#9321: Support for waiting on multiple MVars -------------------------------------+------------------------------------- Reporter: schyler | Owner: simonmar Type: feature request | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 7.8.3 Keywords: mvar | Differential Revisions: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: -------------------------------------+------------------------------------- A lot of code in servers uses MVars because they seem to have more desirable scalability characteristics than STM. Unfortunately, unlike STM which is composable (i.e. `readTChan chan1 <|> readTChan chan2`), `MVar`s often require extra inefficient intermediate steps to funnel many-to-one. A common thing for people to do when they need to funnel N `MVar`s into one is to create 1 `MVar` and N forks where each fork attempts to read from its associated `MVar` and then writes it into the one `MVar` where another fork is waiting to process the data. This is such a waste; it produces more forks and another `MVar` where contention can occur. In some ways it would be better if the internal representation of an `MVar` had a pointer to the "next `MVar`" so that we could use a function like `eitherMVar` to merge two (or more) `MVar`s together into one which can be waited on and yield values from any of the containing `MVar`s. (I believe) the runtime would need to provide appropriate support in the scheduler so that the list is traversed instead of only the single `MVar` checked. The overhead for code which does not use this feature would probably be only 1 branch, which is acceptable. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9321 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler