linux-kernel - Re: [rfc][patch] queued spinlocks (i386)

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Message-ID: <Pine.LNX.4.64.0703281452130.21150@alien.or.mcafeemobile.com>
Date:	Wed, 28 Mar 2007 15:00:21 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Ingo Molnar <mingo@...e.hu>, Nikita Danilov <nikita@...sterfs.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ravikiran G Thirumalai <kiran@...lex86.org>
Subject: Re: [rfc][patch] queued spinlocks (i386)

On Wed, 28 Mar 2007, Davide Libenzi wrote:

> On Wed, 28 Mar 2007, Nick Piggin wrote:
> 
> > On Sat, Mar 24, 2007 at 06:29:59PM +0100, Ingo Molnar wrote:
> > > 
> > > * Nikita Danilov <nikita@...sterfs.com> wrote:
> > > 
> > > > Indeed, this technique is very well known. E.g., 
> > > > http://citeseer.ist.psu.edu/anderson01sharedmemory.html has a whole 
> > > > section (3. Local-spin Algorithms) on them, citing papers from the 
> > > > 1990 onward.
> > > 
> > > that is a cool reference! So i'd suggest to do (redo?) the patch based 
> > > on those concepts and that terminology and not use 'queued spinlocks' 
> > > that are commonly associated with MS's stuff. And as a result the 
> > > contended case would be optimized some more via local-spin algorithms. 
> > > (which is not a key thing for us, but which would be nice to have 
> > > nevertheless)
> > 
> > Firstly, the terminology in that paper _is_ "queue lock", which isn't
> > really surprising. I don't really know or care about what MS calls their
> > locks, but I'd suggest that their queued spinlock is probably named in
> > reference to its queueing property rather than its local spin property.
> 
> The method you propose is otherwise called "Ticket Lock":
> 
> http://en.wikipedia.org/wiki/Ticket_lock
> http://www.cs.rochester.edu/research/synchronization/pseudocode/ss.html#ticket

That this work prio-art dates to 1991:

http://www.cs.rochester.edu/u/scott/papers/1991_TOCS_synch.pdf

So I would not worry to much about patents here. At least W2K MS ones ;)
What I would worry though, is to add another class of locks. There's no 
reason why Ticket Locks would perform worse than our spinlock, in both 
contended and not-contended case, AFAICS. And they have a nice FIFO 
behaviour.



- Davide


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