to back packet
> > trains that are subject to multi-packet synchronized loss. The problem is
> that
> > intermediate router queues are often close to full, and when a long
> string
> > of packets arrives back to back only the first ones will get in, the rest
> > get dropped.  Normal sends have at least minimal pacing so they are less
> > likely do get synchronized drop.
> 
> Hi Stephen,
> 
> OK. The difference that I could see is that in existing code, the "minimal
> pacing" also could lead to (possibly slighly lesser) loss since sends are
> quick iterations at the IP layer, while in batching sends are iterative at
> the driver layer.
> 
> Is it an issue ? Any suggestions ?

Not an immediate issue, but it is the kind of thing that could cause performance
regression reports if it was used on every interface by default.
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