Introduce LifoSem

Summary: A LIFO semaphore is useful for building a thread pool because we can keep cpu caches warmer by continually reusing the last used thread. This diff introduces a LIFO semaphore to folly.

Test Plan: unit tests

Reviewed By: davejwatson@fb.com

FB internal diff: D1280405

folly/LifoSem.cpp

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2014 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "LifoSem.h"
+
+/// Raw node storage is preallocated in a contiguous memory segment,
+/// but we use an anonymous mmap so the physical memory used (RSS) will
+/// only reflect the maximum number of waiters that actually existed
+/// concurrently.  For blocked threads the max node count is limited by the
+/// number of threads, so we can conservatively estimate that this will be
+/// < 10k.  For LifoEventSem, however, we could potentially have many more.
+///
+/// On a 64-bit architecture each LifoSemRawNode takes 16 bytes.  We make
+/// the pool 1 million entries.
+
+LIFOSEM_DECLARE_POOL(std::atomic, 1000000)
+
+namespace folly {
+
+ShutdownSemError::ShutdownSemError(const std::string& msg)
+  : std::runtime_error(msg)
+{}
+
+ShutdownSemError::~ShutdownSemError() noexcept {
+}
+
+}