Changes between Initial Version and Version 1 of Commentary/Rts/IOManager

Timestamp:

10/17/11 15:06:45 (19 months ago)

Author:

dterei

Comment:

--

Commentary/Rts/IOManager

@@ +1 @@
+= The I/O Manager =
+
+This page describes the internals of the I/O manager, the latest version of which can be found in [http://darcs.haskell.org/libraries/base/GHC/Conc.lhs GHC.Conc], in the "Thread IO API" section. The I/O manager's job is to to provide a blocking I/O API to the user without forcing the RTS to create one operating system thread per Haskell thread. We here focus on the ''threaded'' RTS on non-Windows platforms.
+
+The RTS keeps a global list of pending events, unsuprising called `pendingEvents`, containing a elements of the following data type:
+
+{{{
+data IOReq
+  = Read   {-# UNPACK #-} !Fd {-# UNPACK #-} !(MVar ())
+  | Write  {-# UNPACK #-} !Fd {-# UNPACK #-} !(MVar ())
+}}}
+
+When a thread wants to read from a file descriptor `fd` it calls `threadWaitRead` which in turn calls `waitForReadEvent`.
+
+{{{
+waitForReadEvent :: Fd -> IO ()
+waitForReadEvent fd = do
+  m <- newEmptyMVar
+  atomicModifyIORef pendingEvents (\xs -> (Read fd m : xs, ()))
+  prodServiceThread
+  takeMVar m
+}}}
+
+`waitForReadEvent` creates a new `MVar`, adds it to `pendingEvents` and finally blocks on it. `pendingEvents` gets read by the I/O manager thread which runs the event loop, in GHC called `service_loop`. It roughly performs these steps:
+
+ 1. Pick up new I/O requests from `pendingRequests` and set the variable to the empty list.
+ 2. Create data structures appropriate for calling `select`.
+ 3. For each `Read` request in `pendingEvents` check if the file descriptor is in the ready set returned by `select`. If so perform a `putMVar` on the `MVar` associated with that request to wake up the blocked thread.
+ 4. Repeat from step 1.