From 0226613853133c081b55bb892a41bb5eacc0bc94 Mon Sep 17 00:00:00 2001 From: Max Krasnyansky Date: Tue, 17 Sep 2024 01:19:46 -0700 Subject: [PATCH] threadpool : skip polling for unused threads (#9461) * threadpool: skip polling for unused threads Currently all threads do N polling rounds even if only 1 thread is active (n_threads_cur == 1). This commit adds a check to skip the polling for unused threads (ith >= n_threads_cur). n_threads_cur is now an atomic_int to explicitly tell thread sanitizer that it is written from one thread and read from other threads (not a race conditions). * threadpool: further simplify and improve ggml_barrier 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. * threads: add simple barrier test This test does lots of small, parallel matmul ops where the barriers in between dominate the overhead. * threadpool: improve thread sync for new-graphs Using the same tricks as ggml_barrier. All the polling is done with relaxed memory order to keep it efficient, once the new graph is detected we do full fence using read-modify-write with strict memory order. * threadpool: improve abort handling Do not use threadpool->ec (exit code) to decide whether to exit the compute loop. threadpool->ec is not atomic which makes thread-sanitizer rightfully unhappy about it. Instead introduce atomic threadpool->abort flag used for this. This is consistent with how we handle threadpool->stop or pause. While at it add an explicit atomic_load for n_threads_cur for consistency. * test-barrier: release threadpool before releasing the context fixes use-after-free detected by gcc thread-sanitizer on x86-64 for some reason llvm sanitizer is not detecting this issue. --- ggml/src/ggml.c | 122 +++++++++++++++++++++++++---------------- tests/CMakeLists.txt | 1 + tests/test-barrier.cpp | 93 +++++++++++++++++++++++++++++++ 3 files changed, 168 insertions(+), 48 deletions(-) create mode 100644 tests/test-barrier.cpp diff --git a/ggml/src/ggml.c b/ggml/src/ggml.c index 3a8aadae85a78..bccb62377d739 100644 --- a/ggml/src/ggml.c +++ b/ggml/src/ggml.c @@ -2013,10 +2013,11 @@ struct ggml_threadpool { // these are atomic as an annotation for thread-sanitizer atomic_bool stop; // Used for stopping the threadpool altogether atomic_bool pause; // Used for pausing the threadpool or individual threads + atomic_bool abort; // Used for aborting processing of a graph struct ggml_compute_state * workers; // per thread state int n_threads_max; // number of threads in the pool - int n_threads_cur; // number of threads used in the current graph + atomic_int n_threads_cur; // number of threads used in the current graph int32_t prio; // Scheduling priority uint32_t poll; // Polling level (0 - no polling) @@ -3178,41 +3179,36 @@ inline static void ggml_critical_section_start(void) { } } -#ifdef GGML_USE_OPENMP -static void ggml_barrier(struct ggml_threadpool * threadpool) { - if (threadpool->n_threads_cur == 1) { +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 #pragma omp barrier -} #else -static void ggml_barrier(struct ggml_threadpool * threadpool) { - if (threadpool->n_threads_cur == 1) { - return; - } - - atomic_int * n_barrier = &threadpool->n_barrier; - atomic_int * n_barrier_passed = &threadpool->n_barrier_passed; + int n_passed = atomic_load_explicit(&tp->n_barrier_passed, memory_order_relaxed); - int n_threads = threadpool->n_threads_cur; - int passed_old = atomic_load_explicit(n_barrier_passed, memory_order_relaxed); + // enter barrier (full seq-cst fence) + int n_barrier = atomic_fetch_add_explicit(&tp->n_barrier, 1, memory_order_seq_cst); - 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 @@ -19933,34 +19929,33 @@ struct ggml_cplan ggml_graph_plan( static thread_ret_t ggml_graph_compute_thread(void * data) { struct ggml_compute_state * state = (struct ggml_compute_state *) data; + struct ggml_threadpool * tp = state->threadpool; - const struct ggml_cgraph * cgraph = state->threadpool->cgraph; - const struct ggml_cplan * cplan = state->threadpool->cplan; + const struct ggml_cgraph * cgraph = tp->cgraph; + const struct ggml_cplan * cplan = tp->cplan; set_numa_thread_affinity(state->ith); struct ggml_compute_params params = { /*.ith =*/ state->ith, - /*.nth =*/ state->threadpool->n_threads_cur, + /*.nth =*/ atomic_load_explicit(&tp->n_threads_cur, memory_order_relaxed), /*.wsize =*/ cplan->work_size, /*.wdata =*/ cplan->work_data, - /*.threadpool=*/ state->threadpool, + /*.threadpool=*/ tp, }; - for (int node_n = 0; node_n < cgraph->n_nodes; node_n++) { + for (int node_n = 0; node_n < cgraph->n_nodes && !tp->abort; node_n++) { struct ggml_tensor * node = cgraph->nodes[node_n]; ggml_compute_forward(¶ms, node); - if (state->ith == 0 && cplan->abort_callback && cplan->abort_callback(cplan->abort_callback_data)) { - state->threadpool->ec = GGML_STATUS_ABORTED; + if (state->ith == 0 && cplan->abort_callback && + cplan->abort_callback(cplan->abort_callback_data)) { + tp->abort = true; + tp->ec = GGML_STATUS_ABORTED; } ggml_barrier(state->threadpool); - - if (state->threadpool->ec != GGML_STATUS_SUCCESS) { - break; - } } return 0; @@ -19968,7 +19963,15 @@ static thread_ret_t ggml_graph_compute_thread(void * data) { #ifndef GGML_USE_OPENMP -static inline bool ggml_graph_compute_ready(struct ggml_compute_state * state) { +// check if thread is active +static inline bool ggml_graph_compute_thread_active(struct ggml_compute_state * state) { + struct ggml_threadpool * threadpool = state->threadpool; + int n_threads = atomic_load_explicit(&threadpool->n_threads_cur, memory_order_relaxed); + return (state->ith < n_threads); +} + +// check if thread is ready to proceed (exit from polling or sleeping) +static inline bool ggml_graph_compute_thread_ready(struct ggml_compute_state * state) { struct ggml_threadpool * threadpool = state->threadpool; if (state->pending || threadpool->stop || threadpool->pause) { return true; } @@ -19976,21 +19979,34 @@ static inline bool ggml_graph_compute_ready(struct ggml_compute_state * state) { // check for new graph/work int new_graph = atomic_load_explicit(&threadpool->n_graph, memory_order_relaxed); if (new_graph != state->last_graph) { - state->pending = (state->ith < threadpool->n_threads_cur); + state->pending = ggml_graph_compute_thread_active(state); state->last_graph = new_graph; } return state->pending; } +// 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); +} + static inline bool ggml_graph_compute_poll_for_work(struct ggml_compute_state * state) { struct ggml_threadpool * threadpool = state->threadpool; + // Skip polling for unused threads + if (!ggml_graph_compute_thread_active(state)) { + return state->pending; + } + // This seems to make 0 ... 100 a decent range for polling level across modern processors. // Perhaps, we can adjust it dynamically based on load and things. const uint64_t n_rounds = 1024UL * 128 * threadpool->poll; - for (uint64_t i=0; !ggml_graph_compute_ready(state) && ithreadpool; if (ggml_graph_compute_poll_for_work(state)) { + ggml_graph_compute_thread_sync(state); return state->pending; } ggml_mutex_lock_shared(&threadpool->mutex); - while (!ggml_graph_compute_ready(state)) { + while (!ggml_graph_compute_thread_ready(state)) { // No new work. Wait for the signal. - GGML_PRINT_DEBUG("thread #%d waiting for work\n", state->ith); + GGML_PRINT_DEBUG("thread #%d waiting for work (sleeping)\n", state->ith); ggml_cond_wait(&threadpool->cond, &threadpool->mutex); } ggml_mutex_unlock_shared(&threadpool->mutex); @@ -20055,13 +20072,20 @@ static thread_ret_t ggml_graph_compute_secondary_thread(void* data) { } // Start processing new graph -static void ggml_graph_compute_kickoff(struct ggml_threadpool * threadpool) +static void ggml_graph_compute_kickoff(struct ggml_threadpool * threadpool, int n_threads) { - // always take the mutex here because the worker threads are doing hybrid poll/wait + // Always take the mutex here because the worker threads are doing hybrid poll/wait ggml_mutex_lock(&threadpool->mutex); - atomic_fetch_add_explicit(&threadpool->n_graph, 1, memory_order_relaxed); + GGML_PRINT_DEBUG("threadpool: n_threads_cur %d n_threads %d\n", threadpool->n_threads_cur, n_threads); + + // Update the number of active threads + atomic_store_explicit(&threadpool->n_threads_cur, n_threads, memory_order_relaxed); + + // Indicate the graph is ready to be processed + // We need the full seq-cst fence here because of the polling threads (used in thread_sync) + atomic_fetch_add_explicit(&threadpool->n_graph, 1, memory_order_seq_cst); if (threadpool->pause) { // Update main thread prio and affinity to match the threadpool settings @@ -20120,6 +20144,7 @@ static struct ggml_threadpool * ggml_threadpool_new_impl( threadpool->current_chunk = 0; threadpool->stop = false; threadpool->pause = tpp->paused; + threadpool->abort = false; threadpool->workers = NULL; threadpool->n_threads_max = tpp->n_threads; threadpool->n_threads_cur = tpp->n_threads; @@ -20195,15 +20220,11 @@ enum ggml_status ggml_graph_compute(struct ggml_cgraph * cgraph, struct ggml_cpl // No worker threads should be accessing the parameters below at this stage threadpool->cgraph = cgraph; threadpool->cplan = cplan; - threadpool->n_threads_cur = n_threads; threadpool->current_chunk = 0; + threadpool->abort = false; threadpool->ec = GGML_STATUS_SUCCESS; } - if (n_threads > threadpool->n_threads_max) { - GGML_PRINT("WARNING: cplan is requesting more threads than the threadpool contains. Expect a bad time!\n"); - } - #ifdef GGML_USE_OPENMP if (n_threads > 1) { #pragma omp parallel num_threads(n_threads) @@ -20212,7 +20233,7 @@ enum ggml_status ggml_graph_compute(struct ggml_cgraph * cgraph, struct ggml_cpl { // update the number of threads from the actual number of threads that we got from OpenMP n_threads = omp_get_num_threads(); - threadpool->n_threads_cur = n_threads; + atomic_store_explicit(&threadpool->n_threads_cur, n_threads, memory_order_relaxed); } ggml_graph_compute_thread(&threadpool->workers[omp_get_thread_num()]); @@ -20221,8 +20242,13 @@ enum ggml_status ggml_graph_compute(struct ggml_cgraph * cgraph, struct ggml_cpl ggml_graph_compute_thread(&threadpool->workers[0]); } #else + if (n_threads > threadpool->n_threads_max) { + GGML_PRINT("WARNING: cplan requested more threads (%d) than available (%d)\n", n_threads, threadpool->n_threads_max); + n_threads = threadpool->n_threads_max; + } + // Kick all threads to start the new graph - ggml_graph_compute_kickoff(threadpool); + ggml_graph_compute_kickoff(threadpool, n_threads); // This is a work thread too ggml_graph_compute_thread(&threadpool->workers[0]); diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt index 7dcd3fce827bd..08ad66b49fdd4 100644 --- a/tests/CMakeLists.txt +++ b/tests/CMakeLists.txt @@ -119,6 +119,7 @@ llama_target_and_test(test-grammar-parser.cpp) llama_target_and_test(test-llama-grammar.cpp) llama_target_and_test(test-grammar-integration.cpp) llama_target_and_test(test-grad0.cpp) +llama_target_and_test(test-barrier.cpp) # llama_target_and_test(test-opt.cpp) # SLOW llama_target_and_test(test-backend-ops.cpp) diff --git a/tests/test-barrier.cpp b/tests/test-barrier.cpp new file mode 100644 index 0000000000000..cf54237db87b2 --- /dev/null +++ b/tests/test-barrier.cpp @@ -0,0 +1,93 @@ +#include "ggml.h" +#include "ggml-backend.h" + +#include +#include +#include +#include +#include +#include + +#define MAX_NARGS 2 + +int main(int argc, char *argv[]) { + + int n_threads = 4; + int n_rounds = 100; + + if (argc > 1) { + n_threads = std::atoi(argv[1]); + } + + if (argc > 2) { + n_rounds = std::atoi(argv[2]); + } + + struct ggml_init_params params = { + /* .mem_size = */ 1024*1024*1024, + /* .mem_buffer = */ NULL, + /* .no_alloc = */ false, + }; + + struct ggml_context * ctx = ggml_init(params); + + // Create graph + struct ggml_cgraph * gf = ggml_new_graph(ctx); + + // Lots of small, parallel ops where barriers in between will dominate + struct ggml_tensor * out = ggml_new_tensor_1d(ctx, GGML_TYPE_F32, 64); + for (int i = 0; i < 1000; i++) { + struct ggml_tensor * a = ggml_new_tensor_2d(ctx, GGML_TYPE_Q4_0, 64, 128); + out = ggml_mul_mat(ctx, a, out); + + struct ggml_tensor * d = ggml_new_tensor_2d(ctx, GGML_TYPE_Q4_0, 128, 64); + out = ggml_mul_mat(ctx, d, out); + } + + ggml_build_forward_expand(gf, out); + int n_nodes = ggml_graph_n_nodes(gf); + + // Create threadpool + struct ggml_threadpool_params tpp = ggml_threadpool_params_default(n_threads); + struct ggml_threadpool* threadpool = ggml_threadpool_new(&tpp); + if (!threadpool) { + fprintf(stderr, "threadpool create failed : n_threads %d\n", n_threads); + exit(1); + } + + // Create compute plan + struct ggml_cplan cplan = ggml_graph_plan(gf, n_threads, threadpool); + + std::vector work_data(cplan.work_size); + cplan.work_data = work_data.data(); + + std::cerr << "graph-compute with" + << "\n n_threads: " << n_threads + << "\n n_nodes: " << n_nodes + << "\n n_rounds: " << n_rounds + << "\n"; + // ggml_graph_print(gf); + + // Warmup + ggml_graph_compute(gf, &cplan); + + auto t0 = std::chrono::high_resolution_clock::now(); + + for (int i=0; i < n_rounds; i++) { + ggml_graph_compute(gf, &cplan); + } + + auto t1 = std::chrono::high_resolution_clock::now(); + + auto usec = std::chrono::duration_cast(t1-t0).count(); + auto nsec = std::chrono::duration_cast(t1-t0).count(); + std::cerr << "graph-compute took " << usec << " usec " + << "\n " << (float) usec / n_rounds << " usec per-iter" + << "\n " << (float) nsec / (n_rounds * n_nodes) << " nsec per-node" + << "\n"; + + ggml_threadpool_free(threadpool); + ggml_free(ctx); + + return 0; +}