Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

[Stephan Diestelhorst]

On Thursday 06 October 2011, 13:40:01 Jeremy Fitzhardinge wrote:
> On 10/06/2011 07:04 AM, Stephan Diestelhorst wrote:
> > On Wednesday 28 September 2011, 14:49:56 Linus Torvalds wrote:
> >> Which certainly should *work*, but from a conceptual standpoint, isn't
> >> it just *much* nicer to say "we actually know *exactly* what the upper
> >> bits were".
> > Well, we really do NOT want atomicity here. What we really rather want
> > is sequentiality: free the lock, make the update visible, and THEN
> > check if someone has gone sleeping on it.
> >
> > Atomicity only conveniently enforces that the three do not happen in a
> > different order (with the store becoming visible after the checking
> > load).
> >
> > This does not have to be atomic, since spurious wakeups are not a
> > problem, in particular not with the FIFO-ness of ticket locks.
> >
> > For that the fence, additional atomic etc. would be IMHO much cleaner
> > than the crazy overflow logic.
> 
> All things being equal I'd prefer lock-xadd just because its easier to
> analyze the concurrency for, crazy overflow tests or no.  But if
> add+mfence turned out to be a performance win, then that would obviously
> tip the scales.
> 
> However, it looks like locked xadd is also has better performance:  on
> my Sandybridge laptop (2 cores, 4 threads), the add+mfence is 20% slower
> than locked xadd, so that pretty much settles it unless you think
> there'd be a dramatic difference on an AMD system.

Indeed, the fences are usually slower than locked RMWs, in particular,
if you do not need to add an instruction. I originally missed that
amazing stunt the GCC pulled off with replacing the branch with carry
flag magic. It seems that two twisted minds have found each other
here :)

One of my concerns was adding a branch in here... so that is settled,
and if everybody else feels like this is easier to reason about...
go ahead :) (I'll keep my itch to myself then.)

Stephan
-- 
Stephan Diestelhorst, AMD Operating System Research Center
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