vulkan: Hybrid waitForFences/getFenceStatus to reduce fence latency

There seems to be a bubble waking up from waitForFences, which costs a few
percent performance and also increased variance in performance. This change
inserts an "almost_ready" fence when the graph is about 80% complete and we
waitForFences for the almost_ready fence and then spin (with _mm_pauses) waiting
for the final fence to be signaled.

Author: jeffbolznv

ggml/src/ggml-vulkan/ggml-vulkan.cpp

@@ -23,6 +23,7 @@
 #include <mutex>
 #include <future>
 #include <thread>
+#include <immintrin.h>
 
 #include "ggml-impl.h"
 #include "ggml-backend-impl.h"
@@ -770,7 +771,8 @@ struct ggml_backend_vk_context {
     ggml_vk_garbage_collector gc;
     size_t prealloc_size_x, prealloc_size_y, prealloc_size_split_k;
     vk_buffer prealloc_x, prealloc_y, prealloc_split_k;
-    vk::Fence fence;
+    vk::Fence fence, almost_ready_fence;
+    bool almost_ready_fence_pending {};
 
     vk_buffer buffer_pool[MAX_VK_BUFFERS];
 
@@ -861,6 +863,39 @@ typedef void (*ggml_vk_func_t)(ggml_backend_vk_context * ctx, vk_context& subctx
 
 static void ggml_backend_vk_free(ggml_backend_t backend);
 
+// Wait for ctx->fence to be signaled.
+static void ggml_vk_wait_for_fence(ggml_backend_vk_context * ctx) {
+    // Use waitForFences while most of the graph executes. Hopefully the CPU can sleep
+    // during this wait.
+    if (ctx->almost_ready_fence_pending) {
+        VK_CHECK(ctx->device->device.waitForFences({ ctx->almost_ready_fence }, true, UINT64_MAX), "almost_ready_fence");
+        ctx->device->device.resetFences({ ctx->almost_ready_fence });
+        ctx->almost_ready_fence_pending = false;
+    }
+
+    // Spin (w/pause) waiting for the graph to finish executing.
+    vk::Result result;
+    while ((result = ctx->device->device.getFenceStatus(ctx->fence)) != vk::Result::eSuccess) {
+        if (result != vk::Result::eNotReady) {
+            fprintf(stderr, "ggml_vulkan: error %s at %s:%d\n", to_string(result).c_str(), __FILE__, __LINE__);
+            exit(1);
+        }
+        for (uint32_t i = 0; i < 100; ++i) {
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+            _mm_pause();
+        }
+    }
+    ctx->device->device.resetFences({ ctx->fence });
+}
+
 // variables to track number of compiles in progress
 static uint32_t compile_count = 0;
 static std::mutex compile_count_mutex;
@@ -3229,6 +3264,7 @@ static void ggml_vk_init(ggml_backend_vk_context * ctx, size_t idx) {
     ctx->prealloc_size_split_k = 0;
 
     ctx->fence = ctx->device->device.createFence({});
+    ctx->almost_ready_fence = ctx->device->device.createFence({});
 
 #ifdef GGML_VULKAN_CHECK_RESULTS
     const char* skip_checks = getenv("GGML_VULKAN_SKIP_CHECKS");
@@ -7522,11 +7558,11 @@ static void ggml_vk_preallocate_buffers(ggml_backend_vk_context * ctx) {
     }
 }
 
-static bool ggml_vk_compute_forward(ggml_backend_vk_context* ctx, ggml_tensor* tensor, int tensor_idx, bool use_fence);
+static bool ggml_vk_compute_forward(ggml_backend_vk_context* ctx, ggml_tensor* tensor, int tensor_idx, bool use_fence, bool almost_ready);
 
 // Returns true if node has enqueued work into the queue, false otherwise
 // If submit is true the current all operations queued so far are being submitted to Vulkan to overlap cmdlist creation and GPU execution.
-static bool ggml_vk_build_graph(ggml_backend_vk_context * ctx, ggml_tensor * node, int node_idx, ggml_tensor *node_begin, int node_idx_begin, bool dryrun, bool last_node, bool submit){
+static bool ggml_vk_build_graph(ggml_backend_vk_context * ctx, ggml_tensor * node, int node_idx, ggml_tensor *node_begin, int node_idx_begin, bool dryrun, bool last_node, bool almost_ready, bool submit){
     if (ggml_is_empty(node) || !node->buffer) {
         return false;
     }
@@ -7898,7 +7934,7 @@ static bool ggml_vk_build_graph(ggml_backend_vk_context * ctx, ggml_tensor * nod
 
         ctx->compute_ctx.reset();
 
-        bool ok = ggml_vk_compute_forward(ctx, node_begin, node_idx_begin, false);
+        bool ok = ggml_vk_compute_forward(ctx, node_begin, node_idx_begin, false, almost_ready);
         if (!ok) {
             if (node->op == GGML_OP_UNARY) {
                 std::cerr << __func__ << ": error: op not supported UNARY " << node->name << " (" << ggml_unary_op_name(static_cast<ggml_unary_op>(node->op_params[0])) << ")" << std::endl;
@@ -7912,7 +7948,7 @@ static bool ggml_vk_build_graph(ggml_backend_vk_context * ctx, ggml_tensor * nod
     return true;
 }
 
-static bool ggml_vk_compute_forward(ggml_backend_vk_context * ctx, ggml_tensor * tensor, int tensor_idx, bool use_fence = true){
+static bool ggml_vk_compute_forward(ggml_backend_vk_context * ctx, ggml_tensor * tensor, int tensor_idx, bool use_fence = true, bool almost_ready = false) {
     ggml_backend_buffer * buf = nullptr;
 
     switch (tensor->op) {
@@ -8015,12 +8051,15 @@ static bool ggml_vk_compute_forward(ggml_backend_vk_context * ctx, ggml_tensor *
             memcpy(cpy.dst, cpy.src, cpy.n);
         }
 
-        ggml_vk_submit(subctx, use_fence ? ctx->fence : vk::Fence{});
+        if (almost_ready && !ctx->almost_ready_fence_pending && !use_fence) {
+            ggml_vk_submit(subctx, ctx->almost_ready_fence);
+            ctx->almost_ready_fence_pending = true;
+        } else {
+            ggml_vk_submit(subctx, use_fence ? ctx->fence : vk::Fence{});
+        }
 
         if (use_fence) {
-            VK_CHECK(ctx->device->device.waitForFences({ ctx->fence }, true, UINT64_MAX), "ggml_vk_compute_forward waitForFences");
-
-            ctx->device->device.resetFences({ ctx->fence });
+            ggml_vk_wait_for_fence(ctx);
         }
 #ifdef GGML_VULKAN_CHECK_RESULTS
         ggml_vk_check_results_1(tensor);
@@ -8106,6 +8145,7 @@ static void ggml_vk_cleanup(ggml_backend_vk_context * ctx) {
     ctx->gc.events.clear();
 
     ctx->device->device.destroyFence(ctx->fence);
+    ctx->device->device.destroyFence(ctx->almost_ready_fence);
 }
 
 static int ggml_vk_get_device_count() {
@@ -8452,8 +8492,7 @@ static void ggml_backend_vk_synchronize(ggml_backend_t backend) {
     }
 
     ggml_vk_submit(transfer_ctx, ctx->fence);
-    VK_CHECK(ctx->device->device.waitForFences({ ctx->fence }, true, UINT64_MAX), "ggml_backend_vk_synchronize waitForFences");
-    ctx->device->device.resetFences({ ctx->fence });
+    ggml_vk_wait_for_fence(ctx);
 
     for (auto& cpy : transfer_ctx->out_memcpys) {
         memcpy(cpy.dst, cpy.src, cpy.n);
@@ -8472,7 +8511,7 @@ static ggml_status ggml_backend_vk_graph_compute(ggml_backend_t backend, ggml_cg
 
     uint64_t total_mat_mul_bytes = 0;
     for (int i = 0; i < cgraph->n_nodes; i++) {
-        ggml_vk_build_graph(ctx, cgraph->nodes[i], i, nullptr, 0, true, false, false);
+        ggml_vk_build_graph(ctx, cgraph->nodes[i], i, nullptr, 0, true, false, false, false);
         if (cgraph->nodes[i]->op == GGML_OP_MUL_MAT || cgraph->nodes[i]->op == GGML_OP_MUL_MAT_ID) {
             total_mat_mul_bytes += ggml_nbytes(cgraph->nodes[i]->src[0]);
         }
@@ -8514,11 +8553,14 @@ static ggml_status ggml_backend_vk_graph_compute(ggml_backend_t backend, ggml_cg
             mul_mat_bytes += ggml_nbytes(cgraph->nodes[i]->src[0]);
         }
 
+        // Signal the almost_ready fence when the graph is mostly complete (< 20% remaining)
+        bool almost_ready = (cgraph->n_nodes - i) < cgraph->n_nodes / 5;
         bool submit = (submitted_nodes >= nodes_per_submit) ||
                       (mul_mat_bytes >= mul_mat_bytes_per_submit) ||
-                      (i == last_node);
+                      (i == last_node) ||
+                      (almost_ready && !ctx->almost_ready_fence_pending);
 
-        bool enqueued = ggml_vk_build_graph(ctx, cgraph->nodes[i], i, cgraph->nodes[submit_node_idx], submit_node_idx, false, i == last_node, submit);
+        bool enqueued = ggml_vk_build_graph(ctx, cgraph->nodes[i], i, cgraph->nodes[submit_node_idx], submit_node_idx, false, i == last_node, almost_ready, submit);
 
         if (enqueued) {
             ++submitted_nodes;
@@ -8530,7 +8572,7 @@ static ggml_status ggml_backend_vk_graph_compute(ggml_backend_t backend, ggml_cg
 #endif
         }
 
-        if (submit) {
+        if (submit && enqueued) {
             first_node_in_batch = true;
             submitted_nodes = 0;
             mul_mat_bytes = 0;