On Monday 10 October 2011, 07:00:50 Stephan Diestelhorst wrote:
> 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.)
Just that I can't... if performance is a concern, adding the LOCK
prefix to the addb outperforms the xadd significantly:
With mean over 100 runs... this comes out as follows
(on my Phenom II)
locked-add 0.648500 s 80%
add-rmwtos 0.707700 s 88%
locked-xadd 0.807600 s 100%
add-barrier 1.270000 s 157%
With huge read contention added in (as cheaply as possible):
locked-add.openmp 0.640700 s 84%
add-rmwtos.openmp 0.658400 s 86%
locked-xadd.openmp 0.763800 s 100%
And the numbers for write contention are crazy, but also feature the
locked-add version:
locked-add.openmp 0.571400 s 71%
add-rmwtos.openmp 0.699900 s 87%
locked-xadd.openmp 0.800200 s 100%
Stephan
--
Stephan Diestelhorst, AMD Operating System Research Center
stephan.diestelhorst@xxxxxxx, Tel. +49 (0)351 448 356 719
Advanced Micro Devices GmbH
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Germany
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add-rmwtos.c
Description: add-rmwtos.c
add-rmwtos.openmp.c
Description: add-rmwtos.openmp.c
locked-add.c
Description: locked-add.c
locked-xadd.openmp.c
Description: locked-xadd.openmp.c
locked-add.openmp.c
Description: locked-add.openmp.c
locked-xadd.c
Description: locked-xadd.c
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