Re: [rfc][patch] queued spinlocks (i386)

[Nick Piggin]

On Wed, Mar 28, 2007 at 03:00:21PM -0700, Davide Libenzi wrote:
> On Wed, 28 Mar 2007, Davide Libenzi wrote:
> 
> > 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 

No, as you see from my patch I just change the spinlock implementation
to a queued one. I agree it doesn't make sense to add a new type of lock.


> 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.

In most cases, no. For the uncontended case they should be about the
same. They have the same spinning behaviour. However there is a little
window where they might be a bit slower I think... actually perhaps I'm
wrong!

Currently if you have 4 CPUs spinning and the lock is released, all 4
CPU cachelines will be invalidated, then they will be loaded again, and
found to be 0, so they all try to atomic_dec_return the counter, each
one invalidating others' cachelines. 1 gets through.

With my queued locks, all 4 cachelines are invalidated and loaded, but
only one will be allowed to proceed, and there are 0 atomic operations
or stores of any kind.

So I take that back: our current spinlocks have a worse thundering herd
behaviour under contention than my queued ones. So I'll definitely
push the patch through.
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