Default to cores/2 threads in JNI layer

extension/android/jni/BUCK

@@ -1,4 +1,5 @@
 load("@fbsource//tools/build_defs/android:fb_android_cxx_library.bzl", "fb_android_cxx_library")
+load("@fbsource//xplat/executorch/backends/xnnpack/third-party:third_party_libs.bzl", "third_party_dep")
 load("@fbsource//xplat/executorch/codegen:codegen.bzl", "executorch_generated_lib")
 
 oncall("executorch")
@@ -41,6 +42,8 @@ fb_android_cxx_library(
         "//xplat/executorch/extension/module:module_static",
         "//xplat/executorch/extension/runner_util:inputs_static",
         "//xplat/executorch/extension/tensor:tensor_static",
+        "//xplat/executorch/extension/threadpool:threadpool_static",
+        third_party_dep("cpuinfo"),
     ],
 )
 

extension/android/jni/jni_layer.cpp

@@ -25,6 +25,11 @@
 #include <executorch/runtime/platform/platform.h>
 #include <executorch/runtime/platform/runtime.h>
 
+#ifdef ET_USE_THREADPOOL
+#include <cpuinfo.h>
+#include <executorch/extension/threadpool/threadpool.h>
+#endif
+
 #include <fbjni/ByteBuffer.h>
 #include <fbjni/fbjni.h>
 
@@ -260,6 +265,25 @@ class ExecuTorchJni : public facebook::jni::HybridClass<ExecuTorchJni> {
     }
 
     module_ = std::make_unique<Module>(modelPath->toStdString(), load_mode);
+
+#ifdef ET_USE_THREADPOOL
+    // Default to using cores/2 threadpool threads. The long-term plan is to
+    // improve performant core detection in CPUInfo, but for now we can use
+    // cores/2 as a sane default.
+    //
+    // Based on testing, this is almost universally faster than using all
+    // cores, as efficiency cores can be quite slow. In extreme cases, using
+    // all cores can be 10x slower than using cores/2.
+    //
+    // TODO Allow overriding this default from Java.
+    auto threadpool = executorch::extension::threadpool::get_threadpool();
+    if (threadpool) {
+      int thread_count = cpuinfo_get_processors_count() / 2;
+      if (thread_count > 0) {
+        threadpool->_unsafe_reset_threadpool(thread_count);
+      }
+    }
+#endif
   }
 
   facebook::jni::local_ref<facebook::jni::JArrayClass<JEValue>> forward(