linux-kernel - Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

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Message-ID: <1406100346.5076.98.camel@marge.simpson.net>
Date:	Wed, 23 Jul 2014 09:25:46 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-doc@...r.kernel.org, Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

On Wed, 2014-07-23 at 08:57 +0200, Peter Zijlstra wrote: 
> On Wed, Jul 23, 2014 at 06:55:03AM +0200, Mike Galbraith wrote:
> > On Mon, 2014-07-21 at 09:42 -0700, Andi Kleen wrote:
> > 
> > > FWIW the main problem is currently that switch-through-idle is so 
> > > slow. I think improving that would give a boost to far more
> > > situations.
> > 
> > Two high frequency idle enter/exit suckage spots:
> > 
> > 1) nohz (tick) - it's expensive to start/stop tick on every micro-idle,
> > throttle it or something.
> 
> Yeah, so the idea was to use the cpuidle idle guestimator to control
> this, and now that we've moved it somewhat closer to the scheduler that
> might become possible.
> 
> > 2) ondemand governor - tweak silly default settings to reflect the
> > reality that we routinely schedule communicating threads cross core.
> 
> Yeah, so the plan is to shoot cpufreq in the head and base the
> replacement on smp aware metrics ;-) Its on a todo list somewhere..

It never ceases to amaze me that people aren't screaming bloody murder
about those two spots.  Watching performance of lightly loaded boxen is
enough to make a grown man cry.

SUSE (and I in all of my many regression testing trees) puts tourniquets
on both of these blood spurting gashes, laptops be damned.

I also resurrect mwait_idle(), as while you may consider it obsolete, I
still love my lovely little Q6600 box (power sucking pig) dearly :)

-Mike

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