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

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Date:	Fri, 31 Oct 2008 11:40:16 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	David Miller <davem@...emloft.net>
cc:	shemminger@...tta.com, zbr@...emap.net, rjw@...k.pl, mingo@...e.hu,
	s0mbre@...rvice.net.ru, a.p.zijlstra@...llo.nl,
	LKML <linux-kernel@...r.kernel.org>,
	Netdev <netdev@...r.kernel.org>, efault@....de,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [tbench regression fixes]: digging out smelly deadmen.

On Fri, 31 Oct 2008, David Miller wrote:

> 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.

Let me remind that it is just a single process, so no ping-pong & other 
lock related cache effects should play any significant role here, no? (I'm 
no expert though :-)).

One thing I didn't mention earlier, is that I also turned on 
tcp_tw_recycle to get the binding to work without giving
-ESOMETHING very early (also did some, possibly meaningless
things, like drop_caches before each test run, might be
significant only because of the test harness cause minor
variantions). I intend to try w/o binding of the client end
but I guess I might again get more variation between different
test runs.

-- 
 i.