• sof's avatar
    [project @ 2002-02-13 08:48:06 by sof] · e289780e
    sof authored
    Revised implementation of multi-threaded callouts (and callins):
    - unified synchronisation story for threaded and SMP builds,
      following up on SimonM's suggestion. The following synchro
      variables are now used inside the Scheduler:
        + thread_ready_cond - condition variable that is signalled
          when a H. thread has become runnable (via the THREAD_RUNNABLE()
          macro) and there are available capabilities. Waited on:
             + upon schedule() entry (iff no caps. available).
    	 + when a thread inside of the Scheduler spots that there
    	   are no runnable threads to service, but one or more
    	   external call is in progress.
    	 + in resumeThread(), waiting for a capability to become
          Prior to waiting on thread_ready_cond, a counter rts_n_waiting_tasks
          is incremented, so that we can keep track of the number of
          readily available worker threads (need this in order to make
          an informed decision on whether or not to create a new thread
          when an external call is made).
        + returning_worker_cond - condition variable that is waited
          on by an OS thread that has finished executing and external
          call & now want to feed its result back to the H thread
          that made the call. Before doing so, the counter
          rts_n_returning_workers is incremented.
          Upon entry to the Scheduler, this counter is checked for &
          if it is non-zero, the thread gives up its capability and
          signals returning_worker_cond before trying to re-grab a
          capability. (releaseCapability() takes care of this).
        + sched_mutex - protect Scheduler data structures.
        + gc_pending_cond - SMP-only condition variable for signalling
          completion of GCs.
    - initial implementation of call-ins, i.e., multiple OS threads
      may concurrently call into the RTS without interfering with
      each other. Implementation uses cheesy locking protocol to
      ensure that only one OS thread at a time can construct a
      function application -- stop-gap measure until the RtsAPI
      is revised (as discussed last month) *and* a designated
      block is used for allocating these applications.
    - In the implementation of call-ins, the OS thread blocks
      waiting for an RTS worker thread to complete the evaluation
      of the function application. Since main() also uses the
      RtsAPI, provide a separate entry point for it (rts_mainEvalIO()),
      which avoids creating a separate thread to evaluate Main.main,
      that can be done by the thread exec'ing main() directly.
      [Maybe there's a tidier way of doing this, a bit ugly the
      way it is now..]
    There are a couple of dark corners that needs to be looked at,
    such as conditions for shutting down (and how) + consider what
    ought to happen when async I/O is thrown into the mix (I know
    what will happen, but that's maybe not what we want).
    Other than that, things are in a generally happy state & I hope
    to declare myself done before the week is up.
Capability.h 1.75 KB