linux-kernel - Re: [PATCH -v8][RFC] mutex: implement adaptive spinning

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Date:	Mon, 12 Jan 2009 19:07:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH -v8][RFC] mutex: implement adaptive spinning

On Mon, 2009-01-12 at 19:33 +0200, Boaz Harrosh wrote:

> Which brings me back to my initial reaction to this work. Do we need
> two flavors of Mutex? some program sections need Fairness, some need
> performance. Some need low-latency, some need absolute raw CPU power.

Thing is, its the kernel, we cannot have such knobs per task. So we have
to pick one and stick with it -- the 'best' we can do is what PREEMPT_RT
does and replace the thing whole sale at build time.

> Because at the end of the day spinning in a saturated CPU work-load
> that does not care about latency, eats away cycles that could be spent
> on computation. Think multi-threaded video processing for example. 
> Thing I would like to measure is 
> 1 - how many times we spin and at the end get a lock
> 2 - how many times we spin and at the end sleep.
> 3 - how many times we sleep like before.
> vs. In old case CPU spent on scheduling. Just to see if we are actually loosing
> cycles at the end.

Feel free to do so ;-)

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