[petsc-dev] Generality of VecScatter

[Jed Brown]

On Thu, Nov 24, 2011 at 16:01, Barry Smith <bsmith at mcs.anl.gov> wrote:

>   Yes, but they can only get access to that shared variable one at a time:
> first get's it, then second get's it, then third gets, .... Ok for a couple
> of cores but not for dozens.
>
>   Take a look at src/sys/objects/pthread.c for the various ways we have
> coded for "waking" the threads. Maybe I am missing something but this is
> the best Kerry and I could figure out.
>

What exactly should I be looking at? Can't you have all the threads spin on
a normal shared variable (not a mutex) that is only written by the thread
that needs to spark them? Or use a fetch-and-add atomic if you want to keep
track of how many are running or limit the number? The latter could use a
tree to get logarithmic cost, but if they are stored next to each other,
you would still have O(P) cache invalidations.
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