Remove spinning for fibers::Baton when waiting from a thread

Summary: This is not a good default. We can build a dedicated SpinningBaton if needed

Reviewed By: andriigrynenko

Differential Revision: D15387138

fbshipit-source-id: 433ce2b1af7c661edd2c0e264dc618cc9c680aa3

folly/fibers/Baton.cpp

@@ -59,11 +59,6 @@ void Baton::wait(TimeoutHandler& timeoutHandler) {
 }
 
 void Baton::waitThread() {
-  if (spinWaitForEarlyPost()) {
-    assert(waiter_.load(std::memory_order_acquire) == POSTED);
-    return;
-  }
-
   auto waiter = waiter_.load();
 
   if (LIKELY(
@@ -90,31 +85,7 @@ void Baton::waitThread() {
   throw std::logic_error("Other waiter is already waiting on this baton");
 }
 
-bool Baton::spinWaitForEarlyPost() {
-  static_assert(
-      PreBlockAttempts > 0,
-      "isn't this assert clearer than an uninitialized variable warning?");
-  for (int i = 0; i < PreBlockAttempts; ++i) {
-    if (try_wait()) {
-      // hooray!
-      return true;
-    }
-    // The pause instruction is the polite way to spin, but it doesn't
-    // actually affect correctness to omit it if we don't have it.
-    // Pausing donates the full capabilities of the current core to
-    // its other hyperthreads for a dozen cycles or so
-    asm_volatile_pause();
-  }
-
-  return false;
-}
-
 bool Baton::timedWaitThread(std::chrono::milliseconds timeout) {
-  if (spinWaitForEarlyPost()) {
-    assert(waiter_.load(std::memory_order_acquire) == POSTED);
-    return true;
-  }
-
   auto waiter = waiter_.load();
 
   if (LIKELY(

folly/fibers/Baton.h

@@ -236,25 +236,6 @@ class Baton {
  private:
   class FiberWaiter;
 
-  enum {
-    /**
-     * Must be positive.  If multiple threads are actively using a
-     * higher-level data structure that uses batons internally, it is
-     * likely that the post() and wait() calls happen almost at the same
-     * time.  In this state, we lose big 50% of the time if the wait goes
-     * to sleep immediately.  On circa-2013 devbox hardware it costs about
-     * 7 usec to FUTEX_WAIT and then be awoken (half the t/iter as the
-     * posix_sem_pingpong test in BatonTests).  We can improve our chances
-     * of early post by spinning for a bit, although we have to balance
-     * this against the loss if we end up sleeping any way.  Spins on this
-     * hw take about 7 nanos (all but 0.5 nanos is the pause instruction).
-     * We give ourself 300 spins, which is about 2 usec of waiting.  As a
-     * partial consolation, since we are using the pause instruction we
-     * are giving a speed boost to the colocated hyperthread.
-     */
-    PreBlockAttempts = 300,
-  };
-
   explicit Baton(intptr_t state) : waiter_(state) {}
 
   void postHelper(intptr_t new_value);
@@ -263,13 +244,6 @@ class Baton {
 
   template <typename F>
   inline void waitFiber(FiberManager& fm, F&& mainContextFunc);
-  /**
-   * Spin for "some time" (see discussion on PreBlockAttempts) waiting
-   * for a post.
-   * @return true if we received a post the spin wait, false otherwise. If the
-   *         function returns true then Baton state is guaranteed to be POSTED
-   */
-  bool spinWaitForEarlyPost();
 
   bool timedWaitThread(std::chrono::milliseconds timeout);