vulkan: sort graph to allow more parallel execution (ggml-org#15850)

* vulkan: sort graph to allow more parallel execution

Add a backend proc to allow the backend to modify the graph. The
vulkan implementation looks at which nodes depend on each other
and greedily reorders them to group together nodes that don't
depend on each other. It only reorders the nodes, doesn't change
the contents of any of them.

With ggml-org#15489, this reduces the number of synchronizations needed.

* call optimize_graph per-split

Author: jeffbolznv

ggml/src/ggml-backend-impl.h

@@ -114,6 +114,9 @@ extern "C" {
         void (*event_record)(ggml_backend_t backend, ggml_backend_event_t event);
         // wait for an event on on a different stream
         void (*event_wait)  (ggml_backend_t backend, ggml_backend_event_t event);
+
+        // (optional) sort/optimize the nodes in the graph
+        void                      (*optimize_graph)    (ggml_backend_t backend, struct ggml_cgraph * cgraph);
     };
 
     struct ggml_backend {

ggml/src/ggml-backend.cpp

@@ -463,6 +463,13 @@ void ggml_backend_event_wait(ggml_backend_t backend, ggml_backend_event_t event)
     backend->iface.event_wait(backend, event);
 }
 
+static void ggml_backend_optimize_graph(ggml_backend_t backend, struct ggml_cgraph * cgraph) {
+    GGML_ASSERT(backend);
+    if (backend->iface.optimize_graph != NULL) {
+        backend->iface.optimize_graph(backend, cgraph);
+    }
+}
+
 // Backend device
 
 const char * ggml_backend_dev_name(ggml_backend_dev_t device) {
@@ -1298,6 +1305,10 @@ void ggml_backend_sched_split_graph(ggml_backend_sched_t sched, struct ggml_cgra
         struct ggml_backend_sched_split * split = &sched->splits[i];
         split->graph = ggml_graph_view(graph, split->i_start, split->i_end);
 
+        // Optimize this split of the graph. This needs to happen before we make graph_copy,
+        // so they are in sync.
+        ggml_backend_optimize_graph(sched->backends[split->backend_id], &split->graph);
+
         // add inputs to the graph copy so that they are allocated by ggml-alloc at the start of the split
         for (int j = 0; j < split->n_inputs; j++) {
             assert(graph_copy->size > (graph_copy->n_nodes + 1));

ggml/src/ggml-blas/ggml-blas.cpp

@@ -270,6 +270,7 @@ static struct ggml_backend_i blas_backend_i = {
     /* .graph_compute           = */ ggml_backend_blas_graph_compute,
     /* .event_record            = */ NULL,
     /* .event_wait              = */ NULL,
+    /* .optimize_graph          = */ NULL,
 };
 
 static ggml_guid_t ggml_backend_blas_guid(void) {

ggml/src/ggml-cann/ggml-cann.cpp

@@ -2690,6 +2690,7 @@ static const ggml_backend_i ggml_backend_cann_interface = {
     /* .graph_compute           = */ ggml_backend_cann_graph_compute,
     /* .event_record            = */ ggml_backend_cann_event_record,
     /* .event_wait              = */ ggml_backend_cann_event_wait,
+    /* .optimize_graph          = */ NULL,
 };
 
 /**

ggml/src/ggml-cpu/ggml-cpu.cpp

@@ -190,6 +190,7 @@ static const struct ggml_backend_i ggml_backend_cpu_i = {
     /* .graph_compute           = */ ggml_backend_cpu_graph_compute,
     /* .event_record            = */ NULL,
     /* .event_wait              = */ NULL,
+    /* .optimize_graph          = */ NULL,
 };
 
 static ggml_guid_t ggml_backend_cpu_guid(void) {

ggml/src/ggml-cuda/ggml-cuda.cu

@@ -3135,6 +3135,7 @@ static const ggml_backend_i ggml_backend_cuda_interface = {
     /* .graph_compute           = */ ggml_backend_cuda_graph_compute,
     /* .event_record            = */ ggml_backend_cuda_event_record,
     /* .event_wait              = */ ggml_backend_cuda_event_wait,
+    /* .optimize_graph          = */ NULL,
 };
 
 static ggml_guid_t ggml_backend_cuda_guid() {

ggml/src/ggml-metal/ggml-metal.m

@@ -6275,6 +6275,7 @@ static void ggml_backend_metal_set_n_cb(ggml_backend_t backend, int n_cb) {
     /* .graph_compute           = */ ggml_backend_metal_graph_compute,
     /* .event_record            = */ NULL,
     /* .event_wait              = */ NULL,
+    /* .optimize_graph          = */ NULL,
 };
 
 static ggml_guid_t ggml_backend_metal_guid(void) {

ggml/src/ggml-opencl/ggml-opencl.cpp

@@ -2838,6 +2838,7 @@ static ggml_backend_i ggml_backend_opencl_i = {
     /* .graph_compute           = */ ggml_backend_opencl_graph_compute,
     /* .event_record            = */ NULL,
     /* .event_wait              = */ NULL,
+    /* .optimize_graph          = */ NULL,
 };
 
 ggml_backend_t ggml_backend_opencl_init(void) {

ggml/src/ggml-rpc/ggml-rpc.cpp

@@ -795,6 +795,7 @@ static ggml_backend_i ggml_backend_rpc_interface = {
     /* .graph_compute           = */ ggml_backend_rpc_graph_compute,
     /* .event_record            = */ NULL,
     /* .event_wait              = */ NULL,
+    /* .optimize_graph          = */ NULL,
 };
 
 ggml_backend_buffer_type_t ggml_backend_rpc_buffer_type(const char * endpoint) {

ggml/src/ggml-sycl/ggml-sycl.cpp

@@ -4063,6 +4063,7 @@ static ggml_backend_i ggml_backend_sycl_interface = {
     /* .graph_compute           = */ ggml_backend_sycl_graph_compute,
     /* .event_record            = */ ggml_backend_sycl_event_record,
     /* .event_wait              = */ ggml_backend_sycl_event_wait,
+    /* .optimize_graph          = */ NULL,
 };
 
 static ggml_guid_t ggml_backend_sycl_guid() {