linux-kernel - Re: [GIT PULL] adaptive spinning mutexes

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Message-ID: <20090116030306.GK17810@wotan.suse.de>
Date:	Fri, 16 Jan 2009 04:03:06 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Matthew Wilcox <matthew@....cx>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	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>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [GIT PULL] adaptive spinning mutexes

On Thu, Jan 15, 2009 at 10:16:53AM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 15 Jan 2009, Ingo Molnar wrote:
> > 
> > btw., i think spin-mutexes have a design advantage here: in a lot of code 
> > areas it's quite difficult to use spinlocks - cannot allocate memory, 
> > cannot call any code that can sporadically block (but does not _normally_ 
> > block), etc.
> > 
> > With mutexes those atomicity constraints go away - and the performance 
> > profile should now be quite close to that of spinlocks as well.
> 
> Umm. Except if you wrote the code nicely and used spinlocks, you wouldn't 
> hold the lock over all those unnecessary and complex operations.
> 
> IOW, if you do pre-allocation instead of holding a lock over the 
> allocation, you win. So yes, spin-mutexes makes it easier to write the 
> code, but it also makes it easier to just plain be lazy.

Yeah, I agree often it is harder to get the locking right but you end up
with a better result. With mutexes, on the off chance you do have t oblock
while holding the lock, performance and latency of other threads will tank.
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