[GHC] #15267: Extend TVar/MVar to N capacity / Add primitive channal type

#15267: Extend TVar/MVar to N capacity / Add primitive channal type -------------------------------------+------------------------------------- Reporter: winter | Owner: (none) Type: feature | Status: new request | Priority: high | Milestone: 8.6.1 Component: Runtime | Version: 8.4.3 System | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The current concurrent primitives `TVar` and `MVar` have a fixed single capacity for holding the value to be synchronized. This pose great limitations on implementing many higher level concurrent structures efficiently, e.g. bound channel, semaphore, resource pool, etc. For example current http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Concurrent.QSem.html#signalQSem> is not ideal due to the extensive usage of multiple `MVar` s. If we have a good `BoundedChan a` type, semaphores can be easily implemented with `BoundedChan ()`. Currently we have various bound channel implementations, but these implementations are often not very efficient or too complex. For example the `BoundedChan` from http://hackage.haskell.org/package/BoundedChan-1.0.3.0/docs/Control- Concurrent-BoundedChan.html> is based on an array of `MVar` s, thus consumes much more memory than a primitve channel which only have to record parked `StgTSO` s. I'm thinking adding something like: ``` typedef struct { StgHeader header; struct StgMVarTSOQueue_ *head; struct StgMVarTSOQueue_ *tail; StgInt capacity; // the channel's capacity StgInt read_index; // the reading end's index StgInt write_index; // the writing end's index StgClosure *payload[]; // payload array in continuous memory } StgMChan; ``` I still can't work out all the details, but I'm confident something similar to this will work. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15267 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler