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

base/synchronization/lock_impl_posix.cc

Index: base/synchronization/lock_impl_posix.cc
diff --git a/base/synchronization/lock_impl_posix.cc b/base/synchronization/lock_impl_posix.cc
index e54595b87f10a353647d4a0ee40aa30d2a649780..fe5d21cf0d46a2e717e0baadf4a534eff06e26a3 100644
--- a/base/synchronization/lock_impl_posix.cc
+++ b/base/synchronization/lock_impl_posix.cc
@@ -11,6 +11,10 @@
 #include "base/logging.h"
 #include "base/synchronization/lock.h"
 
+#if defined(OS_MACOSX)
+#include <pthread_spis.h>
+#endif
+
 namespace base {
 namespace internal {
 
@@ -42,6 +46,11 @@ LockImpl::LockImpl() {
   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);