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104 :: Shared heap data between threads

• exhu (orginal) [2012-11-02T17:08:13+01:00] view original

  I know there are separate GC heaps per thread and there's allocShared-deallocShared, but is it possible to allocated a shared garbage collected data?

  I need either garbage collecteur to manage shared pointers live or to implement "hazard pointers" myself like in C for a lock-free algorithm.

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• Araq (orginal) [2012-11-02T20:07:27+01:00] view original

  A shared garbage collected heap is planned.

  Manual memory management for a shared data structure is IMHO not as bad as it sounds because both manual memory management and memory sharing between threads require the programmer to have a decent understanding about ownership, so a GC doesn't buy you that much for concurrency. And yes I'm aware of all the lockfree algorithms that "require" a GC. But lockfree algorithms are overrated IMHO.

  What's often forgotten: You can share GC'ed memory without message passing in Nimrod already via some global variable. You have to ensure that the data structure is created by the main thread and not modified by other threads though. Basically this means that large lookup tables computed at program startup are supported with 0 cost.

  The compiler implements a novel program analysis to warn you should you screw up: "Warning: possible inconsistency of thread local heaps [DifferentHeaps]". Take this warning seriously. It will become an error message.

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• exhu (orginal) [2012-11-02T21:08:51+01:00] view original

  I've used lockfree queue in java and the program gained performance boost over a lock-based one, although using java's "synchronized" keyword led to the same performance as the lock-free implementation, probably jvm already uses lock-free technique.

  Anyway I don't need it for a real project now, just interested if I could easily translate my queue implementation from java into nimrod to play with and test if it works here. And now I see it won't work.

  I have not looked in nimrod message passing implemenentation yet.

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• Araq (orginal) [2012-11-07T19:16:35+01:00] view original

  And now I see it won't work.

  Depends. Freeing a list item uses a lock but you can easily add the node to a free list instead. In fact a GC does not really help for lock free programming if you can live with batch deallocations... ;-)

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