Re: [PATCH -v8][RFC] mutex: implement adaptive spinning

Nick Piggin
2009-01-15T00:50:42Z

On Wed, Jan 14, 2009 at 07:23:12PM +0200, Avi Kivity wrote:
> Nick Piggin wrote:
> >>(no they're not, Nick's ticket locks still spin on a shared cacheline
> >>IIRC -- the MCS locks mentioned could fix this)
> >>
> >
> >It reminds me. I wrote a basic variation of MCS spinlocks a while back. And
> >converted dcache lock to use it, which showed large dbench improvements on
> >a big machine (of course for different reasons than the dbench improvements
> >in this threaed).
> >
> >http://lkml.org/lkml/2008/8/28/24
> >
> >Each "lock" object is sane in size because given set of spin-local queues
> >may
> >only be used once per lock stack. But any spinlocks within a mutex
> >acquisition
> >will always be at the bottom of such a stack anyway, by definition.
> >
> >If you can use any code or concept for your code, that would be great.
> >
>
> Does it make sense to replace 'nest' with a per-cpu counter that's
> incremented on each lock? I guest you'd have to search for the value of
> nest on unlock, but it would a very short search (typically length 1, 2
> if lock sorting is used to avoid deadlocks).
>
> I think you'd need to make the lock store the actual node pointer, not
> the cpu number, since the values of nest would be different on each cpu.
>
> That would allow you to replace spinlocks with mcs_locks wholesale.

nest could get quite large (and basically is unbounded), though. I
think there would definitely be variations (NMCS locks is interesting).
But OTOH I think that for _most_ spinlocks, optimising for spinning
case is wrong. For some really tricky global spinlocks, yes I think
MCS is a good idea. But moreso as a stopgap until hopefully more
scalable algorithms are written.