Support read and write from blocking and non-blocking pipes and sockets

[folly.git] / folly / MicroSpinLock.h

diff --git a/folly/MicroSpinLock.h b/folly/MicroSpinLock.h
index fde93357bc610238d5ae589a4fd820358f17a7f8..4a691fd1ff54cb5da397f926c01e7efc25d7367a 100644 (file)
--- a/folly/MicroSpinLock.h
+++ b/folly/MicroSpinLock.h
@@ -14,6 +14,22 @@
  * limitations under the License.
  */
 
+/*
+ * N.B. You most likely do _not_ want to use MicroSpinLock or any
+ * other kind of spinlock.  Consider MicroLock instead.
+ *
+ * In short, spinlocks in preemptive multi-tasking operating systems
+ * have serious problems and fast mutexes like std::mutex are almost
+ * certainly the better choice, because letting the OS scheduler put a
+ * thread to sleep is better for system responsiveness and throughput
+ * than wasting a timeslice repeatedly querying a lock held by a
+ * thread that's blocked, and you can't prevent userspace
+ * programs blocking.
+ *
+ * Spinlocks in an operating system kernel make much more sense than
+ * they do in userspace.
+ */
+
 #pragma once
 
 /*
@@ -89,6 +105,9 @@ struct MicroSpinLock {
                                                         std::memory_order_relaxed);
   }
 };
+static_assert(
+    std::is_pod<MicroSpinLock>::value,
+    "MicroSpinLock must be kept a POD type.");
 
 //////////////////////////////////////////////////////////////////////