*
* Sparking support for PARALLEL_HASKELL and THREADED_RTS versions of the RTS.
*
- * The implementation uses Double-Ended Queues with lock-free access
- * (thereby often called "deque") as described in
- *
- * D.Chase and Y.Lev, Dynamic Circular Work-Stealing Deque.
- * SPAA'05, July 2005, Las Vegas, USA.
- * ACM 1-58113-986-1/05/0007
- *
- * Author: Jost Berthold MSRC 07-09/2008
- *
- * The DeQue is held as a circular array with known length. Positions
- * of top (read-end) and bottom (write-end) always increase, and the
- * array is accessed with indices modulo array-size. While this bears
- * the risk of overflow, we assume that (with 64 bit indices), a
- * program must run very long to reach that point.
- *
- * The write end of the queue (position bottom) can only be used with
- * mutual exclusion, i.e. by exactly one caller at a time. At this
- * end, new items can be enqueued using pushBottom()/newSpark(), and
- * removed using popBottom()/reclaimSpark() (the latter implying a cas
- * synchronisation with potential concurrent readers for the case of
- * just one element).
- *
- * Multiple readers can steal()/findSpark() from the read end
- * (position top), and are synchronised without a lock, based on a cas
- * of the top position. One reader wins, the others return NULL for a
- * failure.
- *
- * Both popBottom and steal also return NULL when the queue is empty.
- *
-------------------------------------------------------------------------*/
#include "PosixSource.h"
#if defined(THREADED_RTS) || defined(PARALLEL_HASKELL)
-/* internal helpers ... */
-
-static StgWord
-roundUp2(StgWord val)
-{
- StgWord rounded = 1;
-
- /* StgWord is unsigned anyway, only catch 0 */
- if (val == 0) {
- barf("DeQue,roundUp2: invalid size 0 requested");
- }
- /* at least 1 bit set, shift up to its place */
- do {
- rounded = rounded << 1;
- } while (0 != (val = val>>1));
- return rounded;
-}
-
-#define CASTOP(addr,old,new) ((old) == cas(((StgPtr)addr),(old),(new)))
-
-/* -----------------------------------------------------------------------------
- *
- * Initialising spark pools.
- *
- * -------------------------------------------------------------------------- */
-
-/* constructor */
-static SparkPool*
-initPool(StgWord size)
-{
- StgWord realsize;
- SparkPool *q;
-
- realsize = roundUp2(size); /* to compute modulo as a bitwise & */
-
- q = (SparkPool*) stgMallocBytes(sizeof(SparkPool), /* admin fields */
- "newSparkPool");
- q->elements = (StgClosurePtr*)
- stgMallocBytes(realsize * sizeof(StgClosurePtr), /* dataspace */
- "newSparkPool:data space");
- q->top=0;
- q->bottom=0;
- q->topBound=0; /* read by writer, updated each time top is read */
-
- q->size = realsize; /* power of 2 */
- q->moduloSize = realsize - 1; /* n % size == n & moduloSize */
-
- ASSERT_SPARK_POOL_INVARIANTS(q);
- return q;
-}
-
void
initSparkPools( void )
{
/* walk over the capabilities, allocating a spark pool for each one */
nat i;
for (i = 0; i < n_capabilities; i++) {
- capabilities[i].sparks = initPool(RtsFlags.ParFlags.maxLocalSparks);
+ capabilities[i].sparks = newWSDeque(RtsFlags.ParFlags.maxLocalSparks);
}
#else
/* allocate a single spark pool */
- MainCapability->sparks = initPool(RtsFlags.ParFlags.maxLocalSparks);
+ MainCapability->sparks = newWSDeque(RtsFlags.ParFlags.maxLocalSparks);
#endif
}
void
freeSparkPool (SparkPool *pool)
{
- /* should not interfere with concurrent findSpark() calls! And
- nobody should use the pointer any more. We cross our fingers...*/
- stgFree(pool->elements);
- stgFree(pool);
-}
-
-/* -----------------------------------------------------------------------------
- *
- * reclaimSpark: remove a spark from the write end of the queue.
- * Returns the removed spark, and NULL if a race is lost or the pool
- * empty.
- *
- * If only one spark is left in the pool, we synchronise with
- * concurrently stealing threads by using cas to modify the top field.
- * This routine should NEVER be called by a task which does not own
- * the capability. Can this be checked here?
- *
- * -------------------------------------------------------------------------- */
-
-StgClosure *
-reclaimSpark (SparkPool *deque)
-{
- /* also a bit tricky, has to avoid concurrent steal() calls by
- accessing top with cas, when there is only one element left */
- StgWord t, b;
- StgClosurePtr* pos;
- long currSize;
- StgClosurePtr removed;
-
- ASSERT_SPARK_POOL_INVARIANTS(deque);
-
- b = deque->bottom;
- /* "decrement b as a test, see what happens" */
- deque->bottom = --b;
- pos = (deque->elements) + (b & (deque->moduloSize));
- t = deque->top; /* using topBound would give an *upper* bound, we
- need a lower bound. We use the real top here, but
- can update the topBound value */
- deque->topBound = t;
- currSize = b - t;
- if (currSize < 0) { /* was empty before decrementing b, set b
- consistently and abort */
- deque->bottom = t;
- return NULL;
- }
- removed = *pos;
- if (currSize > 0) { /* no danger, still elements in buffer after b-- */
- return removed;
- }
- /* otherwise, has someone meanwhile stolen the same (last) element?
- Check and increment top value to know */
- if ( !(CASTOP(&(deque->top),t,t+1)) ) {
- removed = NULL; /* no success, but continue adjusting bottom */
- }
- deque->bottom = t+1; /* anyway, empty now. Adjust bottom consistently. */
- deque->topBound = t+1; /* ...and cached top value as well */
-
- ASSERT_SPARK_POOL_INVARIANTS(deque);
-
- return removed;
-}
-
-/* -----------------------------------------------------------------------------
- *
- * tryStealSpark: try to steal a spark from a Capability.
- *
- * Returns a valid spark, or NULL if the pool was empty, and can
- * occasionally return NULL if there was a race with another thread
- * stealing from the same pool. In this case, try again later.
- *
- -------------------------------------------------------------------------- */
-
-static StgClosurePtr
-steal(SparkPool *deque)
-{
- StgClosurePtr* pos;
- StgClosurePtr* arraybase;
- StgWord sz;
- StgClosurePtr stolen;
- StgWord b,t;
-
-// Can't do this on someone else's spark pool:
-// ASSERT_SPARK_POOL_INVARIANTS(deque);
-
- b = deque->bottom;
- t = deque->top;
-
- // NB. b and t are unsigned; we need a signed value for the test
- // below.
- if ((long)b - (long)t <= 0 ) {
- return NULL; /* already looks empty, abort */
- }
-
- /* now access array, see pushBottom() */
- arraybase = deque->elements;
- sz = deque->moduloSize;
- pos = arraybase + (t & sz);
- stolen = *pos;
-
- /* now decide whether we have won */
- if ( !(CASTOP(&(deque->top),t,t+1)) ) {
- /* lost the race, someon else has changed top in the meantime */
- return NULL;
- } /* else: OK, top has been incremented by the cas call */
-
-// Can't do this on someone else's spark pool:
-// ASSERT_SPARK_POOL_INVARIANTS(deque);
-
- /* return stolen element */
- return stolen;
-}
-
-StgClosure *
-tryStealSpark (Capability *cap)
-{
- SparkPool *pool = cap->sparks;
- StgClosure *stolen;
-
- do {
- stolen = steal(pool);
- } while (stolen != NULL && !closure_SHOULD_SPARK(stolen));
-
- return stolen;
-}
-
-
-/* -----------------------------------------------------------------------------
- *
- * "guesses" whether a deque is empty. Can return false negatives in
- * presence of concurrent steal() calls, and false positives in
- * presence of a concurrent pushBottom().
- *
- * -------------------------------------------------------------------------- */
-
-rtsBool
-looksEmpty(SparkPool* deque)
-{
- StgWord t = deque->top;
- StgWord b = deque->bottom;
- /* try to prefer false negatives by reading top first */
- return ((long)b - (long)t <= 0);
- /* => array is *never* completely filled, always 1 place free! */
+ freeWSDeque(pool);
}
/* -----------------------------------------------------------------------------
appendToRunQueue(cap,tso);
}
-/* -----------------------------------------------------------------------------
- *
- * Create a new spark
- *
- * -------------------------------------------------------------------------- */
-
-#define DISCARD_NEW
-
-/* enqueue an element. Should always succeed by resizing the array
- (not implemented yet, silently fails in that case). */
-static void
-pushBottom (SparkPool* deque, StgClosurePtr elem)
-{
- StgWord t;
- StgClosurePtr* pos;
- StgWord sz = deque->moduloSize;
- StgWord b = deque->bottom;
-
- ASSERT_SPARK_POOL_INVARIANTS(deque);
-
- /* we try to avoid reading deque->top (accessed by all) and use
- deque->topBound (accessed only by writer) instead.
- This is why we do not just call empty(deque) here.
- */
- t = deque->topBound;
- if ( (StgInt)b - (StgInt)t >= (StgInt)sz ) {
- /* NB. 1. sz == deque->size - 1, thus ">="
- 2. signed comparison, it is possible that t > b
- */
- /* could be full, check the real top value in this case */
- t = deque->top;
- deque->topBound = t;
- if (b - t >= sz) { /* really no space left :-( */
- /* reallocate the array, copying the values. Concurrent steal()s
- will in the meantime use the old one and modify only top.
- This means: we cannot safely free the old space! Can keep it
- on a free list internally here...
-
- Potential bug in combination with steal(): if array is
- replaced, it is unclear which one concurrent steal operations
- use. Must read the array base address in advance in steal().
- */
-#if defined(DISCARD_NEW)
- ASSERT_SPARK_POOL_INVARIANTS(deque);
- return; /* for now, silently fail */
-#else
- /* could make room by incrementing the top position here. In
- * this case, should use CASTOP. If this fails, someone else has
- * removed something, and new room will be available.
- */
- ASSERT_SPARK_POOL_INVARIANTS(deque);
-#endif
- }
- }
- pos = (deque->elements) + (b & sz);
- *pos = elem;
- (deque->bottom)++;
-
- ASSERT_SPARK_POOL_INVARIANTS(deque);
- return;
-}
-
-
/* --------------------------------------------------------------------------
* newSpark: create a new spark, as a result of calling "par"
* Called directly from STG.
*/
p = UNTAG_CLOSURE(p);
- ASSERT_SPARK_POOL_INVARIANTS(pool);
-
if (closure_SHOULD_SPARK(p)) {
- pushBottom(pool,p);
+ pushWSDeque(pool,p);
}
cap->sparks_created++;
- ASSERT_SPARK_POOL_INVARIANTS(pool);
return 1;
}
+/* -----------------------------------------------------------------------------
+ *
+ * tryStealSpark: try to steal a spark from a Capability.
+ *
+ * Returns a valid spark, or NULL if the pool was empty, and can
+ * occasionally return NULL if there was a race with another thread
+ * stealing from the same pool. In this case, try again later.
+ *
+ -------------------------------------------------------------------------- */
+StgClosure *
+tryStealSpark (Capability *cap)
+{
+ SparkPool *pool = cap->sparks;
+ StgClosure *stolen;
+
+ do {
+ stolen = stealWSDeque_(pool);
+ // use the no-loopy version, stealWSDeque_(), since if we get a
+ // spurious NULL here the caller may want to try stealing from
+ // other pools before trying again.
+ } while (stolen != NULL && !closure_SHOULD_SPARK(stolen));
+
+ return stolen;
+}
/* --------------------------------------------------------------------------
* Remove all sparks from the spark queues which should not spark any
pool->topBound = pool->top;
debugTrace(DEBUG_sched,
- "markSparkQueue: current spark queue len=%d; (hd=%ld; tl=%ld)",
+ "markSparkQueue: current spark queue len=%ld; (hd=%ld; tl=%ld)",
sparkPoolSize(pool), pool->bottom, pool->top);
- ASSERT_SPARK_POOL_INVARIANTS(pool);
- elements = pool->elements;
+ ASSERT_WSDEQUE_INVARIANTS(pool);
+
+ elements = (StgClosurePtr *)pool->elements;
/* We have exclusive access to the structure here, so we can reset
bottom and top counters, and prune invalid sparks. Contents are
debugTrace(DEBUG_sched, "pruned %d sparks", pruned_sparks);
debugTrace(DEBUG_sched,
- "new spark queue len=%d; (hd=%ld; tl=%ld)",
+ "new spark queue len=%ld; (hd=%ld; tl=%ld)",
sparkPoolSize(pool), pool->bottom, pool->top);
- ASSERT_SPARK_POOL_INVARIANTS(pool);
+ ASSERT_WSDEQUE_INVARIANTS(pool);
}
/* GC for the spark pool, called inside Capability.c for all
pool = cap->sparks;
- ASSERT_SPARK_POOL_INVARIANTS(pool);
+ ASSERT_WSDEQUE_INVARIANTS(pool);
top = pool->top;
bottom = pool->bottom;
- sparkp = pool->elements;
+ sparkp = (StgClosurePtr*)pool->elements;
modMask = pool->moduloSize;
while (top < bottom) {
}
debugTrace(DEBUG_sched,
- "traversed spark queue, len=%d; (hd=%ld; tl=%ld)",
+ "traversed spark queue, len=%ld; (hd=%ld; tl=%ld)",
sparkPoolSize(pool), pool->bottom, pool->top);
}
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