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

Ilpo Järvinen
2008-10-31T09:40:35Z

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On Fri, 31 Oct 2008, David Miller wrote:

> From: "Ilpo Järvinen" <ilpo.jarvinen@helsinki.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.