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threads: improve ggml_barrier scaling with large number of threads (llama/9598)

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Make sure n_barrier and n_barrier_passed do not share the cache line to avoid cache line bouncing.
This optimization shows performance improvements even for n_threads <= 8 cases.

Resurect TSAN (Thread Sanitizer) check so that we can avoid doing expensive read-modify-write
in the normal case and just use thread-fence as originally intended.
This commit is contained in:
Max Krasnyansky 2024-09-23 11:42:43 -07:00 committed by Georgi Gerganov
parent 05c6139625
commit 08e8414f27
1 changed files with 45 additions and 14 deletions
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53
ggml/src/ggml.c
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@ -63,6 +63,25 @@ int ggml_sve_cnt_b = 0;
#pragma warning(disable: 4702)
#endif
// Note: once we move threading into a separate C++ file
// will use std::hardware_destructive_interference_size instead of hardcoding it here
// and we'll use C++ attribute syntax.
#define GGML_CACHE_LINE 64
#if defined(__clang__) || defined(__GNUC__)
#define GGML_CACHE_ALIGN __attribute__((aligned(GGML_CACHE_LINE)))
#endif
#if defined(__has_feature)
#if __has_feature(thread_sanitizer)
#define GGML_TSAN_ENABLED 1
#endif
#else // __has_feature
#if defined(__SANITIZE_THREAD__)
#define GGML_TSAN_ENABLED 1
#endif
#endif // __has_feature
#if defined(_WIN32)
#define WIN32_LEAN_AND_MEAN
@ -72,6 +91,8 @@ int ggml_sve_cnt_b = 0;
#include <windows.h>
#if !defined(__clang__)
#define GGML_CACHE_ALIGN __declspec(align(GGML_CACHE_LINE))
typedef volatile LONG atomic_int;
typedef atomic_int atomic_bool;
typedef atomic_int atomic_flag;
@ -2006,8 +2027,8 @@ struct ggml_threadpool {
// synchronization primitives
atomic_int n_graph; // incremented when there is work to be done (i.e each graph)
atomic_int n_barrier;
atomic_int n_barrier_passed;
atomic_int GGML_CACHE_ALIGN n_barrier;
atomic_int GGML_CACHE_ALIGN n_barrier_passed;
atomic_int current_chunk; // currently processing chunk during Mat_Mul, shared between all the threads.
// these are atomic as an annotation for thread-sanitizer
@ -3195,20 +3216,27 @@ static void ggml_barrier(struct ggml_threadpool * tp) {
// enter barrier (full seq-cst fence)
int n_barrier = atomic_fetch_add_explicit(&tp->n_barrier, 1, memory_order_seq_cst);
int last = 0;
if (n_barrier == (n_threads - 1)) {
// last thread
atomic_store_explicit(&tp->n_barrier, 0, memory_order_relaxed);
last = 1;
} else {
// exit barrier (fill seq-cst fence)
atomic_fetch_add_explicit(&tp->n_barrier_passed, 1, memory_order_seq_cst);
return;
}
// wait for other threads
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);
// TSAN doesn't support standalone fence yet, we use a dummy read-modify-write instead
#ifdef GGML_TSAN_ENABLED
atomic_fetch_add_explicit(&tp->n_barrier_passed, 0, memory_order_seq_cst);
#else
atomic_thread_fence(memory_order_seq_cst);
#endif
#endif
}
@ -20239,10 +20267,13 @@ static inline bool ggml_graph_compute_thread_ready(struct ggml_compute_state * s
// sync thread state after polling
static inline void ggml_graph_compute_thread_sync(struct ggml_compute_state * state) {
struct ggml_threadpool * threadpool = state->threadpool;
// this should just be atomic_thread_fence(seq_cst) but it confuses thread-sanitizer
// so instead we just use a dummy read-modify-write
atomic_fetch_add_explicit(&threadpool->n_graph, 0, memory_order_seq_cst);
// TSAN doesn't support standalone fence yet, we use a dummy read-modify-write instead
#ifdef GGML_TSAN_ENABLED
atomic_fetch_add_explicit(&state->threadpool->n_graph, 0, memory_order_seq_cst);
#else
atomic_thread_fence(memory_order_seq_cst);
#endif
UNUSED(state);
}
static inline bool ggml_graph_compute_poll_for_work(struct ggml_compute_state * state) {