Tweak concurrency limits

Create capacity for 2x max, but lie about it

src/common/threadpool.cpp

@@ -39,7 +39,7 @@ namespace tools
 threadpool::threadpool() : running(true), active(0) {
   boost::thread::attributes attrs;
   attrs.set_stack_size(THREAD_STACK_SIZE);
-  max = tools::get_max_concurrency();
+  max = tools::get_max_concurrency() * 2;
   size_t i = max;
   while(i--) {
     threads.push_back(boost::thread(attrs, boost::bind(&threadpool::run, this)));
@@ -73,6 +73,10 @@ void threadpool::submit(waiter *obj, std::function<void()> f) {
   }
 }
 
+int threadpool::get_max_concurrency() {
+  return max / 2;
+}
+
 void threadpool::waiter::wait() {
   boost::unique_lock<boost::mutex> lock(mt);
   while(num) cv.wait(lock);

src/common/threadpool.h

@@ -65,7 +65,7 @@ public:
   // task to finish.
   void submit(waiter *waiter, std::function<void()> f);
 
-  int get_max_concurrency() { return max; }
+  int get_max_concurrency();
 
   private:
     threadpool();