[OpenMP][libomp] avoid spin wait and yield on arm64 macOS

[danieljdouglas-deco]

Summary

This patch changes the default behavior to avoid spin waiting and yielding. (See “Don’t Keep Threads Active And Idle” section here: https://developer.apple.com/documentation/apple-silicon/tuning-your-code-s-performance-for-apple-silicon)

We verified using instruments traces that the changes improve scheduling behavior on macOS.

We also collected results using EPCC schedbench (https://github.com/LangdalP/EPCC-OpenMP-micro-benchmarks) that are attached here that show a reduction in standard deviation and max test run time across all scheduling types. Static scheduling sees dramatic improvements with these changes, we see a 2-4x average runtime improvement in the benchmark.

[jlpeyton]

LGTM

[tlwilmar]

LGTM

[jlpeyton]

Daniel, do you need us to commit on your behalf?

[danieljdouglas-deco]

In D126510#3602789, @jlpeyton wrote:

Daniel, do you need us to commit on your behalf?

Yes please, and thanks for the review!

openmp/runtime/src/kmp.h

extern int
     __kmp_storage_map; /* True means print storage map for threads and teams */
 extern int __kmp_storage_map_verbose; /* True means storage map includes
                                          placement info */
 extern int __kmp_storage_map_verbose_specified;
 
 #if KMP_ARCH_X86 || KMP_ARCH_X86_64
 extern kmp_cpuinfo_t __kmp_cpuinfo;
 static inline bool __kmp_is_hybrid_cpu() { return __kmp_cpuinfo.flags.hybrid; }
+#elif KMP_OS_DARWIN && KMP_ARCH_AARCH64
+static inline bool __kmp_is_hybrid_cpu() { return true; }
 #else
 static inline bool __kmp_is_hybrid_cpu() { return false; }
 #endif
 
 extern volatile int __kmp_init_serial;
 extern volatile int __kmp_init_gtid;
 extern volatile int __kmp_init_common;
 extern volatile int __kmp_init_middle;

openmp/runtime/src/kmp_global.cpp

 int __kmp_env_blocktime = FALSE; /* KMP_BLOCKTIME specified? */
 int __kmp_env_checks = FALSE; /* KMP_CHECKS specified?    */
 int __kmp_env_consistency_check = FALSE; /* KMP_CONSISTENCY_CHECK specified? */
 
 // From KMP_USE_YIELD:
 // 0 = never yield;
 // 1 = always yield (default);
 // 2 = yield only if oversubscribed
+#if KMP_OS_DARWIN && KMP_ARCH_AARCH64
+// Set to 0 for environments where yield is slower
+kmp_int32 __kmp_use_yield = 0;
+#else
 kmp_int32 __kmp_use_yield = 1;
+#endif
+
 // This will be 1 if KMP_USE_YIELD environment variable was set explicitly
 kmp_int32 __kmp_use_yield_exp_set = 0;
 
 kmp_uint32 __kmp_yield_init = KMP_INIT_WAIT;
 kmp_uint32 __kmp_yield_next = KMP_NEXT_WAIT;
 kmp_uint64 __kmp_pause_init = 1; // for tpause
 
 /* ------------------------------------------------------ */

openmp/runtime/src/kmp_runtime.cpp

case library_serial: {
     KMP_INFORM(LibraryIsSerial);
   } break;
   case library_turnaround:
     if (__kmp_use_yield == 1 && !__kmp_use_yield_exp_set)
       __kmp_use_yield = 2; // only yield when oversubscribed
     break;
   case library_throughput:
     if (__kmp_dflt_blocktime == KMP_MAX_BLOCKTIME)
-      __kmp_dflt_blocktime = 200;
+      __kmp_dflt_blocktime = KMP_DEFAULT_BLOCKTIME;
     break;
   default:
     KMP_FATAL(UnknownLibraryType, arg);
   }
 }
 
 /* Getting team information common for all team API */
 // Returns NULL if not in teams construct