Optimize memory management in Trilinos sparsity pattern accessors.

[bangerth]

No description provided.

[peterrum]

The following tests FAILED:
	7340 - trilinos/sparsity_pattern_06.mpirun=3.debug (Failed)
Errors while running CTest

[masterleinad]

Is this thread-safe? The use count could be increased on a different thread after the check but before resize, right?

[bangerth]

Hm, good question. My primary motivation was that when you do

  SparsityPattern::iterator p = sp.begin();
  ++p;

and if ++p moves you to the next row of the matrix, then the current design deallocates the std::vector only to allocate another one. That's wasteful. Note that here, only one iterator is ever around, so the use count of these shared pointers is always one.

Your question pertains to the situation where another thread makes a copy of an iterator owned by the first thread (bumping the use count to two) at the very inopportune moment that the first thread has just passed the use_count() check. So perhaps something like this:

  SparsityPattern::iterator p = sp.begin();
  auto t = std::thread([&p]() { auto x = p; ++x; });
  ++p;
  t.join();

Note that here, the lambda function must capture p by reference -- if it had captured it by value, the capture would be a second copy, resulting in a use count of two, so the optimization would not have applied.

So I think you're right that the optimization is not thread-safe. Good catch!

The question is what we want to do about the situation. I would really like to optimize the use case here because typically one will only have one iterator object sitting around, no copies being made, and it seems silly to release and re-allocate the memory all the time. I could guard access to that variable with a mutex. What would you suggest?

[masterleinad]

The question is what we want to do about the situation. I would really like to optimize the use case here because typically one will only have one iterator object sitting around, no copies being made, and it seems silly to release and re-allocate the memory all the time. I could guard access to that variable with a mutex. What would you suggest?

Yeah, I would just use a static std::mutex and lock it for any access to colnum_cache.