linux-kernel - Re: 2.6.19-rc1: Volanomark slowdown

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Date:	Wed, 8 Nov 2006 14:58:15 -0800
From:	Stephen Hemminger <shemminger@...l.org>
To:	Olaf Kirch <okir@...e.de>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, davem@...set.davemloft.net,
	kuznet@....inr.ac.ru, netdev@...r.kernel.org
Subject: Re: 2.6.19-rc1: Volanomark slowdown

On Wed, 8 Nov 2006 23:10:28 +0100
Olaf Kirch <okir@...e.de> wrote:

> On Wed, Nov 08, 2006 at 10:38:52AM -0800, Tim Chen wrote:
> > The patch in question affects purely TCP and not the scheduler.  I don't
> 
> I know.
> 
> > think the scheduler has anything to do with the slowdown seen after
> > the patch is applied.
> 
> In fixing performance issues, the most obvious explanation isn't always
> the right one. It's quite possible you're right, sure.
> 
> What I'm saying though is that it doesn't rhyme with what I've seen of
> Volanomark - we ran 2.6.16 on a 4p Intel box for instance and it didn't
> come close to saturating a Gigabit pipe before it maxed out on CPU load.
> 
> > The total number of messages being exchanged around the chatrooms in 
> > Volanomark remain unchanged.  But ACKS increase by 3.5 times and
> > segments received increase by 38% from netstat.  
> 
> > So I think it is reasonable to conclude that the increase in TCP traffic
> > reduce the bandwidth and throughput in Volanomark.
> 
> You could count the number of outbound packets dropped on the server.
> 
> Olaf

Also under benchmark stress, the load can get so high that timers go
off that normally don't. For example, I have seen delayed ack timer
cause extra ack's when at lower loads the response happened quick enough
that the ACK was piggybacked.


-- 
Stephen Hemminger <shemminger@...l.org>
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