In the threaded pack-objects code the main thread and the worker threads
must mutually signal that they have assigned a new pack of work or have
completed their work, respectively. Previously, the code used mutexes that
were locked in one thread and unlocked from a different thread, which is
bogus (and happens to work on Linux).
Here we rectify the implementation by using condition variables: There is
one condition variable on which the main thread waits until a thread
requests new work; and each worker thread has its own condition variable
on which it waits until it is assigned new work or signaled to terminate.
As a cleanup, the worker threads are spawned only after the initial work
packages have been assigned.
Signed-off-by: Johannes Sixt <johannes.sixt@xxxxxxxxxx>
---
builtin-pack-objects.c | 129 +++++++++++++++++++++++++++++------------------
1 files changed, 79 insertions(+), 50 deletions(-)
diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
index 7dd0d7f..451e48e 100644
--- a/builtin-pack-objects.c
+++ b/builtin-pack-objects.c
@@ -1594,6 +1594,15 @@ static void find_deltas(struct object_entry **list, unsigned *list_size,
#ifdef THREADED_DELTA_SEARCH
+/*
+ * The main thread waits on the condition that (at least) one of the workers
+ * has stopped working (which is indicated in the .working member of
+ * struct thread_params).
+ * When a work thread has completed its work, it sets .working to 0 and
+ * signals the main thread and waits on the condition that .data_ready
+ * becomes 1.
+ */
+
struct thread_params {
pthread_t thread;
struct object_entry **list;
@@ -1601,37 +1610,50 @@ struct thread_params {
unsigned remaining;
int window;
int depth;
+ int working;
+ int data_ready;
+ pthread_mutex_t mutex;
+ pthread_cond_t cond;
unsigned *processed;
};
-static pthread_mutex_t data_request = PTHREAD_MUTEX_INITIALIZER;
-static pthread_mutex_t data_ready = PTHREAD_MUTEX_INITIALIZER;
-static pthread_mutex_t data_provider = PTHREAD_MUTEX_INITIALIZER;
-static struct thread_params *data_requester;
+static pthread_cond_t progress_cond = PTHREAD_COND_INITIALIZER;
static void *threaded_find_deltas(void *arg)
{
struct thread_params *me = arg;
- for (;;) {
- pthread_mutex_lock(&data_request);
- data_requester = me;
- pthread_mutex_unlock(&data_provider);
- pthread_mutex_lock(&data_ready);
- pthread_mutex_unlock(&data_request);
-
- if (!me->remaining)
- return NULL;
-
+ while (me->remaining) {
find_deltas(me->list, &me->remaining,
me->window, me->depth, me->processed);
+
+ progress_lock();
+ me->working = 0;
+ progress_unlock();
+ pthread_cond_signal(&progress_cond);
+
+ /*
+ * We must not set ->data_ready before we wait on the
+ * condition because the main thread may have set it to 1
+ * before we get here. In order to be sure that new
+ * work is available if we see 1 in ->data_ready, it
+ * was initialized to 0 before this thread was spawned
+ * and we reset it to 0 right away.
+ */
+ pthread_mutex_lock(&me->mutex);
+ while (!me->data_ready)
+ pthread_cond_wait(&me->cond, &me->mutex);
+ me->data_ready = 0;
+ pthread_mutex_unlock(&me->mutex);
}
+ /* leave ->working 1 so that this doesn't get more work assigned */
+ return NULL;
}
static void ll_find_deltas(struct object_entry **list, unsigned list_size,
int window, int depth, unsigned *processed)
{
- struct thread_params *target, p[delta_search_threads];
+ struct thread_params p[delta_search_threads];
int i, ret, active_threads = 0;
if (delta_search_threads <= 1) {
@@ -1639,49 +1661,42 @@ static void ll_find_deltas(struct object_entry **list, unsigned list_size,
return;
}
- pthread_mutex_lock(&data_provider);
- pthread_mutex_lock(&data_ready);
-
- /* Start work threads. */
+ /* Partition the work amongst work threads. */
for (i = 0; i < delta_search_threads; i++) {
+ unsigned sub_size = list_size / (delta_search_threads - i);
+
p[i].window = window;
p[i].depth = depth;
p[i].processed = processed;
- p[i].remaining = 0;
- ret = pthread_create(&p[i].thread, NULL,
- threaded_find_deltas, &p[i]);
- if (ret)
- die("unable to create thread: %s", strerror(ret));
- active_threads++;
- }
-
- /* Then partition the work amongst them. */
- for (i = 0; i < delta_search_threads; i++) {
- unsigned sub_size = list_size / (delta_search_threads - i);
-
- pthread_mutex_lock(&data_provider);
- target = data_requester;
- if (!sub_size) {
- pthread_mutex_unlock(&data_ready);
- pthread_join(target->thread, NULL);
- active_threads--;
- continue;
- }
+ p[i].working = 1;
+ p[i].data_ready = 0;
+ pthread_mutex_init(&p[i].mutex, NULL);
+ pthread_cond_init(&p[i].cond, NULL);
/* try to split chunks on "path" boundaries */
while (sub_size < list_size && list[sub_size]->hash &&
list[sub_size]->hash == list[sub_size-1]->hash)
sub_size++;
- target->list = list;
- target->list_size = sub_size;
- target->remaining = sub_size;
- pthread_mutex_unlock(&data_ready);
+ p[i].list = list;
+ p[i].list_size = sub_size;
+ p[i].remaining = sub_size;
list += sub_size;
list_size -= sub_size;
}
+ /* Start work threads. */
+ for (i = 0; i < delta_search_threads; i++) {
+ if (!p[i].list_size)
+ continue;
+ ret = pthread_create(&p[i].thread, NULL,
+ threaded_find_deltas, &p[i]);
+ if (ret)
+ die("unable to create thread: %s", strerror(ret));
+ active_threads++;
+ }
+
/*
* Now let's wait for work completion. Each time a thread is done
* with its work, we steal half of the remaining work from the
@@ -1690,13 +1705,21 @@ static void ll_find_deltas(struct object_entry **list, unsigned list_size,
* until the remaining object list segments are simply too short
* to be worth splitting anymore.
*/
- do {
+ while (active_threads) {
+ struct thread_params *target = NULL;
struct thread_params *victim = NULL;
unsigned sub_size = 0;
- pthread_mutex_lock(&data_provider);
- target = data_requester;
progress_lock();
+ for (;;) {
+ for (i = 0; !target && i < delta_search_threads; i++)
+ if (!p[i].working)
+ target = &p[i];
+ if (target)
+ break;
+ pthread_cond_wait(&progress_cond, &progress_mutex);
+ };
+
for (i = 0; i < delta_search_threads; i++)
if (p[i].remaining > 2*window &&
(!victim || victim->remaining < p[i].remaining))
@@ -1723,17 +1746,23 @@ static void ll_find_deltas(struct object_entry **list, unsigned list_size,
victim->list_size -= sub_size;
victim->remaining -= sub_size;
}
- progress_unlock();
-
target->list_size = sub_size;
target->remaining = sub_size;
- pthread_mutex_unlock(&data_ready);
+ target->working = 1;
+ progress_unlock();
+
+ pthread_mutex_lock(&target->mutex);
+ target->data_ready = 1;
+ pthread_mutex_unlock(&target->mutex);
+ pthread_cond_signal(&target->cond);
if (!sub_size) {
pthread_join(target->thread, NULL);
+ pthread_cond_destroy(&target->cond);
+ pthread_mutex_destroy(&target->mutex);
active_threads--;
}
- } while (active_threads);
+ }
}
#else
--
1.5.3.6.954.g0c63
-
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