[gin] Provide number of available cores to V8

Provide V8 with the number of processors (-1) available to
avoid posting too many tasks to Chromium.

BUG=

Review URL: https://codereview.chromium.org/1677963003

Cr-Commit-Position: refs/heads/master@{#379640}

2 files changed, 15 insertions, 0 deletions

diff --git a/gin/public/v8_platform.h b/gin/public/v8_platform.h
index d1e703e..ad37a5c 100644
--- a/gin/public/v8_platform.h
+++ b/gin/public/v8_platform.h
@@ -19,6 +19,7 @@ class GIN_EXPORT V8Platform : public NON_EXPORTED_BASE(v8::Platform) {
   static V8Platform* Get();
 
   // v8::Platform implementation.
+  size_t NumberOfAvailableBackgroundThreads() override;
   void CallOnBackgroundThread(
       v8::Task* task,
       v8::Platform::ExpectedRuntime expected_runtime) override;
diff --git a/gin/v8_platform.cc b/gin/v8_platform.cc
index be7f5c2..556f7fc 100644
--- a/gin/v8_platform.cc
+++ b/gin/v8_platform.cc
@@ -6,6 +6,7 @@
 
 #include "base/bind.h"
 #include "base/location.h"
+#include "base/sys_info.h"
 #include "base/threading/worker_pool.h"
 #include "base/trace_event/trace_event.h"
 #include "gin/per_isolate_data.h"
@@ -25,6 +26,19 @@ V8Platform::V8Platform() {}
 
 V8Platform::~V8Platform() {}
 
+size_t V8Platform::NumberOfAvailableBackgroundThreads() {
+  // WorkerPool will currently always create additional threads for posted
+  // background tasks, unless there are threads sitting idle (on posix).
+  // Indicate that V8 should create no more than the number of cores available,
+  // reserving one core for the main thread.
+  const size_t available_cores =
+    static_cast<size_t>(base::SysInfo::NumberOfProcessors());
+  if (available_cores > 1) {
+    return available_cores - 1;
+  }
+  return 1;
+}
+
 void V8Platform::CallOnBackgroundThread(
     v8::Task* task,
     v8::Platform::ExpectedRuntime expected_runtime) {