Reduce the calls to ink_get_hrtime in the event loop

doc/admin-guide/files/records.yaml.en.rst

@@ -383,6 +383,19 @@ Thread Variables
    will create its own domain socket with a ``-<thread id>`` suffix added to the
    end of the path.
 
+.. ts:cv:: CONFIG proxy.config.exec_thread.loop_time_update_probability INT 10
+   :reloadable:
+
+   This dynamically loadable setting controls the rate that exec thread loop timestamps are
+   updated after processing an event given as a percentage from 0 to 100. 0
+   would mean the timestamp is only updated once per event loop, 100 percent
+   means the timestamp is updated after any potential operation that could take
+   time (i.e. processing an event or waiting on IO).  The timestamp is used for
+   queuing events and comparing timestamps for processing.  Updating more often
+   might improve event timer accuracy and event loop metrics, but increases the
+   number of times that the current time is obtained from the OS.  See also
+   `proxy.config.system_clock`
+
 .. ts:cv:: CONFIG proxy.config.accept_threads INT 1
 
    The number of accept threads. If disabled (``0``), then accepts will be done

include/iocore/eventsystem/EThread.h

@@ -352,8 +352,8 @@ class EThread : public Thread
 
   void             execute() override;
   void             execute_regular();
-  void             process_queue(Que(Event, link) * NegativeQueue, int *ev_count, int *nq_count);
-  void             process_event(Event *e, int calling_code);
+  ink_hrtime       process_queue(Que(Event, link) * NegativeQueue, int *ev_count, int *nq_count, ink_hrtime event_time);
+  ink_hrtime       process_event(Event *e, int calling_code, ink_hrtime event_time);
   void             free_event(Event *e);
   LoopTailHandler *tail_cb = &DEFAULT_TAIL_HANDLER;
 

src/iocore/eventsystem/EventSystem.cc

@@ -44,6 +44,9 @@ ink_event_system_init(ts::ModuleVersion v)
 
   RecEstablishStaticConfigInt32(thread_freelist_low_watermark, "proxy.config.allocator.thread_freelist_low_watermark");
 
+  extern int loop_time_update_probability;
+  RecEstablishStaticConfigInt32(loop_time_update_probability, "proxy.config.exec_thread.loop_time_update_probability");
+
   int chunk_sizes[DEFAULT_BUFFER_SIZES] = {0};
   {
     auto chunk_sizes_string{RecGetRecordStringAlloc("proxy.config.allocator.iobuf_chunk_sizes")};

src/iocore/eventsystem/UnixEThread.cc

@@ -33,6 +33,7 @@
 /////////////////////////////////////////////////////////////////////
 #include "P_EventSystem.h"
 #include "iocore/eventsystem/Lock.h"
+#include "tscore/ink_hrtime.h"
 
 #if HAVE_EVENTFD
 #include <sys/eventfd.h>
@@ -53,6 +54,7 @@ char const *const EThread::Metrics::Slice::STAT_NAME[] = {
 };
 
 int thread_max_heartbeat_mseconds = THREAD_MAX_HEARTBEAT_MSECONDS;
+int loop_time_update_probability  = 10;
 
 // To define a class inherits from Thread:
 //   1) Define an independent thread_local static member
@@ -144,26 +146,33 @@ EThread::set_event_type(EventType et)
   event_types |= (1 << static_cast<int>(et));
 }
 
-void
-EThread::process_event(Event *e, int calling_code)
+ink_hrtime
+EThread::process_event(Event *e, int calling_code, ink_hrtime event_time)
 {
   ink_assert((!e->in_the_prot_queue && !e->in_the_priority_queue));
   WEAK_MUTEX_TRY_LOCK(lock, e->mutex, this);
   if (!lock.is_locked()) {
-    e->timeout_at = ink_get_hrtime() + DELAY_FOR_RETRY;
+    e->timeout_at = event_time + DELAY_FOR_RETRY;
     EventQueueExternal.enqueue_local(e);
   } else {
     if (e->cancelled) {
       MUTEX_RELEASE(lock);
       free_event(e);
-      return;
+      return event_time;
     }
     Continuation *c_temp = e->continuation;
 
     // Restore the client IP debugging flags
     set_cont_flags(e->continuation->control_flags);
 
     e->continuation->handleEvent(calling_code, e);
+    if (loop_time_update_probability == 100) {
+      event_time = ink_get_hrtime();
+    } else if (loop_time_update_probability > 0) {
+      if (static_cast<int>(generator.random() % 100) < loop_time_update_probability) {
+        event_time = ink_get_hrtime();
+      }
+    }
     ink_assert(!e->in_the_priority_queue);
     ink_assert(c_temp == e->continuation);
     MUTEX_RELEASE(lock);
@@ -172,18 +181,19 @@ EThread::process_event(Event *e, int calling_code)
         if (e->period < 0) {
           e->timeout_at = e->period;
         } else {
-          e->timeout_at = ink_get_hrtime() + e->period;
+          e->timeout_at = event_time + e->period;
         }
         EventQueueExternal.enqueue_local(e);
       }
     } else if (!e->in_the_prot_queue && !e->in_the_priority_queue) {
       free_event(e);
     }
   }
+  return event_time;
 }
 
-void
-EThread::process_queue(Que(Event, link) * NegativeQueue, int *ev_count, int *nq_count)
+ink_hrtime
+EThread::process_queue(Que(Event, link) * NegativeQueue, int *ev_count, int *nq_count, ink_hrtime event_time)
 {
   Event *e;
 
@@ -198,9 +208,9 @@ EThread::process_queue(Que(Event, link) * NegativeQueue, int *ev_count, int *nq_
       free_event(e);
     } else if (!e->timeout_at) { // IMMEDIATE
       ink_assert(e->period == 0);
-      process_event(e, e->callback_event);
+      event_time = process_event(e, e->callback_event, event_time);
     } else if (e->timeout_at > 0) { // INTERVAL
-      EventQueue.enqueue(e, ink_get_hrtime());
+      EventQueue.enqueue(e, event_time);
     } else { // NEGATIVE
       Event *p = nullptr;
       Event *a = NegativeQueue->head;
@@ -216,6 +226,7 @@ EThread::process_queue(Que(Event, link) * NegativeQueue, int *ev_count, int *nq_
     }
     ++(*nq_count);
   }
+  return event_time;
 }
 
 void
@@ -228,8 +239,10 @@ EThread::execute_regular()
   ink_hrtime loop_start_time;  // Time the loop started.
   ink_hrtime loop_finish_time; // Time at the end of the loop.
 
+  loop_start_time = ink_get_hrtime();
+
   // Track this so we can update on boundary crossing.
-  auto prev_slice = this->metrics.prev_slice(metrics._slice.data() + (ink_get_hrtime() / HRTIME_SECOND) % Metrics::N_SLICES);
+  auto prev_slice = this->metrics.prev_slice(metrics._slice.data() + (loop_start_time / HRTIME_SECOND) % Metrics::N_SLICES);
 
   int nq_count;
   int ev_count;
@@ -241,9 +254,8 @@ EThread::execute_regular()
 
   // give priority to immediate events
   while (!TSSystemState::is_event_system_shut_down()) {
-    loop_start_time = ink_get_hrtime();
-    nq_count        = 0; // count # of elements put on negative queue.
-    ev_count        = 0; // # of events handled.
+    nq_count = 0; // count # of elements put on negative queue.
+    ev_count = 0; // # of events handled.
 
     current_slice = metrics._slice.data() + (loop_start_time / HRTIME_SECOND) % Metrics::N_SLICES;
     metrics.current_slice.store(current_slice, std::memory_order_release);
@@ -256,37 +268,37 @@ EThread::execute_regular()
     }
     ++(current_slice->_count); // loop started, bump count.
 
-    process_queue(&NegativeQueue, &ev_count, &nq_count);
+    ink_hrtime event_time = process_queue(&NegativeQueue, &ev_count, &nq_count, loop_start_time);
 
     bool done_one;
     do {
       done_one = false;
       // execute all the eligible internal events
-      EventQueue.check_ready(loop_start_time, this);
-      while ((e = EventQueue.dequeue_ready(ink_get_hrtime()))) {
+      EventQueue.check_ready(event_time, this);
+      while ((e = EventQueue.dequeue_ready(event_time))) {
         ink_assert(e);
         ink_assert(e->timeout_at > 0);
         if (e->cancelled) {
           free_event(e);
         } else {
-          done_one = true;
-          process_event(e, e->callback_event);
+          done_one   = true;
+          event_time = process_event(e, e->callback_event, event_time);
         }
       }
     } while (done_one);
 
     // execute any negative (poll) events
     if (NegativeQueue.head) {
-      process_queue(&NegativeQueue, &ev_count, &nq_count);
+      event_time = process_queue(&NegativeQueue, &ev_count, &nq_count, event_time);
 
       // execute poll events
       while ((e = NegativeQueue.dequeue())) {
-        process_event(e, EVENT_POLL);
+        event_time = process_event(e, EVENT_POLL, event_time);
       }
     }
 
     next_time             = EventQueue.earliest_timeout();
-    ink_hrtime sleep_time = next_time - ink_get_hrtime();
+    ink_hrtime sleep_time = next_time - event_time;
     if (sleep_time > 0) {
       if (EventQueueExternal.localQueue.empty()) {
         sleep_time = std::min(sleep_time, HRTIME_MSECONDS(thread_max_heartbeat_mseconds));
@@ -301,7 +313,7 @@ EThread::execute_regular()
     }
 
     // drained the queue by this point
-    ink_hrtime post_drain  = ink_get_hrtime();
+    ink_hrtime post_drain  = event_time;
     ink_hrtime drain_queue = post_drain - loop_start_time;
 
     tail_cb->waitForActivity(sleep_time);
@@ -311,7 +323,8 @@ EThread::execute_regular()
     // @a delta can be negative due to time of day adjustments (which apparently happen quite frequently). I
     // tried using the monotonic clock to get around this but it was *very* stuttery (up to hundreds
     // of milliseconds), far too much to be actually used.
-    delta = std::max<ink_hrtime>(0, loop_finish_time - loop_start_time);
+    delta           = std::max<ink_hrtime>(0, loop_finish_time - loop_start_time);
+    loop_start_time = loop_finish_time;
 
     metrics.decay();
     metrics.record_loop_time(delta);

src/records/RecordsConfig.cc

@@ -21,6 +21,7 @@
   limitations under the License.
  */
 
+#include "records/RecDefs.h"
 #include "tscore/ink_config.h"
 #include "tscore/Filenames.h"
 #include "records/RecordsConfig.h"
@@ -117,6 +118,8 @@ static constexpr RecordElement RecordsConfig[] =
   ,
   {RECT_CONFIG, "proxy.config.exec_thread.listen", RECD_INT, "0", RECU_RESTART_TS, RR_NULL, RECC_INT, "[0-1]", RECA_READ_ONLY}
   ,
+  {RECT_CONFIG, "proxy.config.exec_thread.loop_time_update_probability", RECD_INT, "10", RECU_DYNAMIC, RR_NULL, RECC_INT, "[0-100]", RECA_NULL}
+  ,
   {RECT_CONFIG, "proxy.config.accept_threads", RECD_INT, "1", RECU_RESTART_TS, RR_NULL, RECC_INT, "[0-" TS_STR(TS_MAX_NUMBER_EVENT_THREADS) "]", RECA_READ_ONLY}
   ,
   {RECT_CONFIG, "proxy.config.task_threads", RECD_INT, "2", RECU_RESTART_TS, RR_NULL, RECC_INT, "[1-" TS_STR(TS_MAX_NUMBER_EVENT_THREADS) "]", RECA_READ_ONLY}