linux-kernel - Re: [PATCH 3/3] RT: CPU priority management

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Date:	Thu, 25 Oct 2007 13:26:38 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Darren Hart <dvhltc@...ibm.com>
Subject: Re: [PATCH 3/3] RT: CPU priority management

Oh crap.  I just realized this is an older version of the patch..mustv'e
forgot to refresh...grr.  Ill send out the refreshed one.

But anyway, I digress.  

On Thu, 2007-10-25 at 11:27 -0400, Steven Rostedt wrote:

> 
> The cpu_priority and the cp->lock will be aboslutely horrible for
> cacheline bouncing.

In the form presented here in this email, perhaps.  I think you will see
some significant improvements in the refreshed version.  The big change
is that the global lock is gone.

>   Ironically, this will kill performance for the very
> machines this code is to help with.  The larger the number of CPUs you
> have the more cacheline bouncing this code will create.

Don't forget:  The same is precisely true for the current -rt2
algorithm.  For instance, the -rt2 algorithm aside from being linear in
general, scales cacheline bouncing linearly as well.  Each cpu is going
to trash rq->rt.highest_prio and then we will walk them for each scan.

The fact is, you can't maintain a global dynamic policy without bouncing
cachelines, period.  But hopefully we can minimize it, and I just want
to see the fastest code here.

> 
> I still don't see the benefit from the cpupri code.

I still owe you timing data, but at this juncture I think I can beat
linear (especially as we start throwing in big-iron) ;)  I originally
got involved in this scheduler rework from observations of poor scaling
on our 8/16-ways, so I want it to scale as much as you ;)  If this alg
doesn't pan out, that's cool.  But I think it will in the end.  Linear
algs in the fast path just make my skin crawl.  Perhaps it will still be
the best design in the end, but I am not giving up that easy until I
prove it to myself.

Regards,
-Greg

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