Thread Safety

[SSoelvsten]

In many contexts, BDD packages should be threadsafe such that they can be used as part of parallel algorithms. It is not possible for such a program to know early how many threads may exist before it starts its calculations. Hence, Adiar has to adapt on-the-fly to the number of concurrent calls.

Luckily for us, the write-once/read-only design of Adiar's files makes thread-safety much easier (since we have no unique node table to lock). Yet, instead we have one global pool of M memory in TPIE that we are allowed to use. Hence, we need some kind of locking/scheduler to make sure every thread can claim a share of M "early". Otherwise, another thread can by accident overtake it and both ask TPIE for the same "free" memory.

• Add ADIAR_THREADSAFE to CMake (default: OFF). If on, then the logic for thread-safety below is included.
• Add memory_share class that uses RAII to claim a share of memory.
  • Claim up-to half of the unclaimed memory.
  • Claim at least the minimum amount of memory (128 MiB, or maybe a more sophisticated number?). If not currently available, then the thread has to wait for someone else to finish their calculations.
  • If the internal-memory cases use less than is claimed, then relinquish the unused amount early. Alternatively, first claim memory after all internal-memory calculations have been made.

[SSoelvsten]

While talking to Riko Jacob, it can be a bad idea to just naively split the amount of memory (as described above). Iterating on his point, we should consider to:

1. Have an upper bound on the number of concurrent jobs (set by the user / CPU specification). This could quite easily be integrated into the above solution by (1) making the minimum claimed memory be M/P where P being the maximum number of concurrent jobs and/or (2) make it also track the number of concurrently active jobs (with a semaphore).
2. Schedule/Throttle the concurrent jobs (in a FIFO fashion?)

[SSoelvsten]

Note, the memory_share class almost acts as a semaphore (but one you can read and where the increment/decrement is not 1 but the amount of memory you want to use).