Change dispenso default for Linux to event-based wakeup

Summary: For software which has infrequent use of the thread pool, this results in an order of magnitude less system resources being used due to strongly reduced polling.  In some benchmarks, it can lead to a small percentage reduction in throughput.

Differential Revision: D63671846

fbshipit-source-id: 752d639fa6199094992fc4382c89ad3a1d77219e

Author: graphicsMan

dispenso/thread_pool.h

@@ -31,29 +31,32 @@
 
 namespace dispenso {
 
+#if !defined(DISPENSO_WAKEUP_ENABLE)
+#if defined(_WIN32) || defined(__linux__)
+#define DISPENSO_WAKEUP_ENABLE 1
+#else
+// TODO(bbudge):  For now, only enable Linux and Windows.  On Mac still need to figure out how to
+// wake more quickly (see e.g.
+// https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/KernelProgramming/scheduler/scheduler.html)
+#define DISPENSO_WAKEUP_ENABLE 0
+#endif // Linux or Windows
+#endif // DISPENSO_WAKEUP_ENABLE
+
 #if !defined(DISPENSO_POLL_PERIOD_US)
 #if defined(_WIN32)
 #define DISPENSO_POLL_PERIOD_US 1000
 #else
-#if !defined(DISPENSO_WAKEUP_ENABLE)
+#if !(DISPENSO_WAKEUP_ENABLE)
 #define DISPENSO_POLL_PERIOD_US 200
 #else
-#define DISPENSO_POLL_PERIOD_US 8000
+#define DISPENSO_POLL_PERIOD_US (1 << 15) // Determined empirically good on dual Xeon Linux
 #endif // DISPENSO_WAKEUP_ENABLE
 #endif // PLATFORM
 #endif // DISPENSO_POLL_PERIOD_US
 
 constexpr uint32_t kDefaultSleepLenUs = DISPENSO_POLL_PERIOD_US;
 
-#if defined(DISPENSO_WAKEUP_ENABLE)
 constexpr bool kDefaultWakeupEnable = DISPENSO_WAKEUP_ENABLE;
-#else
-#if defined(__WIN32)
-constexpr bool kDefaultWakeupEnable = true;
-#else
-constexpr bool kDefaultWakeupEnable = false;
-#endif // PLATFORM
-#endif // DISPENSO_WAKEUP_ENABLE
 
 /**
  * A simple tag specifier that can be fed to TaskSets and