/* ---------------------------------------------------------------------------
*
- * (c) The GHC Team, 2001
+ * (c) The GHC Team, 2002
*
* Capabilities
*
- * The notion of a capability is used when operating in multi-threaded
- * environments (which the SMP and Threads builds of the RTS do), to
- * hold all the state an OS thread/task needs to run Haskell code:
- * its STG registers, a pointer to its TSO, a nursery etc. During
- * STG execution, a pointer to the capabilitity is kept in a
+ * A Capability represent the token required to execute STG code,
+ * and all the state an OS thread/task needs to run Haskell code:
+ * its STG registers, a pointer to its TSO, a nursery etc. During
+ * STG execution, a pointer to the capabilitity is kept in a
* register (BaseReg).
*
- * Only in an SMP build will there be multiple capabilities, the threaded
- * RTS and other non-threaded builds, there is one global capability,
- * namely MainRegTable.
- *
+ * Only in an SMP build will there be multiple capabilities, for
+ * the threaded RTS and other non-threaded builds, there is only
+ * one global capability, namely MainCapability.
*
* --------------------------------------------------------------------------*/
#include "PosixSource.h"
#include "Rts.h"
#include "RtsUtils.h"
+#include "OSThreads.h"
#include "Capability.h"
+#include "Schedule.h" /* to get at EMPTY_RUN_QUEUE() */
+#include "Signals.h" /* to get at handleSignalsInThisThread() */
+
+#if !defined(SMP)
+Capability MainCapability; /* for non-SMP, we have one global capability */
+#endif
+
+nat rts_n_free_capabilities;
+
+#if defined(RTS_SUPPORTS_THREADS)
+/* returning_worker_cond: when a worker thread returns from executing an
+ * external call, it needs to wait for an RTS Capability before passing
+ * on the result of the call to the Haskell thread that made it.
+ *
+ * returning_worker_cond is signalled in Capability.releaseCapability().
+ *
+ */
+Condition returning_worker_cond = INIT_COND_VAR;
+
+/*
+ * To avoid starvation of threads blocked on worker_thread_cond,
+ * the task(s) that enter the Scheduler will check to see whether
+ * there are one or more worker threads blocked waiting on
+ * returning_worker_cond.
+ */
+nat rts_n_waiting_workers = 0;
+
+/* thread_ready_cond: when signalled, a thread has become runnable for a
+ * task to execute.
+ *
+ * In the non-SMP case, it also implies that the thread that is woken up has
+ * exclusive access to the RTS and all its data structures (that are not
+ * locked by the Scheduler's mutex).
+ *
+ * thread_ready_cond is signalled whenever noCapabilities doesn't hold.
+ *
+ */
+Condition thread_ready_cond = INIT_COND_VAR;
+/*
+ * To be able to make an informed decision about whether or not
+ * to create a new task when making an external call, keep track of
+ * the number of tasks currently blocked waiting on thread_ready_cond.
+ * (if > 0 => no need for a new task, just unblock an existing one).
+ *
+ * waitForWorkCapability() takes care of keeping it up-to-date;
+ * Task.startTask() uses its current value.
+ */
+nat rts_n_waiting_tasks = 0;
+#endif
+/* -----------------------------------------------------------------------------
+ Initialisation
+ -------------------------------------------------------------------------- */
static
void
initCapability( Capability *cap )
{
- cap->f.stgChk0 = (F_)__stg_chk_0;
- cap->f.stgChk1 = (F_)__stg_chk_1;
cap->f.stgGCEnter1 = (F_)__stg_gc_enter_1;
- cap->f.stgUpdatePAP = (F_)__stg_update_PAP;
+ cap->f.stgGCFun = (F_)__stg_gc_fun;
}
-#ifdef SMP
+#if defined(SMP)
static void initCapabilities_(nat n);
#endif
/*
+ * Function: initCapabilities()
+ *
+ * Purpose: set up the Capability handling. For the SMP build,
+ * we keep a table of them, the size of which is
+ * controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
+ *
+ * Pre-conditions: no locks assumed held.
*/
void
initCapabilities()
{
+#if defined(RTS_SUPPORTS_THREADS)
+ initCondition(&returning_worker_cond);
+ initCondition(&thread_ready_cond);
+#endif
+
#if defined(SMP)
initCapabilities_(RtsFlags.ParFlags.nNodes);
#else
initCapability(&MainCapability);
+ rts_n_free_capabilities = 1;
#endif
return;
}
-/* Free capability list.
- * Locks required: sched_mutex.
- */
#if defined(SMP)
+/* Free capability list. */
static Capability *free_capabilities; /* Available capabilities for running threads */
+static Capability *returning_capabilities;
+ /* Capabilities being passed to returning worker threads */
#endif
+/* -----------------------------------------------------------------------------
+ Acquiring capabilities
+ -------------------------------------------------------------------------- */
+
+/*
+ * Function: grabCapability(Capability**)
+ *
+ * Purpose: the act of grabbing a capability is easy; just
+ * remove one from the free capabilities list (which
+ * may just have one entry). In threaded builds, worker
+ * threads are prevented from doing so willy-nilly
+ * via the condition variables thread_ready_cond and
+ * returning_worker_cond.
+ *
+ */
void grabCapability(Capability** cap)
{
+#ifdef RTS_SUPPORTS_THREADS
+ ASSERT(rts_n_free_capabilities > 0);
+#endif
#if !defined(SMP)
+ rts_n_free_capabilities = 0;
*cap = &MainCapability;
+ handleSignalsInThisThread();
#else
*cap = free_capabilities;
free_capabilities = (*cap)->link;
#endif
}
-void releaseCapability(Capability** cap)
+/*
+ * Function: releaseCapability(Capability*)
+ *
+ * Purpose: Letting go of a capability. Causes a
+ * 'returning worker' thread or a 'waiting worker'
+ * to wake up, in that order.
+ *
+ */
+void releaseCapability(Capability* cap
+#if !defined(SMP)
+ STG_UNUSED
+#endif
+)
+{ // Precondition: sched_mutex must be held
+#if defined(RTS_SUPPORTS_THREADS)
+#ifndef SMP
+ ASSERT(rts_n_free_capabilities == 0);
+#endif
+ /* Check to see whether a worker thread can be given
+ the go-ahead to return the result of an external call..*/
+ if (rts_n_waiting_workers > 0) {
+ /* Decrement the counter here to avoid livelock where the
+ * thread that is yielding its capability will repeatedly
+ * signal returning_worker_cond.
+ */
+#if defined(SMP)
+ // SMP variant untested
+ cap->link = returning_capabilities;
+ returning_capabilities = cap;
+#else
+#endif
+ rts_n_waiting_workers--;
+ signalCondition(&returning_worker_cond);
+ } else /*if ( !EMPTY_RUN_QUEUE() )*/ {
+#if defined(SMP)
+ cap->link = free_capabilities;
+ free_capabilities = cap;
+ rts_n_free_capabilities++;
+#else
+ rts_n_free_capabilities = 1;
+#endif
+ /* Signal that a capability is available */
+ signalCondition(&thread_ready_cond);
+ }
+#endif
+ return;
+}
+
+#if defined(RTS_SUPPORTS_THREADS)
+/*
+ * When a native thread has completed the execution of an external
+ * call, it needs to communicate the result back. This is done
+ * as follows:
+ *
+ * - in resumeThread(), the thread calls grabReturnCapability().
+ * - If no capabilities are readily available, grabReturnCapability()
+ * increments a counter rts_n_waiting_workers, and blocks
+ * waiting for the condition returning_worker_cond to become
+ * signalled.
+ * - upon entry to the Scheduler, a worker thread checks the
+ * value of rts_n_waiting_workers. If > 0, the worker thread
+ * will yield its capability to let a returning worker thread
+ * proceed with returning its result -- this is done via
+ * yieldToReturningWorker().
+ * - the worker thread that yielded its capability then tries
+ * to re-grab a capability and re-enter the Scheduler.
+ */
+
+/*
+ * Function: grabReturnCapability(Capability**)
+ *
+ * Purpose: when an OS thread returns from an external call,
+ * it calls grabReturnCapability() (via Schedule.resumeThread())
+ * to wait for permissions to enter the RTS & communicate the
+ * result of the external call back to the Haskell thread that
+ * made it.
+ *
+ * Pre-condition: pMutex is held.
+ * Post-condition: pMutex is still held and a capability has
+ * been assigned to the worker thread.
+ */
+void
+grabReturnCapability(Mutex* pMutex, Capability** pCap)
{
+ IF_DEBUG(scheduler,
+ fprintf(stderr,"worker (%ld): returning, waiting for lock.\n", osThreadId()));
+ IF_DEBUG(scheduler,
+ fprintf(stderr,"worker (%ld): returning; workers waiting: %d\n",
+ osThreadId(), rts_n_waiting_workers));
+ if ( noCapabilities() ) {
+ rts_n_waiting_workers++;
+ wakeBlockedWorkerThread();
+ context_switch = 1; // make sure it's our turn soon
+ waitCondition(&returning_worker_cond, pMutex);
#if defined(SMP)
- (*cap)->link = free_capabilities;
- free_capabilities = *cap;
- rts_n_free_capabilities++;
+ *pCap = returning_capabilities;
+ returning_capabilities = (*pCap)->link;
+#else
+ *pCap = &MainCapability;
+ ASSERT(rts_n_free_capabilities == 0);
+ handleSignalsInThisThread();
#endif
+ } else {
+ grabCapability(pCap);
+ }
+ return;
+}
+
+
+/* -----------------------------------------------------------------------------
+ Yielding/waiting for capabilities
+ -------------------------------------------------------------------------- */
+
+/*
+ * Function: yieldToReturningWorker(Mutex*,Capability*)
+ *
+ * Purpose: when, upon entry to the Scheduler, an OS worker thread
+ * spots that one or more threads are blocked waiting for
+ * permission to return back their result, it gives up
+ * its Capability.
+ *
+ * Pre-condition: pMutex is assumed held and the thread possesses
+ * a Capability.
+ * Post-condition: pMutex is held and the Capability has
+ * been given back.
+ */
+void
+yieldToReturningWorker(Mutex* pMutex, Capability** pCap)
+{
+ if ( rts_n_waiting_workers > 0 ) {
+ IF_DEBUG(scheduler,
+ fprintf(stderr,"worker thread (%p): giving up RTS token\n", osThreadId()));
+ releaseCapability(*pCap);
+ /* And wait for work */
+ waitForWorkCapability(pMutex, pCap, rtsFalse);
+ IF_DEBUG(scheduler,
+ fprintf(stderr,"worker thread (%p): got back RTS token (after yieldToReturningWorker)\n",
+ osThreadId()));
+ }
return;
}
+
+/*
+ * Function: waitForWorkCapability(Mutex*, Capability**, rtsBool)
+ *
+ * Purpose: wait for a Capability to become available. In
+ * the process of doing so, updates the number
+ * of tasks currently blocked waiting for a capability/more
+ * work. That counter is used when deciding whether or
+ * not to create a new worker thread when an external
+ * call is made.
+ *
+ * Pre-condition: pMutex is held.
+ * Post-condition: pMutex is held and *pCap is held by the current thread
+ */
+void
+waitForWorkCapability(Mutex* pMutex, Capability** pCap, rtsBool runnable)
+{
+ while ( noCapabilities() || (runnable && EMPTY_RUN_QUEUE()) ) {
+ rts_n_waiting_tasks++;
+ waitCondition(&thread_ready_cond, pMutex);
+ rts_n_waiting_tasks--;
+ }
+ grabCapability(pCap);
+ return;
+}
+
+#endif /* RTS_SUPPORTS_THREADS */
+
#if defined(SMP)
-/* Allocate 'n' capabilities */
+/*
+ * Function: initCapabilities_(nat)
+ *
+ * Purpose: upon startup, allocate and fill in table
+ * holding 'n' Capabilities. Only for SMP, since
+ * it is the only build that supports multiple
+ * capabilities within the RTS.
+ */
static void
initCapabilities_(nat n)
{
}
free_capabilities = cap;
rts_n_free_capabilities = n;
+ returning_capabilities = NULL;
IF_DEBUG(scheduler,fprintf(stderr,"scheduler: Allocated %d capabilities\n", n_free_capabilities););
}
#endif /* SMP */
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