Export of internal Abseil changes
--
4e789887c5e2d3e531499c6c761a10bf60272969 by Abseil Team <absl-team@google.com>:
Remove a branch from MutexDelay.
Store the mutex sleep limit in the static so that it's not calculated every
time. Remove num_cpus from the static data because it's not used anymore. The
size of static data is 2 QWORDS after this change.
PiperOrigin-RevId: 350069849
GitOrigin-RevId: 4e789887c5e2d3e531499c6c761a10bf60272969
Change-Id: Ic374c63c8a7011b5f8931459e44a7841d4b2a2e6
diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 9e01393..82b631a 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -124,35 +124,44 @@
symbolizer.Store(fn);
}
+namespace {
+// Represents the strategy for spin and yield.
+// See the comment in GetMutexGlobals() for more information.
+enum DelayMode { AGGRESSIVE, GENTLE };
+
struct ABSL_CACHELINE_ALIGNED MutexGlobals {
absl::once_flag once;
- int num_cpus = 0;
int spinloop_iterations = 0;
+ int32_t mutex_sleep_limit[2] = {};
};
-static const MutexGlobals& GetMutexGlobals() {
+const MutexGlobals &GetMutexGlobals() {
ABSL_CONST_INIT static MutexGlobals data;
absl::base_internal::LowLevelCallOnce(&data.once, [&]() {
- data.num_cpus = absl::base_internal::NumCPUs();
- data.spinloop_iterations = data.num_cpus > 1 ? 1500 : 0;
+ const int num_cpus = absl::base_internal::NumCPUs();
+ data.spinloop_iterations = num_cpus > 1 ? 1500 : 0;
+ // If this a uniprocessor, only yield/sleep. Otherwise, if the mode is
+ // aggressive then spin many times before yielding. If the mode is
+ // gentle then spin only a few times before yielding. Aggressive spinning
+ // is used to ensure that an Unlock() call, which must get the spin lock
+ // for any thread to make progress gets it without undue delay.
+ if (num_cpus > 1) {
+ data.mutex_sleep_limit[AGGRESSIVE] = 5000;
+ data.mutex_sleep_limit[GENTLE] = 250;
+ } else {
+ data.mutex_sleep_limit[AGGRESSIVE] = 0;
+ data.mutex_sleep_limit[GENTLE] = 0;
+ }
});
return data;
}
-
-// Spinlock delay on iteration c. Returns new c.
-namespace {
- enum DelayMode { AGGRESSIVE, GENTLE };
-};
+} // namespace
namespace synchronization_internal {
+// Returns the Mutex delay on iteration `c` depending on the given `mode`.
+// The returned value should be used as `c` for the next call to `MutexDelay`.
int MutexDelay(int32_t c, int mode) {
- // If this a uniprocessor, only yield/sleep. Otherwise, if the mode is
- // aggressive then spin many times before yielding. If the mode is
- // gentle then spin only a few times before yielding. Aggressive spinning is
- // used to ensure that an Unlock() call, which must get the spin lock for
- // any thread to make progress gets it without undue delay.
- const int32_t limit =
- GetMutexGlobals().num_cpus > 1 ? (mode == AGGRESSIVE ? 5000 : 250) : 0;
+ const int32_t limit = GetMutexGlobals().mutex_sleep_limit[mode];
if (c < limit) {
// Spin.
c++;
