[wasm] Allow users to set threads count at runtime

tfjs-backend-wasm/README.md

@@ -91,6 +91,30 @@ If the steps above are correctly done, you can check the Network tab from the
 console and make sure the
 <code>tfjs-backend-wasm-<b>threaded-simd</b>.wasm</code> WASM binary is loaded.
 
+## Setting threads count
+
+By default, the backend will use the number of logical CPU cores as the
+threads count when creating the threadpool used by XNNPACK. You can use the
+`setThreadsCount` API to manually set it (must be called before calling
+`tf.setBackend('wasm')`).
+
+### Via NPM
+
+```js
+import * as tf from '@tensorflow/tfjs';
+import {setThreadsCount} from '@tensorflow/tfjs-backend-wasm';
+
+setThreadsCount(2);
+tf.setBackend('wasm').then(() => {...});
+```
+
+### Via script tag
+
+```js
+tf.wasm.setThreadsCount(2);
+tf.setBackend('wasm').then(() => {...});
+```
+
 ## Running MobileNet
 
 ```js

tfjs-backend-wasm/src/backend_wasm.ts

@@ -42,9 +42,9 @@ export class BackendWasm extends KernelBackend {
   private dataIdNextNumber = 1;
   dataIdMap: DataStorage<TensorData>;
 
-  constructor(public wasm: BackendWasmModule | BackendWasmThreadedSimdModule) {
+  constructor(public wasm: BackendWasmModule|BackendWasmThreadedSimdModule) {
     super();
-    this.wasm.tfjs.init();
+    this.wasm.tfjs.initWithThreadsCount(threadsCount);
     this.dataIdMap = new DataStorage(this, engine());
   }
 
@@ -210,7 +210,7 @@ export class BackendWasm extends KernelBackend {
 }
 
 function createInstantiateWasmFunc(path: string) {
-  // this will be replace by rollup plugin patchWechatWebAssembly in 
+  // this will be replace by rollup plugin patchWechatWebAssembly in
   // minprogram's output.
   // tslint:disable-next-line:no-any
   return (imports: any, callback: any) => {
@@ -346,6 +346,8 @@ export async function init(): Promise<{wasm: BackendWasmModule}> {
       // Using the tfjs namespace to avoid conflict with emscripten's API.
       module.tfjs = {
         init: module.cwrap('init', null, []),
+        initWithThreadsCount:
+            module.cwrap('init_with_threads_count', null, ['number']),
         registerTensor: module.cwrap(
             'register_tensor', null,
             [
@@ -474,3 +476,15 @@ export function resetWasmPath(): void {
   customFetch = false;
   initAborted = false;
 }
+
+let threadsCount = -1;
+
+/**
+ * Sets the number of threads that will be used by XNNPACK to create
+ * threadpool (default to the number of logical CPU cores).
+ *
+ * This must be called before calling `tf.setBackend('wasm')`.
+ */
+export function setThreadsCount(numThreads: number) {
+  threadsCount = numThreads;
+}

tfjs-backend-wasm/src/base.ts

@@ -21,7 +21,7 @@ import {registerBackend} from '@tensorflow/tfjs-core';
 
 import {BackendWasm, init} from './backend_wasm';
 
-export {BackendWasm, setWasmPath, setWasmPaths} from './backend_wasm';
+export {BackendWasm, setThreadsCount, setWasmPath, setWasmPaths} from './backend_wasm';
 export {version as version_wasm} from './version';
 
 const WASM_PRIORITY = 2;

tfjs-backend-wasm/src/cc/BUILD

@@ -58,9 +58,12 @@ cc_binary(
         "-s MALLOC=emmalloc",
         "-s USE_PTHREADS=1",
         "-s PROXY_TO_PTHREAD=1",
-        # Many x86-64 processors have 2 threads per core, so we divide by 2.
-        "-s PTHREAD_POOL_SIZE=" +
-        "'Math.min(4, Math.max(1, (navigator.hardwareConcurrency || 1) / 2))'",
+        # Pre-create 8 webworkers (threads). The actual number of threads that
+        # will be used by XNNPACK for creating the threadpool might be fewer. It
+        # is by default set to the number of logical cores which in most cases
+        # should be fewer than 8. It can also be set manually by calling the
+        # `setThreadsCount` API.
+        "-s PTHREAD_POOL_SIZE=8",
     ],
     deps = [
         ":all_kernels",

tfjs-backend-wasm/src/cc/backend.cc

@@ -52,18 +52,7 @@ TensorInfo &get_tensor_info_out(const size_t tensor_id) {
 
 size_t xnn_operator_count = 0;
 
-// emscripten_num_logical_cores corresponds to navigator.hardwareConcurrency.
-// Many x86-64 processors have 2 threads per core, so we are dividing by 2.
-#ifdef __EMSCRIPTEN_PTHREADS__
-int num_cores = emscripten_num_logical_cores() / 2;
-#else
-int num_cores = 1;
-#endif
-
-int min_num_threads = 1;
-int max_num_threads = 4;
-pthreadpool *threadpool = pthreadpool_create(
-    std::min(std::max(num_cores, min_num_threads), max_num_threads));
+pthreadpool *threadpool = NULL;
 
 // Registers a disposal callback for a tensor id with a given callback function.
 void register_disposal_callback(const size_t tensor_id,
@@ -82,13 +71,42 @@ const size_t num_tensors() { return data.size(); }
 }  // namespace backend
 
 namespace wasm {
+
+// emscripten_num_logical_cores corresponds to navigator.hardwareConcurrency.
+// Many x86-64 processors have 2 threads per core, so we are dividing by 2.
+#ifdef __EMSCRIPTEN_PTHREADS__
+int num_cores = emscripten_num_logical_cores() / 2;
+#else
+int num_cores = 1;
+#endif
+
+int min_num_threads = 1;
+// In cc/BUILD, we pre-created 8 webworker threads which is the maximum number
+// of threads that the threadpool could use here.
+int max_num_threads = 8;
+
 // We use C-style API to interface with Javascript.
+
 extern "C" {
 
 #ifdef __EMSCRIPTEN__
 EMSCRIPTEN_KEEPALIVE
 #endif
-void init() { xnn_initialize(nullptr); }
+void init() { init_with_threads_count(num_cores); }
+
+#ifdef __EMSCRIPTEN__
+EMSCRIPTEN_KEEPALIVE
+#endif
+void init_with_threads_count(const int threads_count) {
+  int count = threads_count;
+  if (threads_count < 0) {
+    count = num_cores;
+  }
+  int capped_num_threads = std::min(
+      std::min(std::max(count, min_num_threads), max_num_threads), num_cores);
+  tfjs::backend::threadpool = pthreadpool_create(capped_num_threads);
+  xnn_initialize(nullptr);
+}
 
 #ifdef __EMSCRIPTEN__
 EMSCRIPTEN_KEEPALIVE
@@ -139,6 +157,9 @@ void dispose() {
 
   data.clear();
   disposal_callbacks.clear();
+
+  pthreadpool_destroy(tfjs::backend::threadpool);
+  tfjs::backend::threadpool = NULL;
 }
 
 }  // extern "C"

tfjs-backend-wasm/src/cc/backend.h

@@ -99,6 +99,9 @@ extern "C" {
 // Initializes the WASM backend.
 void init();
 
+// Initializes the WASM backend with the given threads count.
+void init_with_threads_count(const int threads_count);
+
 // Registers a tensor with a tensor ID, size, and the pointer to where the
 // tensor data lives.
 void register_tensor(const size_t tensor_id, const size_t size,

tfjs-backend-wasm/wasm-out/tfjs-backend-wasm.d.ts

@@ -19,6 +19,7 @@ export interface BackendWasmModule extends EmscriptenModule {
   // Using the tfjs namespace to avoid conflict with emscripten's API.
   tfjs: {
     init(): void,
+    initWithThreadsCount(threadsCount: number): void,
     registerTensor(id: number, size: number, memoryOffset: number): void,
     // Disposes the data behind the data bucket.
     disposeData(id: number): void,