Switch to unfair locks on macOS for better performance (?)

base/synchronization/lock_impl_posix.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/synchronization/lock_impl.h"
 
 #include <errno.h>
 #include <string.h>
 
 #include "base/debug/activity_tracker.h"
 #include "base/logging.h"
 #include "base/synchronization/lock.h"
 
+#if defined(OS_MACOSX)
+#include <pthread_spis.h>
+#endif
+
 namespace base {
 namespace internal {
 
 // Determines which platforms can consider using priority inheritance locks. Use
 // this define for platform code that may not compile if priority inheritance
 // locks aren't available. For this platform code,
 // PRIORITY_INHERITANCE_LOCKS_POSSIBLE() is a necessary but insufficient check.
 // Lock::PriorityInheritanceAvailable still must be checked as the code may
 // compile but the underlying platform still may not correctly support priority
 // inheritance locks.
@@ skipping 11 matching lines @@
   if (PriorityInheritanceAvailable()) {
     rv = pthread_mutexattr_setprotocol(&mta, PTHREAD_PRIO_INHERIT);
     DCHECK_EQ(rv, 0) << ". " << strerror(rv);
   }
 #endif
 #ifndef NDEBUG
   // In debug, setup attributes for lock error checking.
   rv = pthread_mutexattr_settype(&mta, PTHREAD_MUTEX_ERRORCHECK);
   DCHECK_EQ(rv, 0) << ". " << strerror(rv);
 #endif
+#if defined(OS_MACOSX)
+  // macOS defaults to fair locks which can be orders of magnitude slower in the
+  // contention case, so default to first fit, which avoids queuing.
+  pthread_mutexattr_setpolicy_np(&mta, _PTHREAD_MUTEX_POLICY_FIRSTFIT);
+#endif
   rv = pthread_mutex_init(&native_handle_, &mta);
   DCHECK_EQ(rv, 0) << ". " << strerror(rv);
   rv = pthread_mutexattr_destroy(&mta);
   DCHECK_EQ(rv, 0) << ". " << strerror(rv);
 }
 
 LockImpl::~LockImpl() {
   int rv = pthread_mutex_destroy(&native_handle_);
   DCHECK_EQ(rv, 0) << ". " << strerror(rv);
 }
@@ skipping 34 matching lines @@
   //     Fixed in glibc 2.17.
   //     Priority inheritance mutexes may deadlock with condition variables
   //     during recacquisition of the mutex after the condition variable is
   //     signalled.
   return false;
 #endif
 }
 
 }  // namespace internal
 }  // namespace base