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

[David Miller <davem@davemloft.net>]

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.