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mark-sweep collector allows parallel mutators
This commit is contained in:
Andy Wingo
parent
ded3b3c7a3
commit
e837d51f53
1 changed files with 171 additions and 17 deletions
188
mark-sweep.h
188
mark-sweep.h
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@ -1,3 +1,4 @@
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#include <stdatomic.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <string.h>
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@ -102,6 +103,13 @@ struct gcobj {
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};
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struct mark_space {
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pthread_mutex_t lock;
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pthread_cond_t collector_cond;
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pthread_cond_t mutator_cond;
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int collecting;
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int multithreaded;
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size_t active_mutator_count;
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size_t mutator_count;
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struct gcobj_freelists small_objects;
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// Unordered list of large objects.
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struct gcobj_free_large *large_objects;
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@ -110,6 +118,7 @@ struct mark_space {
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uintptr_t heap_base;
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size_t heap_size;
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uintptr_t sweep;
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struct handle *global_roots;
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struct mutator_mark_buf *mutator_roots;
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void *mem;
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size_t mem_size;
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@ -204,12 +213,61 @@ static void clear_global_freelists(struct mark_space *space) {
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space->large_objects = NULL;
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}
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static int space_has_multiple_mutators(struct mark_space *space) {
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return atomic_load_explicit(&space->multithreaded, memory_order_relaxed);
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}
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static int mutators_are_stopping(struct mark_space *space) {
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return atomic_load_explicit(&space->collecting, memory_order_relaxed);
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}
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static inline void mark_space_lock(struct mark_space *space) {
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pthread_mutex_lock(&space->lock);
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}
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static inline void mark_space_unlock(struct mark_space *space) {
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pthread_mutex_unlock(&space->lock);
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}
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static void add_mutator(struct heap *heap, struct mutator *mut) {
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mut->heap = heap;
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struct mark_space *space = heap_mark_space(heap);
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mark_space_lock(space);
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// We have no roots. If there is a GC currently in progress, we have
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// nothing to add. Just wait until it's done.
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while (mutators_are_stopping(space))
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pthread_cond_wait(&space->mutator_cond, &space->lock);
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if (space->mutator_count == 1)
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space->multithreaded = 1;
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space->active_mutator_count++;
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space->mutator_count++;
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mark_space_unlock(space);
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}
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static void remove_mutator(struct heap *heap, struct mutator *mut) {
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mut->heap = NULL;
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struct mark_space *space = heap_mark_space(heap);
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mark_space_lock(space);
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space->active_mutator_count--;
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space->mutator_count--;
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// We have no roots. If there is a GC stop currently in progress,
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// maybe tell the controller it can continue.
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if (mutators_are_stopping(space) && space->active_mutator_count == 0)
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pthread_cond_signal(&space->collector_cond);
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mark_space_unlock(space);
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}
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static void request_mutators_to_stop(struct mark_space *space) {
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ASSERT(!mutators_are_stopping(space));
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atomic_store_explicit(&space->collecting, 1, memory_order_relaxed);
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}
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static void allow_mutators_to_continue(struct mark_space *space) {
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ASSERT(mutators_are_stopping(space));
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ASSERT(space->active_mutator_count == 0);
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space->active_mutator_count++;
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atomic_store_explicit(&space->collecting, 0, memory_order_relaxed);
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ASSERT(!mutators_are_stopping(space));
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pthread_cond_broadcast(&space->mutator_cond);
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}
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static void mutator_mark_buf_grow(struct mutator_mark_buf *buf) {
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@ -253,7 +311,9 @@ static void mutator_mark_buf_destroy(struct mutator_mark_buf *buf) {
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munmap(buf->objects, bytes);
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}
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static void mark_mutator_roots(struct mutator *mut) {
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// Mark the roots of a mutator that is stopping for GC. We can't
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// enqueue them directly, so we send them to the controller in a buffer.
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static void mark_stopping_mutator_roots(struct mutator *mut) {
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struct mark_space *space = mutator_mark_space(mut);
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struct mutator_mark_buf *local_roots = &mut->mark_buf;
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for (struct handle *h = mut->roots; h; h = h->next) {
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@ -263,20 +323,78 @@ static void mark_mutator_roots(struct mutator *mut) {
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}
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// Post to global linked-list of thread roots.
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struct mutator_mark_buf *next = space->mutator_roots;
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local_roots->next = next;
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space->mutator_roots = local_roots;
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struct mutator_mark_buf *next =
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atomic_load_explicit(&space->mutator_roots, memory_order_acquire);
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do {
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local_roots->next = next;
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} while (!atomic_compare_exchange_weak(&space->mutator_roots,
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&next, local_roots));
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}
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static void release_mutator_roots(struct mutator *mut) {
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// Mark the roots of the mutator that causes GC.
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static void mark_controlling_mutator_roots(struct mutator *mut) {
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struct mark_space *space = mutator_mark_space(mut);
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for (struct handle *h = mut->roots; h; h = h->next) {
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struct gcobj *root = h->v;
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if (root && mark_object(space, root))
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marker_enqueue_root(&space->marker, root);
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}
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}
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static void release_stopping_mutator_roots(struct mutator *mut) {
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mutator_mark_buf_release(&mut->mark_buf);
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}
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static void wait_for_mutators_to_stop(struct mark_space *space) {
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space->active_mutator_count--;
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while (space->active_mutator_count)
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pthread_cond_wait(&space->collector_cond, &space->lock);
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}
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static void mark_global_roots(struct mark_space *space) {
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struct mutator_mark_buf *roots = space->mutator_roots;
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for (struct handle *h = space->global_roots; h; h = h->next) {
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struct gcobj *obj = h->v;
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if (obj && mark_object(space, obj))
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marker_enqueue_root(&space->marker, obj);
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}
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struct mutator_mark_buf *roots = atomic_load(&space->mutator_roots);
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for (; roots; roots = roots->next)
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marker_enqueue_roots(&space->marker, roots->objects, roots->size);
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space->mutator_roots = NULL;
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atomic_store(&space->mutator_roots, NULL);
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}
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static void pause_mutator_for_collection(struct mutator *mut) NEVER_INLINE;
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static void pause_mutator_for_collection(struct mutator *mut) {
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struct mark_space *space = mutator_mark_space(mut);
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ASSERT(mutators_are_stopping(space));
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mark_stopping_mutator_roots(mut);
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mark_space_lock(space);
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ASSERT(space->active_mutator_count);
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space->active_mutator_count--;
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if (space->active_mutator_count == 0)
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pthread_cond_signal(&space->collector_cond);
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// Go to sleep and wake up when the collector is done. Note,
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// however, that it may be that some other mutator manages to
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// trigger collection before we wake up. In that case we need to
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// mark roots, not just sleep again. To detect a wakeup on this
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// collection vs a future collection, we use the global GC count.
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// This is safe because the count is protected by the space lock,
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// which we hold.
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long epoch = space->count;
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do
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pthread_cond_wait(&space->mutator_cond, &space->lock);
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while (mutators_are_stopping(space) && space->count == epoch);
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space->active_mutator_count++;
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mark_space_unlock(space);
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release_stopping_mutator_roots(mut);
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}
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static inline void maybe_pause_mutator_for_collection(struct mutator *mut) {
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while (mutators_are_stopping(mutator_mark_space(mut)))
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pause_mutator_for_collection(mut);
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}
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static void reset_sweeper(struct mark_space *space) {
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@ -286,14 +404,16 @@ static void reset_sweeper(struct mark_space *space) {
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static void collect(struct mark_space *space, struct mutator *mut) {
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DEBUG("start collect #%ld:\n", space->count);
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marker_prepare(space);
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mark_mutator_roots(mut);
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request_mutators_to_stop(space);
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mark_controlling_mutator_roots(mut);
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wait_for_mutators_to_stop(space);
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mark_global_roots(space);
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marker_trace(space);
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marker_release(space);
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clear_global_freelists(space);
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reset_sweeper(space);
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space->count++;
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release_mutator_roots(mut);
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allow_mutators_to_continue(space);
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clear_mutator_freelists(mut);
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DEBUG("collect done\n");
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}
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@ -443,8 +563,12 @@ static int sweep(struct mark_space *space,
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static void* allocate_large(struct mutator *mut, enum alloc_kind kind,
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size_t granules) {
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struct mark_space *space = mutator_mark_space(mut);
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struct gcobj_freelists *small_objects = &mut->small_objects;
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struct gcobj_freelists *small_objects = space_has_multiple_mutators(space) ?
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&space->small_objects : &mut->small_objects;
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maybe_pause_mutator_for_collection(mut);
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mark_space_lock(space);
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int swept_from_beginning = 0;
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while (1) {
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struct gcobj_free_large *already_scanned = NULL;
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@ -456,6 +580,7 @@ static void* allocate_large(struct mutator *mut, enum alloc_kind kind,
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if (large->granules >= granules) {
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unlink_large_object(prev, large);
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split_large_object(space, small_objects, large, granules);
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mark_space_unlock(space);
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struct gcobj *obj = (struct gcobj *)large;
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obj->tag = tag_live(kind);
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return large;
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@ -469,7 +594,13 @@ static void* allocate_large(struct mutator *mut, enum alloc_kind kind,
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fprintf(stderr, "ran out of space, heap size %zu\n", space->heap_size);
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abort();
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} else {
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collect(space, mut);
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if (mutators_are_stopping(space)) {
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mark_space_unlock(space);
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pause_mutator_for_collection(mut);
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mark_space_lock(space);
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} else {
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collect(space, mut);
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}
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swept_from_beginning = 1;
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}
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}
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@ -497,6 +628,7 @@ static int fill_small_from_local(struct gcobj_freelists *small_objects,
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return 0;
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}
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// with space lock
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static int fill_small_from_large(struct mark_space *space,
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struct gcobj_freelists *small_objects,
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enum small_object_size kind) {
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@ -516,6 +648,8 @@ static int fill_small_from_large(struct mark_space *space,
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static int fill_small_from_global_small(struct mark_space *space,
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struct gcobj_freelists *small_objects,
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enum small_object_size kind) {
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if (!space_has_multiple_mutators(space))
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return 0;
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struct gcobj_freelists *global_small = &space->small_objects;
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if (*get_small_object_freelist(global_small, kind)
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|| fill_small_from_local(global_small, kind)) {
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@ -538,6 +672,9 @@ static void fill_small_from_global(struct mutator *mut,
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struct gcobj_freelists *small_objects = &mut->small_objects;
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struct mark_space *space = mutator_mark_space(mut);
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maybe_pause_mutator_for_collection(mut);
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mark_space_lock(space);
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int swept_from_beginning = 0;
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while (1) {
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if (fill_small_from_global_small(space, small_objects, kind))
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@ -551,7 +688,13 @@ static void fill_small_from_global(struct mutator *mut,
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fprintf(stderr, "ran out of space, heap size %zu\n", space->heap_size);
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abort();
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} else {
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collect(space, mut);
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if (mutators_are_stopping(space)) {
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mark_space_unlock(space);
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pause_mutator_for_collection(mut);
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mark_space_lock(space);
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} else {
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collect(space, mut);
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}
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swept_from_beginning = 1;
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}
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}
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@ -561,6 +704,7 @@ static void fill_small_from_global(struct mutator *mut,
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if (fill_small_from_local(small_objects, kind))
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break;
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}
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mark_space_unlock(space);
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}
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static void fill_small(struct mutator *mut, enum small_object_size kind) {
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@ -644,6 +788,10 @@ static int initialize_gc(size_t size, struct heap **heap,
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size_t mark_bytes_size = (size + GRANULE_SIZE) / (GRANULE_SIZE + 1);
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size_t overhead = align_up(mark_bytes_size, GRANULE_SIZE);
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pthread_mutex_init(&space->lock, NULL);
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pthread_cond_init(&space->mutator_cond, NULL);
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pthread_cond_init(&space->collector_cond, NULL);
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space->heap_base = ((uintptr_t) mem) + overhead;
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space->heap_size = size - overhead;
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space->sweep = space->heap_base + space->heap_size;
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@ -659,12 +807,18 @@ static int initialize_gc(size_t size, struct heap **heap,
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}
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static struct mutator* initialize_gc_for_thread(uintptr_t *stack_base,
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struct heap *parent) {
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fprintf(stderr,
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"Multiple mutator threads not yet implemented.\n");
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exit(1);
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struct heap *heap) {
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struct mutator *ret = calloc(1, sizeof(struct mutator));
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if (!ret)
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abort();
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add_mutator(heap, ret);
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return ret;
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}
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static void finish_gc_for_thread(struct mutator *heap) {
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static void finish_gc_for_thread(struct mutator *mut) {
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remove_mutator(mutator_heap(mut), mut);
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mutator_mark_buf_destroy(&mut->mark_buf);
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free(mut);
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}
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static inline void print_start_gc_stats(struct heap *heap) {
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