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threaded pack-objects: Use condition variables for thread communication. 

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@telecom.at>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
maint
Johannes Sixt 2007-12-16 20:45:34 +01:00 committed by Junio C Hamano
parent 3eb2a15eb3
commit 50f22ada52
1 changed files with 80 additions and 51 deletions
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131
builtin-pack-objects.c
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@ -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;
pthread_cond_signal(&progress_cond);
progress_unlock();

/*
* 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. */
for (i = 0; i < 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. */
/* Partition the work amongst work threads. */
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].window = window;
p[i].depth = depth;
p[i].processed = processed;
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_cond_signal(&target->cond);
pthread_mutex_unlock(&target->mutex);

if (!sub_size) {
pthread_join(target->thread, NULL);
pthread_cond_destroy(&target->cond);
pthread_mutex_destroy(&target->mutex);
active_threads--;
}
} while (active_threads);
}
}

#else