Merge pull request #38 from ChaelCodes/hyper-accurate-timing

Hyper Accurate Timing for Slow Systems
jhawthorn · Nov 13, 2023 · 8e4bc96 · 8e4bc96
2 parents fefcfbe + 56e3af1
commit 8e4bc96
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Showing 2 changed files with 18 additions and 10 deletions.
diff --git a/ext/vernier/vernier.cc b/ext/vernier/vernier.cc
@@ -72,13 +72,17 @@ class TimeStamp {
         return TimeStamp(0);
     }
 
-    static void Sleep(const TimeStamp &time) {
-        struct timespec ts = time.timespec();
+    // SleepUntil a specified timestamp
+    // Highly accurate manual sleep time
+    static void SleepUntil(const TimeStamp &target_time) {
+        if (target_time.zero()) return;
+        struct timespec ts = target_time.timespec();
 
         int res;
         do {
-            res = nanosleep(&ts, &ts);
-        } while (res && errno == EINTR);
+            // do nothing until it's time :)
+            sleep(0);
+        } while (target_time > TimeStamp::Now());
     }
 
     static TimeStamp from_microseconds(uint64_t us) {
@@ -1252,13 +1256,12 @@ class TimeCollector : public BaseCollector {
 
             next_sample_schedule += interval;
 
+            // If sampling falls behind, restart, and check in another interval
             if (next_sample_schedule < sample_complete) {
-                //fprintf(stderr, "fell behind by %ius\n", (sample_complete - next_sample_schedule).microseconds());
                 next_sample_schedule = sample_complete + interval;
             }
 
-            TimeStamp sleep_time = next_sample_schedule - sample_complete;
-            TimeStamp::Sleep(sleep_time);
+            TimeStamp::SleepUntil(next_sample_schedule);
         }
 
         thread_stopped.post();

diff --git a/test/test_time_collector.rb b/test/test_time_collector.rb
@@ -3,8 +3,11 @@
 require "test_helper"
 
 class TestTimeCollector < Minitest::Test
+  SLEEP_SCALE = ENV.fetch("TEST_SLEEP_SCALE", 0.1).to_f # seconds/100ms
+  SAMPLE_SCALE_INTERVAL = 10_000 * SLEEP_SCALE # Microseconds
+
   def bar
-    sleep 0.1
+    sleep SLEEP_SCALE
   end
 
   def foo
@@ -28,7 +31,7 @@ def test_receives_gc_events
   end
 
   def test_time_collector
-    collector = Vernier::Collector.new(:wall, interval: 1000)
+    collector = Vernier::Collector.new(:wall, interval: SAMPLE_SCALE_INTERVAL)
     collector.start
     foo
     result = collector.stop
@@ -45,7 +48,7 @@ def test_time_collector
   end
 
   def test_sleeping_threads
-    collector = Vernier::Collector.new(:wall, interval: 1000)
+    collector = Vernier::Collector.new(:wall, interval: SAMPLE_SCALE_INTERVAL)
     th1 = Thread.new { foo; Thread.current.native_thread_id }
     th2 = Thread.new { foo; Thread.current.native_thread_id }
     collector.start
@@ -54,8 +57,10 @@ def test_sleeping_threads
     result = collector.stop
 
     tally = result.threads.transform_values do |thread|
+      # Number of samples
       thread[:weights].sum
     end.to_h
+
     assert_in_epsilon 200, tally[Thread.current.native_thread_id], generous_epsilon
     assert_in_epsilon 200, tally[th1id], generous_epsilon
     assert_in_epsilon 200, tally[th2id], generous_epsilon