[petsc-dev] Generality of VecScatter

[Dmitry Karpeev]

Barry,

You might want to take a look at this:
http://stackoverflow.com/questions/2994216/linux-pthread-suspend
The solution suggested there (Answer 1) is to use sigwait().  Answer 3,
although not the actual solution,
explains why the  pthread_suspend/pthread_continue-based solution is racy.
I think this may be a general issue
with the problem, which requires the use of mutexes.

What example do you use to test this?  I'd like to take a look.

Thanks.
Dmitry.

On Thu, Nov 24, 2011 at 4:10 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> 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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