Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation

[Peter Zijlstra]

On Tue, Feb 18, 2014 at 07:50:13PM -0500, Waiman Long wrote:
> On 02/18/2014 04:34 PM, Peter Zijlstra wrote:
> >On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:
> >>The #ifdef is harder to take away here. The point is that doing a 32-bit
> >>exchange may accidentally steal the lock with the additional code to handle
> >>that. Doing a 16-bit exchange, on the other hand, will never steal the lock
> >>and so don't need the extra handling code. I could construct a function with
> >>different return values to handle the different cases if you think it will
> >>make the code easier to read.
> >Does it really pay to use xchg() with all those fixup cases? Why not
> >have a single cmpxchg() loop that does just the exact atomic op you
> >want?
> 
> The main reason for using xchg instead of cmpxchg is its performance impact
> when the lock is heavily contended. Under those circumstances, a task may
> need to do several tries of read+atomic-RMV before getting it right. This
> may cause a lot of cacheline contention. With xchg, we need at most 2 atomic
> ops. Using cmpxchg() does simplify the code a bit at the expense of
> performance with heavy contention.

Have you actually measured this?
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