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threadpool: further simplify and improve ggml_barrier

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Avoid using strict memory order while polling, yet make sure that all threads go through
full memory barrier (memory fence) on ggml_barrier entrace and exit.
This commit is contained in:
Max Krasnyansky 2024-09-13 11:21:59 -07:00
parent 2bd9f47800
commit ed094a5211
1 changed files with 18 additions and 24 deletions
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42
ggml/src/ggml.c
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@ -3178,42 +3178,36 @@ inline static void ggml_critical_section_start(void) {
}
}
static void ggml_barrier(struct ggml_threadpool * tp) {
int n_threads = atomic_load_explicit(&tp->n_threads_cur, memory_order_relaxed);
if (n_threads == 1) {
return;
}
#ifdef GGML_USE_OPENMP
static void ggml_barrier(struct ggml_threadpool * threadpool) {
int n_threads = atomic_load_explicit(&threadpool->n_threads_cur, memory_order_relaxed);
if (n_threads == 1) {
return;
}
#pragma omp barrier
}
#else
static void ggml_barrier(struct ggml_threadpool * threadpool) {
int n_threads = atomic_load_explicit(&threadpool->n_threads_cur, memory_order_relaxed);
if (n_threads == 1) {
return;
}
int n_passed = atomic_load_explicit(&tp->n_barrier_passed, memory_order_relaxed);
atomic_int * n_barrier = &threadpool->n_barrier;
atomic_int * n_barrier_passed = &threadpool->n_barrier_passed;
// enter barrier (full seq-cst fence)
int n_barrier = atomic_fetch_add_explicit(&tp->n_barrier, 1, memory_order_seq_cst);
int passed_old = atomic_load_explicit(n_barrier_passed, memory_order_relaxed);
if (atomic_fetch_add(n_barrier, 1) == n_threads - 1) {
int last = 0;
if (n_barrier == (n_threads - 1)) {
// last thread
atomic_store(n_barrier, 0);
atomic_fetch_add_explicit(n_barrier_passed, 1, memory_order_relaxed);
atomic_store_explicit(&tp->n_barrier, 0, memory_order_relaxed);
last = 1;
} else {
// wait for other threads
while (true) {
if (atomic_load_explicit(n_barrier_passed, memory_order_relaxed) != passed_old) {
return;
}
while (atomic_load_explicit(&tp->n_barrier_passed, memory_order_relaxed) == n_passed) {
ggml_thread_cpu_relax();
}
}
}
// exit barrier (full seq-cst fence)
atomic_fetch_add_explicit(&tp->n_barrier_passed, last, memory_order_seq_cst);
#endif
}
// TODO: make this somehow automatically executed
// some sort of "sentry" mechanism