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Date: Fri, 31 Jan 2014 20:51:33 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
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Subject: Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation
Message-ID: <20140131195133.GP5002@laptop.programming.kicks-ass.net>
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On Fri, Jan 31, 2014 at 02:24:33PM -0500, Waiman Long wrote:
> On 01/31/2014 10:08 AM, Peter Zijlstra wrote:
> >Whereas for the straight cmpxchg() you'd get something relatively simple
> >like:
> >
> >   mov %edx,%eax
> >   lock cmpxchg %ecx,(%rbx)
> >   cmp %edx,%eax
> >   jne ...
> 
> I believe the speeds of the lock functions are about the same. However,
> qspinlock has a much simpler unlock function which probably account of most
> of the speed gain.

The unlock path for ticket locks is a single "add[bw] $0x1,()", that should be
as fast as the single "movb 0,()" you have.

> >I suppose we could from the ticket code more and optimize the
> >uncontended path, but that'll make the contended path more expensive
> >again, although probably not as bad as hitting a new cacheline.
> 
> I don't get what you are trying to say.

I said we could probably make the ticket lock function faster for the
uncontended case by making the contended case slightly more expensive.