Date: Fri, 16 Jan 2009 04:03:06 +0100
From: Nick Piggin <npiggin@suse.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>, Matthew Wilcox <matthew@wil.cx>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Gregory Haskins <ghaskins@novell.com>, Andi Kleen <andi@firstfloor.org>,
       Chris Mason <chris.mason@oracle.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>,
       linux-btrfs <linux-btrfs@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Peter Morreale <pmorreale@novell.com>,
       Sven Dietrich <SDietrich@novell.com>,
       Dmitry Adamushko <dmitry.adamushko@gmail.com>,
       Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [GIT PULL] adaptive spinning mutexes
Message-ID: <20090116030306.GK17810@wotan.suse.de>
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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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