rpc : refactor backend

Use structs for RPC request/response messages

ggml/src/ggml-rpc.cpp

@@ -58,7 +58,7 @@ struct socket_t {
 };
 
 // ggml_tensor is serialized into rpc_tensor
-#pragma pack(push, 1)
+#pragma pack(1)
 struct rpc_tensor {
     uint64_t id;
     uint32_t type;
@@ -96,6 +96,17 @@ enum rpc_cmd {
     RPC_CMD_COUNT,
 };
 
+#pragma pack(1)
+struct request_alloc_buffer {
+    uint64_t size;
+};
+
+#pragma pack(1)
+struct response_alloc_buffer {
+    uint64_t remote_ptr;
+    uint64_t remote_size;
+};
+
 // RPC data structures
 
 static ggml_guid_t ggml_backend_rpc_guid() {
@@ -252,30 +263,31 @@ static bool parse_endpoint(const std::string & endpoint, std::string & host, int
 
 // RPC request : | rpc_cmd (1 byte) | request_size (8 bytes) | request_data (request_size bytes) |
 // RPC response: | response_size (8 bytes) | response_data (response_size bytes) |
-static bool send_rpc_cmd(const std::shared_ptr<socket_t> & sock, enum rpc_cmd cmd, const std::vector<uint8_t> & input, std::vector<uint8_t> & output) {
+static bool send_rpc_cmd(const std::shared_ptr<socket_t> & sock, enum rpc_cmd cmd, const void * input, size_t input_size, void * output, size_t output_size) {
     uint8_t cmd_byte = cmd;
     if (!send_data(sock->fd, &cmd_byte, sizeof(cmd_byte))) {
         return false;
     }
-    uint64_t input_size = input.size();
     if (!send_data(sock->fd, &input_size, sizeof(input_size))) {
         return false;
     }
-    if (!send_data(sock->fd, input.data(), input.size())) {
+    if (!send_data(sock->fd, input, input_size)) {
         return false;
     }
-    uint64_t output_size;
-    if (!recv_data(sock->fd, &output_size, sizeof(output_size))) {
+    // TODO: currently the output_size is always known, do we need support for commands with variable output size?
+    // even if we do, we can skip sending output_size from the server for commands with known output size
+    uint64_t out_size;
+    if (!recv_data(sock->fd, &out_size, sizeof(out_size))) {
         return false;
     }
-    if (output_size == 0) {
-        output.clear();
-        return true;
-    }
-    output.resize(output_size);
-    if (!recv_data(sock->fd, output.data(), output_size)) {
+    if (out_size != output_size) {
         return false;
     }
+    if (output_size > 0) {
+        if (!recv_data(sock->fd, output, output_size)) {
+            return false;
+        }
+    }
     return true;
 }
 
@@ -484,25 +496,15 @@ static const char * ggml_backend_rpc_buffer_type_name(ggml_backend_buffer_type_t
 
 static ggml_backend_buffer_t ggml_backend_rpc_buffer_type_alloc_buffer(ggml_backend_buffer_type_t buft, size_t size) {
     ggml_backend_rpc_buffer_type_context * buft_ctx = (ggml_backend_rpc_buffer_type_context *)buft->context;
-    // input serialization format: | size (8 bytes) |
-    int input_size = sizeof(uint64_t);
-    std::vector<uint8_t> input(input_size, 0);
-    memcpy(input.data(), &size, sizeof(size));
-    std::vector<uint8_t> output;
+    request_alloc_buffer request = {size};
+    response_alloc_buffer response;
     auto sock = get_socket(buft_ctx->endpoint);
-    bool status = send_rpc_cmd(sock, RPC_CMD_ALLOC_BUFFER, input, output);
-    GGML_ASSERT(status);
-    GGML_ASSERT(output.size() == 2*sizeof(uint64_t));
-    // output serialization format: | remote_ptr (8 bytes) | remote_size (8 bytes) |
-    uint64_t remote_ptr;
-    memcpy(&remote_ptr, output.data(), sizeof(remote_ptr));
-    size_t remote_size;
-    memcpy(&remote_size, output.data() + sizeof(uint64_t), sizeof(remote_size));
-    if (remote_ptr != 0) {
+    bool status = send_rpc_cmd(sock, RPC_CMD_ALLOC_BUFFER, &request, sizeof(request), &response, sizeof(response));
+    if (response.remote_ptr != 0) {
         ggml_backend_buffer_t buffer = ggml_backend_buffer_init(buft,
             ggml_backend_rpc_buffer_interface,
-            new ggml_backend_rpc_buffer_context{sock, {}, remote_ptr, "RPC[" + std::string(buft_ctx->endpoint) + "]"},
-            remote_size);
+            new ggml_backend_rpc_buffer_context{sock, {}, response.remote_ptr, "RPC[" + std::string(buft_ctx->endpoint) + "]"},
+            response.remote_size);
         return buffer;
     } else {
         return nullptr;