Improve active/idle cycle time tracking

Author: cstlee

src/Perf.h

@@ -17,6 +17,7 @@
 #define HOMA_PERF_H
 
 #include <Homa/Perf.h>
+#include <PerfUtils/Cycles.h>
 
 #include <atomic>
 
@@ -225,6 +226,34 @@ struct ThreadCounters : public Counters {
  */
 extern thread_local ThreadCounters counters;
 
+/**
+ * Provides a convenient way to measure multiple consecutive cycle time
+ * intervals.
+ */
+class Timer {
+  public:
+    /**
+     * Construct a new uninitialized Timer.
+     */
+    Timer()
+        : split_tsc(0)
+    {}
+
+    /**
+     * Return the number of cycles since the last time split was called.
+     */
+    inline uint64_t split()
+    {
+        uint64_t prev_tsc = split_tsc;
+        split_tsc = PerfUtils::Cycles::rdtsc();
+        return split_tsc - prev_tsc;
+    }
+
+  private:
+    /// Cycle time that split was last called.
+    uint64_t split_tsc;
+};
+
 }  // namespace Perf
 }  // namespace Homa
 

src/TransportImpl.cc

@@ -93,13 +93,16 @@ TransportImpl::poll()
 void
 TransportImpl::processPackets()
 {
-    uint64_t start_tsc = PerfUtils::Cycles::rdtsc();
-    bool idle = true;
+    // Keep track of time spent doing active processing versus idle.
+    Perf::Timer activityTimer;
+    activityTimer.split();
+    uint64_t activeTime = 0;
+    uint64_t idleTime = 0;
+
     const int MAX_BURST = 32;
     Driver::Packet* packets[MAX_BURST];
     int numPackets = driver->receivePackets(MAX_BURST, packets);
     for (int i = 0; i < numPackets; ++i) {
-        idle = false;
         Driver::Packet* packet = packets[i];
         assert(packet->length >=
                Util::downCast<int>(sizeof(Protocol::Packet::CommonHeader)));
@@ -140,13 +143,12 @@ TransportImpl::processPackets()
                 sender->handleErrorPacket(packet, driver);
                 break;
         }
+        activeTime += activityTimer.split();
     }
-    uint64_t elapsed_cycles = PerfUtils::Cycles::rdtsc() - start_tsc;
-    if (!idle) {
-        Perf::counters.active_cycles.add(elapsed_cycles);
-    } else {
-        Perf::counters.idle_cycles.add(elapsed_cycles);
-    }
+    idleTime += activityTimer.split();
+
+    Perf::counters.active_cycles.add(activeTime);
+    Perf::counters.idle_cycles.add(idleTime);
 }
 
 }  // namespace Core