From what I understand Barry doesn't want the threads to spin. Also, synchronizing through an unguarded memory location seems to create a race conditions. Isn't cmpxchg instruction-set specific?

Dmitry. On Tue, Nov 29, 2011 at 8:50 AM, Jed Brown <[email protected]> wrote:

You don't need any of this unless your threads need to actually sleep instead of spin. I think we can afford to have them spin, so we don't need any locking. You let the threads continue by doing an ordinary write to a memory location. (You can do more sophisticated things with cmpxchg, which you likely need at synchronization points.) On Nov 29, 2011 8:41 AM, "Dmitry Karpeev" <[email protected]> wrote:

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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 <[email protected]> wrote:

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On Thu, Nov 24, 2011 at 16:01, Barry Smith <[email protected]> wrote:

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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.