count "dud" sparks (expressions that were already evaluated when sparked)
[ghc.git] / rts / Capability.c
index 0415092..9f7d152 100644 (file)
 
 #include "PosixSource.h"
 #include "Rts.h"
-#include "RtsUtils.h"
-#include "RtsFlags.h"
-#include "STM.h"
-#include "OSThreads.h"
+
 #include "Capability.h"
 #include "Schedule.h"
 #include "Sparks.h"
 #include "Trace.h"
+#include "sm/GC.h" // for gcWorkerThread()
+#include "STM.h"
+#include "RtsUtils.h"
 
 // one global capability, this is the Capability for non-threaded
 // builds, and for +RTS -N1
 Capability MainCapability;
 
-nat n_capabilities;
+nat n_capabilities = 0;
 Capability *capabilities = NULL;
 
 // Holds the Capability which last became free.  This is used so that
 // an in-call has a chance of quickly finding a free Capability.
 // Maintaining a global free list of Capabilities would require global
 // locking, so we don't do that.
-Capability *last_free_capability;
+Capability *last_free_capability = NULL;
+
+/* GC indicator, in scope for the scheduler, init'ed to false */
+volatile StgWord waiting_for_gc = 0;
+
+/* Let foreign code get the current Capability -- assuming there is one!
+ * This is useful for unsafe foreign calls because they are called with
+ * the current Capability held, but they are not passed it. For example,
+ * see see the integer-gmp package which calls allocateLocal() in its
+ * stgAllocForGMP() function (which gets called by gmp functions).
+ * */
+Capability * rts_unsafeGetMyCapability (void)
+{
+#if defined(THREADED_RTS)
+  return myTask()->cap;
+#else
+  return &MainCapability;
+#endif
+}
 
 #if defined(THREADED_RTS)
 STATIC_INLINE rtsBool
 globalWorkToDo (void)
 {
-    return blackholes_need_checking
-       || sched_state >= SCHED_INTERRUPTING
-       ;
+    return sched_state >= SCHED_INTERRUPTING
+        || recent_activity == ACTIVITY_INACTIVE; // need to check for deadlock
 }
 #endif
 
 #if defined(THREADED_RTS)
-STATIC_INLINE rtsBool
-anyWorkForMe( Capability *cap, Task *task )
+StgClosure *
+findSpark (Capability *cap)
 {
-    if (task->tso != NULL) {
-       // A bound task only runs if its thread is on the run queue of
-       // the capability on which it was woken up.  Otherwise, we
-       // can't be sure that we have the right capability: the thread
-       // might be woken up on some other capability, and task->cap
-       // could change under our feet.
-       return !emptyRunQueue(cap) && cap->run_queue_hd->bound == task;
-    } else {
-       // A vanilla worker task runs if either there is a lightweight
-       // thread at the head of the run queue, or the run queue is
-       // empty and (there are sparks to execute, or there is some
-       // other global condition to check, such as threads blocked on
-       // blackholes).
-       if (emptyRunQueue(cap)) {
-           return !emptySparkPoolCap(cap)
-               || !emptyWakeupQueue(cap)
-               || globalWorkToDo();
-       } else
-           return cap->run_queue_hd->bound == NULL;
+  Capability *robbed;
+  StgClosurePtr spark;
+  rtsBool retry;
+  nat i = 0;
+
+  if (!emptyRunQueue(cap) || cap->returning_tasks_hd != NULL) {
+      // If there are other threads, don't try to run any new
+      // sparks: sparks might be speculative, we don't want to take
+      // resources away from the main computation.
+      return 0;
+  }
+
+  do {
+      retry = rtsFalse;
+
+      // first try to get a spark from our own pool.
+      // We should be using reclaimSpark(), because it works without
+      // needing any atomic instructions:
+      //   spark = reclaimSpark(cap->sparks);
+      // However, measurements show that this makes at least one benchmark
+      // slower (prsa) and doesn't affect the others.
+      spark = tryStealSpark(cap);
+      if (spark != NULL) {
+          cap->sparks_converted++;
+
+          // Post event for running a spark from capability's own pool.
+          traceEventRunSpark(cap, cap->r.rCurrentTSO);
+
+          return spark;
+      }
+      if (!emptySparkPoolCap(cap)) {
+          retry = rtsTrue;
+      }
+
+      if (n_capabilities == 1) { return NULL; } // makes no sense...
+
+      debugTrace(DEBUG_sched,
+                 "cap %d: Trying to steal work from other capabilities", 
+                 cap->no);
+
+      /* visit cap.s 0..n-1 in sequence until a theft succeeds. We could
+      start at a random place instead of 0 as well.  */
+      for ( i=0 ; i < n_capabilities ; i++ ) {
+          robbed = &capabilities[i];
+          if (cap == robbed)  // ourselves...
+              continue;
+
+          if (emptySparkPoolCap(robbed)) // nothing to steal here
+              continue;
+
+          spark = tryStealSpark(robbed);
+          if (spark == NULL && !emptySparkPoolCap(robbed)) {
+              // we conflicted with another thread while trying to steal;
+              // try again later.
+              retry = rtsTrue;
+          }
+
+          if (spark != NULL) {
+              cap->sparks_converted++;
+
+              traceEventStealSpark(cap, cap->r.rCurrentTSO, robbed->no);
+              
+              return spark;
+          }
+          // otherwise: no success, try next one
+      }
+  } while (retry);
+
+  debugTrace(DEBUG_sched, "No sparks stolen");
+  return NULL;
+}
+
+// Returns True if any spark pool is non-empty at this moment in time
+// The result is only valid for an instant, of course, so in a sense
+// is immediately invalid, and should not be relied upon for
+// correctness.
+rtsBool
+anySparks (void)
+{
+    nat i;
+
+    for (i=0; i < n_capabilities; i++) {
+        if (!emptySparkPoolCap(&capabilities[i])) {
+            return rtsTrue;
+        }
     }
+    return rtsFalse;
 }
 #endif
 
@@ -88,10 +172,10 @@ STATIC_INLINE void
 newReturningTask (Capability *cap, Task *task)
 {
     ASSERT_LOCK_HELD(&cap->lock);
-    ASSERT(task->return_link == NULL);
+    ASSERT(task->next == NULL);
     if (cap->returning_tasks_hd) {
-       ASSERT(cap->returning_tasks_tl->return_link == NULL);
-       cap->returning_tasks_tl->return_link = task;
+       ASSERT(cap->returning_tasks_tl->next == NULL);
+       cap->returning_tasks_tl->next = task;
     } else {
        cap->returning_tasks_hd = task;
     }
@@ -105,11 +189,11 @@ popReturningTask (Capability *cap)
     Task *task;
     task = cap->returning_tasks_hd;
     ASSERT(task);
-    cap->returning_tasks_hd = task->return_link;
+    cap->returning_tasks_hd = task->next;
     if (!cap->returning_tasks_hd) {
        cap->returning_tasks_tl = NULL;
     }
-    task->return_link = NULL;
+    task->next = NULL;
     return task;
 }
 #endif
@@ -135,28 +219,39 @@ initCapability( Capability *cap, nat i )
     initMutex(&cap->lock);
     cap->running_task      = NULL; // indicates cap is free
     cap->spare_workers     = NULL;
-    cap->suspended_ccalling_tasks = NULL;
+    cap->n_spare_workers   = 0;
+    cap->suspended_ccalls  = NULL;
     cap->returning_tasks_hd = NULL;
     cap->returning_tasks_tl = NULL;
-    cap->wakeup_queue_hd    = END_TSO_QUEUE;
-    cap->wakeup_queue_tl    = END_TSO_QUEUE;
+    cap->inbox              = (Message*)END_TSO_QUEUE;
+    cap->sparks_created     = 0;
+    cap->sparks_dud         = 0;
+    cap->sparks_converted   = 0;
+    cap->sparks_pruned      = 0;
 #endif
 
-    cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
-    cap->f.stgGCFun        = (F_)__stg_gc_fun;
+    cap->f.stgEagerBlackholeInfo = (W_)&__stg_EAGER_BLACKHOLE_info;
+    cap->f.stgGCEnter1     = (StgFunPtr)__stg_gc_enter_1;
+    cap->f.stgGCFun        = (StgFunPtr)__stg_gc_fun;
 
     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
                                     RtsFlags.GcFlags.generations,
                                     "initCapability");
+    cap->saved_mut_lists = stgMallocBytes(sizeof(bdescr *) *
+                                          RtsFlags.GcFlags.generations,
+                                          "initCapability");
 
     for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
        cap->mut_lists[g] = NULL;
     }
 
-    cap->free_tvar_wait_queues = END_STM_WAIT_QUEUE;
+    cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
+    cap->free_invariant_check_queues = END_INVARIANT_CHECK_QUEUE;
     cap->free_trec_chunks = END_STM_CHUNK_LIST;
     cap->free_trec_headers = NO_TREC;
     cap->transaction_tokens = 0;
+    cap->context_switch = 0;
+    cap->pinned_object_block = NULL;
 }
 
 /* ---------------------------------------------------------------------------
@@ -214,6 +309,19 @@ initCapabilities( void )
 }
 
 /* ----------------------------------------------------------------------------
+ * setContextSwitches: cause all capabilities to context switch as
+ * soon as possible.
+ * ------------------------------------------------------------------------- */
+
+void setContextSwitches(void)
+{
+    nat i;
+    for (i=0; i < n_capabilities; i++) {
+        contextSwitchCapability(&capabilities[i]);
+    }
+}
+
+/* ----------------------------------------------------------------------------
  * Give a Capability to a Task.  The task must currently be sleeping
  * on its condition variable.
  *
@@ -233,10 +341,9 @@ giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
 {
     ASSERT_LOCK_HELD(&cap->lock);
     ASSERT(task->cap == cap);
-    trace(TRACE_sched | DEBUG_sched,
-         "passing capability %d to %s %p",
-         cap->no, task->tso ? "bound task" : "worker",
-         (void *)task->id);
+    debugTrace(DEBUG_sched, "passing capability %d to %s %p",
+               cap->no, task->incall->tso ? "bound task" : "worker",
+               (void *)task->id);
     ACQUIRE_LOCK(&task->lock);
     task->wakeup = rtsTrue;
     // the wakeup flag is needed because signalCondition() doesn't
@@ -257,7 +364,8 @@ giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
 
 #if defined(THREADED_RTS)
 void
-releaseCapability_ (Capability* cap)
+releaseCapability_ (Capability* cap, 
+                    rtsBool always_wakeup)
 {
     Task *task;
 
@@ -275,12 +383,22 @@ releaseCapability_ (Capability* cap)
        return;
     }
 
+    if (waiting_for_gc == PENDING_GC_SEQ) {
+      last_free_capability = cap; // needed?
+      debugTrace(DEBUG_sched, "GC pending, set capability %d free", cap->no);
+      return;
+    } 
+
+
     // If the next thread on the run queue is a bound thread,
     // give this Capability to the appropriate Task.
     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
        // Make sure we're not about to try to wake ourselves up
-       ASSERT(task != cap->run_queue_hd->bound);
-       task = cap->run_queue_hd->bound;
+       // ASSERT(task != cap->run_queue_hd->bound);
+        // assertion is false: in schedule() we force a yield after
+       // ThreadBlocked, but the thread may be back on the run queue
+       // by now.
+       task = cap->run_queue_hd->bound->task;
        giveCapabilityToTask(cap,task);
        return;
     }
@@ -293,15 +411,16 @@ releaseCapability_ (Capability* cap)
        if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
            debugTrace(DEBUG_sched,
                       "starting new worker on capability %d", cap->no);
-           startWorkerTask(cap, workerStart);
+           startWorkerTask(cap);
            return;
        }
     }
 
     // If we have an unbound thread on the run queue, or if there's
     // anything else to do, give the Capability to a worker thread.
-    if (!emptyRunQueue(cap) || !emptyWakeupQueue(cap)
-             || !emptySparkPoolCap(cap) || globalWorkToDo()) {
+    if (always_wakeup || 
+        !emptyRunQueue(cap) || !emptyInbox(cap) ||
+        !emptySparkPoolCap(cap) || globalWorkToDo()) {
        if (cap->spare_workers) {
            giveCapabilityToTask(cap,cap->spare_workers);
            // The worker Task pops itself from the queue;
@@ -310,14 +429,22 @@ releaseCapability_ (Capability* cap)
     }
 
     last_free_capability = cap;
-    trace(TRACE_sched | DEBUG_sched, "freeing capability %d", cap->no);
+    debugTrace(DEBUG_sched, "freeing capability %d", cap->no);
 }
 
 void
 releaseCapability (Capability* cap USED_IF_THREADS)
 {
     ACQUIRE_LOCK(&cap->lock);
-    releaseCapability_(cap);
+    releaseCapability_(cap, rtsFalse);
+    RELEASE_LOCK(&cap->lock);
+}
+
+void
+releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
+{
+    ACQUIRE_LOCK(&cap->lock);
+    releaseCapability_(cap, rtsTrue);
     RELEASE_LOCK(&cap->lock);
 }
 
@@ -330,20 +457,37 @@ releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
 
     task = cap->running_task;
 
+    // If the Task is stopped, we shouldn't be yielding, we should
+    // be just exiting.
+    ASSERT(!task->stopped);
+
     // If the current task is a worker, save it on the spare_workers
     // list of this Capability.  A worker can mark itself as stopped,
     // in which case it is not replaced on the spare_worker queue.
     // This happens when the system is shutting down (see
     // Schedule.c:workerStart()).
-    // Also, be careful to check that this task hasn't just exited
-    // Haskell to do a foreign call (task->suspended_tso).
-    if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
-       task->next = cap->spare_workers;
-       cap->spare_workers = task;
+    if (!isBoundTask(task))
+    {
+        if (cap->n_spare_workers < MAX_SPARE_WORKERS)
+        {
+            task->next = cap->spare_workers;
+            cap->spare_workers = task;
+            cap->n_spare_workers++;
+        }
+        else
+        {
+            debugTrace(DEBUG_sched, "%d spare workers already, exiting",
+                       cap->n_spare_workers);
+            releaseCapability_(cap,rtsFalse);
+            // hold the lock until after workerTaskStop; c.f. scheduleWorker()
+            workerTaskStop(task);
+            RELEASE_LOCK(&cap->lock);
+            shutdownThread();
+        }
     }
     // Bound tasks just float around attached to their TSOs.
 
-    releaseCapability_(cap);
+    releaseCapability_(cap,rtsFalse);
 
     RELEASE_LOCK(&cap->lock);
 }
@@ -374,17 +518,20 @@ waitForReturnCapability (Capability **pCap, Task *task)
     if (cap == NULL) {
        // Try last_free_capability first
        cap = last_free_capability;
-       if (!cap->running_task) {
+       if (cap->running_task) {
            nat i;
            // otherwise, search for a free capability
+            cap = NULL;
            for (i = 0; i < n_capabilities; i++) {
-               cap = &capabilities[i];
-               if (!cap->running_task) {
+               if (!capabilities[i].running_task) {
+                    cap = &capabilities[i];
                    break;
                }
            }
-           // Can't find a free one, use last_free_capability.
-           cap = last_free_capability;
+            if (cap == NULL) {
+                // Can't find a free one, use last_free_capability.
+                cap = last_free_capability;
+            }
        }
 
        // record the Capability as the one this Task is now assocated with.
@@ -434,7 +581,7 @@ waitForReturnCapability (Capability **pCap, Task *task)
 
     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 
-    trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
+    debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
 
     *pCap = cap;
 #endif
@@ -450,9 +597,13 @@ yieldCapability (Capability** pCap, Task *task)
 {
     Capability *cap = *pCap;
 
-    // The fast path has no locking, if we don't enter this while loop
+    if (waiting_for_gc == PENDING_GC_PAR) {
+        traceEventGcStart(cap);
+        gcWorkerThread(cap);
+        traceEventGcEnd(cap);
+        return;
+    }
 
-    while ( cap->returning_tasks_hd != NULL || !anyWorkForMe(cap,task) ) {
        debugTrace(DEBUG_sched, "giving up capability %d", cap->no);
 
        // We must now release the capability and wait to be woken up
@@ -478,7 +629,7 @@ yieldCapability (Capability** pCap, Task *task)
                continue;
            }
 
-           if (task->tso == NULL) {
+           if (task->incall->tso == NULL) {
                ASSERT(cap->spare_workers != NULL);
                // if we're not at the front of the queue, release it
                // again.  This is unlikely to happen.
@@ -489,15 +640,15 @@ yieldCapability (Capability** pCap, Task *task)
                }
                cap->spare_workers = task->next;
                task->next = NULL;
-           }
+                cap->n_spare_workers--;
+            }
            cap->running_task = task;
            RELEASE_LOCK(&cap->lock);
            break;
        }
 
-       trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
+       debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
        ASSERT(cap->running_task == task);
-    }
 
     *pCap = cap;
 
@@ -507,93 +658,24 @@ yieldCapability (Capability** pCap, Task *task)
 }
 
 /* ----------------------------------------------------------------------------
- * Wake up a thread on a Capability.
+ * prodCapability
  *
- * This is used when the current Task is running on a Capability and
- * wishes to wake up a thread on a different Capability.
+ * If a Capability is currently idle, wake up a Task on it.  Used to 
+ * get every Capability into the GC.
  * ------------------------------------------------------------------------- */
 
 void
-wakeupThreadOnCapability (Capability *cap, StgTSO *tso)
+prodCapability (Capability *cap, Task *task)
 {
-    ASSERT(tso->cap == cap);
-    ASSERT(tso->bound ? tso->bound->cap == cap : 1);
-
     ACQUIRE_LOCK(&cap->lock);
-    if (cap->running_task == NULL) {
-       // nobody is running this Capability, we can add our thread
-       // directly onto the run queue and start up a Task to run it.
-       appendToRunQueue(cap,tso);
-
-       // start it up
-       cap->running_task = myTask(); // precond for releaseCapability_()
-       trace(TRACE_sched, "resuming capability %d", cap->no);
-       releaseCapability_(cap);
-    } else {
-       appendToWakeupQueue(cap,tso);
-       // someone is running on this Capability, so it cannot be
-       // freed without first checking the wakeup queue (see
-       // releaseCapability_).
+    if (!cap->running_task) {
+        cap->running_task = task;
+        releaseCapability_(cap,rtsTrue);
     }
     RELEASE_LOCK(&cap->lock);
 }
 
 /* ----------------------------------------------------------------------------
- * prodCapabilities
- *
- * Used to indicate that the interrupted flag is now set, or some
- * other global condition that might require waking up a Task on each
- * Capability.
- * ------------------------------------------------------------------------- */
-
-static void
-prodCapabilities(rtsBool all)
-{
-    nat i;
-    Capability *cap;
-    Task *task;
-
-    for (i=0; i < n_capabilities; i++) {
-       cap = &capabilities[i];
-       ACQUIRE_LOCK(&cap->lock);
-       if (!cap->running_task) {
-           if (cap->spare_workers) {
-               trace(TRACE_sched, "resuming capability %d", cap->no);
-               task = cap->spare_workers;
-               ASSERT(!task->stopped);
-               giveCapabilityToTask(cap,task);
-               if (!all) {
-                   RELEASE_LOCK(&cap->lock);
-                   return;
-               }
-           }
-       }
-       RELEASE_LOCK(&cap->lock);
-    }
-    return;
-}
-
-void
-prodAllCapabilities (void)
-{
-    prodCapabilities(rtsTrue);
-}
-
-/* ----------------------------------------------------------------------------
- * prodOneCapability
- *
- * Like prodAllCapabilities, but we only require a single Task to wake
- * up in order to service some global event, such as checking for
- * deadlock after some idle time has passed.
- * ------------------------------------------------------------------------- */
-
-void
-prodOneCapability (void)
-{
-    prodCapabilities(rtsFalse);
-}
-
-/* ----------------------------------------------------------------------------
  * shutdownCapability
  *
  * At shutdown time, we want to let everything exit as cleanly as
@@ -609,15 +691,21 @@ prodOneCapability (void)
  * ------------------------------------------------------------------------- */
 
 void
-shutdownCapability (Capability *cap, Task *task)
+shutdownCapability (Capability *cap, Task *task, rtsBool safe)
 {
     nat i;
 
-    ASSERT(sched_state == SCHED_SHUTTING_DOWN);
-
     task->cap = cap;
 
-    for (i = 0; i < 50; i++) {
+    // Loop indefinitely until all the workers have exited and there
+    // are no Haskell threads left.  We used to bail out after 50
+    // iterations of this loop, but that occasionally left a worker
+    // running which caused problems later (the closeMutex() below
+    // isn't safe, for one thing).
+
+    for (i = 0; /* i < 50 */; i++) {
+        ASSERT(sched_state == SCHED_SHUTTING_DOWN);
+
        debugTrace(DEBUG_sched, 
                   "shutting down capability %d, attempt %d", cap->no, i);
        ACQUIRE_LOCK(&cap->lock);
@@ -628,20 +716,74 @@ shutdownCapability (Capability *cap, Task *task)
            continue;
        }
        cap->running_task = task;
+
+        if (cap->spare_workers) {
+            // Look for workers that have died without removing
+            // themselves from the list; this could happen if the OS
+            // summarily killed the thread, for example.  This
+            // actually happens on Windows when the system is
+            // terminating the program, and the RTS is running in a
+            // DLL.
+            Task *t, *prev;
+            prev = NULL;
+            for (t = cap->spare_workers; t != NULL; t = t->next) {
+                if (!osThreadIsAlive(t->id)) {
+                    debugTrace(DEBUG_sched, 
+                               "worker thread %p has died unexpectedly", (void *)t->id);
+                    cap->n_spare_workers--;
+                    if (!prev) {
+                        cap->spare_workers = t->next;
+                    } else {
+                        prev->next = t->next;
+                    }
+                    prev = t;
+                }
+            }
+        }
+
        if (!emptyRunQueue(cap) || cap->spare_workers) {
            debugTrace(DEBUG_sched, 
                       "runnable threads or workers still alive, yielding");
-           releaseCapability_(cap); // this will wake up a worker
+           releaseCapability_(cap,rtsFalse); // this will wake up a worker
            RELEASE_LOCK(&cap->lock);
            yieldThread();
            continue;
        }
-       debugTrace(DEBUG_sched, "capability %d is stopped.", cap->no);
+
+        // If "safe", then busy-wait for any threads currently doing
+        // foreign calls.  If we're about to unload this DLL, for
+        // example, we need to be sure that there are no OS threads
+        // that will try to return to code that has been unloaded.
+        // We can be a bit more relaxed when this is a standalone
+        // program that is about to terminate, and let safe=false.
+        if (cap->suspended_ccalls && safe) {
+           debugTrace(DEBUG_sched, 
+                      "thread(s) are involved in foreign calls, yielding");
+            cap->running_task = NULL;
+           RELEASE_LOCK(&cap->lock);
+            // The IO manager thread might have been slow to start up,
+            // so the first attempt to kill it might not have
+            // succeeded.  Just in case, try again - the kill message
+            // will only be sent once.
+            //
+            // To reproduce this deadlock: run ffi002(threaded1)
+            // repeatedly on a loaded machine.
+            ioManagerDie();
+            yieldThread();
+            continue;
+        }
+
+        traceEventShutdown(cap);
        RELEASE_LOCK(&cap->lock);
        break;
     }
     // we now have the Capability, its run queue and spare workers
     // list are both empty.
+
+    // ToDo: we can't drop this mutex, because there might still be
+    // threads performing foreign calls that will eventually try to 
+    // return via resumeThread() and attempt to grab cap->lock.
+    // closeMutex(&cap->lock);
 }
 
 /* ----------------------------------------------------------------------------
@@ -669,4 +811,81 @@ tryGrabCapability (Capability *cap, Task *task)
 
 #endif /* THREADED_RTS */
 
+static void
+freeCapability (Capability *cap)
+{
+    stgFree(cap->mut_lists);
+    stgFree(cap->saved_mut_lists);
+#if defined(THREADED_RTS)
+    freeSparkPool(cap->sparks);
+#endif
+}
+
+void
+freeCapabilities (void)
+{
+#if defined(THREADED_RTS)
+    nat i;
+    for (i=0; i < n_capabilities; i++) {
+        freeCapability(&capabilities[i]);
+    }
+#else
+    freeCapability(&MainCapability);
+#endif
+}
+
+/* ---------------------------------------------------------------------------
+   Mark everything directly reachable from the Capabilities.  When
+   using multiple GC threads, each GC thread marks all Capabilities
+   for which (c `mod` n == 0), for Capability c and thread n.
+   ------------------------------------------------------------------------ */
+
+void
+markSomeCapabilities (evac_fn evac, void *user, nat i0, nat delta, 
+                      rtsBool no_mark_sparks USED_IF_THREADS)
+{
+    nat i;
+    Capability *cap;
+    InCall *incall;
+
+    // Each GC thread is responsible for following roots from the
+    // Capability of the same number.  There will usually be the same
+    // or fewer Capabilities as GC threads, but just in case there
+    // are more, we mark every Capability whose number is the GC
+    // thread's index plus a multiple of the number of GC threads.
+    for (i = i0; i < n_capabilities; i += delta) {
+       cap = &capabilities[i];
+       evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
+       evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
+#if defined(THREADED_RTS)
+        evac(user, (StgClosure **)(void *)&cap->inbox);
+#endif
+       for (incall = cap->suspended_ccalls; incall != NULL; 
+            incall=incall->next) {
+           evac(user, (StgClosure **)(void *)&incall->suspended_tso);
+       }
+
+#if defined(THREADED_RTS)
+        if (!no_mark_sparks) {
+            traverseSparkQueue (evac, user, cap);
+        }
+#endif
+    }
+
+#if !defined(THREADED_RTS)
+    evac(user, (StgClosure **)(void *)&blocked_queue_hd);
+    evac(user, (StgClosure **)(void *)&blocked_queue_tl);
+    evac(user, (StgClosure **)(void *)&sleeping_queue);
+#endif 
+}
+
+void
+markCapabilities (evac_fn evac, void *user)
+{
+    markSomeCapabilities(evac, user, 0, 1, rtsFalse);
+}
+
+/* -----------------------------------------------------------------------------
+   Messages
+   -------------------------------------------------------------------------- */