netdev - Re: [RFC] napi: limit GRO latency

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Date:	Mon, 08 Oct 2012 21:30:12 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Miller <davem@...emloft.net>, rick.jones2@...com,
	herbert@...dor.apana.org.au, netdev@...r.kernel.org,
	jesse@...ira.com
Subject: Re: [RFC] napi: limit GRO latency

On Mon, 2012-10-08 at 12:12 -0700, Stephen Hemminger wrote:
> On Mon, 08 Oct 2012 15:10:35 -0400 (EDT)
> David Miller <davem@...emloft.net> wrote:
> 
> > From: Stephen Hemminger <shemminger@...tta.com>
> > Date: Mon, 8 Oct 2012 11:58:35 -0700
> > 
> > > Limit the latency of pending GRO in NAPI processing to 2*HZ.
> > > When the system is under heavy network load, NAPI will go into
> > > poll mode via soft irq, and only stay in the loop for
> > > two jiffies. If this occurs, process the GRO pending list
> > > to make sure and not delay outstanding TCP frames for too long.
> > > 
> > > Rearrange the exit path to get rid of unnecessary goto logic.
> > > 
> > > Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> > 
> > Did you see Eric's patch I just applied which limits it to 1ms?
> 
> Think this is a different problem.
> After leaving softirq, it may be a long time until
> ksoftirqd is run which is a different problem.

System is under stress, you dont want to increase load at this point,
but increase bandwidth.

Really try this on a 40Gbe link and see how bad it is.

On a server, we probably dedicate one or several cpus only to run NAPI,
so yes we hit net_rx_action() break, but we'll reenter it a few us
later.



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