linux-kernel - Re: [tbench regression fixes]: digging out smelly deadmen.

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Message-Id: <20081031.005219.141937694.davem@davemloft.net>
Date:	Fri, 31 Oct 2008 00:52:19 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	shemminger@...tta.com, zbr@...emap.net, rjw@...k.pl, mingo@...e.hu,
	s0mbre@...rvice.net.ru, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	efault@....de, akpm@...ux-foundation.org
Subject: Re: [tbench regression fixes]: digging out smelly deadmen.

From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Thu, 30 Oct 2008 21:01:19 +0200 (EET)

> On Thu, 30 Oct 2008, Stephen Hemminger wrote:
> 
> > Has anyone looked into the impact of port randomization on this benchmark.
> > If it is generating lots of sockets quickly there could be an impact:
> >   * port randomization causes available port space to get filled non-uniformly
> >     and what was once a linear scan may have to walk over existing ports.
> >     (This could be improved by a hint bitmap)
> > 
> >   * port randomization adds at least one modulus operation per socket
> >     creation. This could be optimized by using a loop instead.
> 
> I did something with AIM9's tcp_test recently (1-2 days ago depending on 
> how one calculates that so didn't yet have time summarize the details in 
> the AIM9 thread) by deterministicly binding in userspace and got much more 
> sensible numbers than with randomized ports (2-4%/5-7% vs 25% variation 
> some difference in variation in different kernel versions even with 
> deterministic binding). Also, I'm still to actually oprofile and bisect 
> the remaining ~4% regression (around 20% was reported by Christoph). For 
> oprofiling I might have to change aim9 to do predefined number of loops 
> instead of a deadline to get more consistent view on changes in per func 
> runtime.

Yes, it looks like port selection cache and locking effects are
a very real issue.

Good find.
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