[tests] Use memory diffs from baseline in observer helper module stats (#668)

Absolute values of RSS memory are not really useful as the setup may be
different before the observer is run so we now initialize these value
when the observer is created and we only record the relative difference
of the context-managed block.

Author: sgnn7

tests/performance/test_statsd_throughput.py

@@ -108,7 +108,7 @@ class TestDogStatsdThroughput(unittest.TestCase):
 
     RUN_MESSAGE = (
         "Run #{:2d}/{:2d}: {:.4f}s (latency: {:.2f}μs, cpu: {:.4f},"
-        + " mem.rss: {:.0f}kb, recv: {:.2f}%)"
+        + " mem.rss_diff: {:.0f}kb, recv: {:.2f}%)"
     )
 
     def setUp(self):
@@ -192,22 +192,22 @@ def test_statsd_performance(self):
                     duration,
                     avg_latency,
                     sys_stats["cpu.user"] + sys_stats["cpu.system"],
-                    sys_stats["mem.rss_kb"],
+                    sys_stats["mem.rss_diff_kb"],
                     received_packet_pct,
                 )
             )
 
             run_durations.append(duration)
             run_cpu_stats.append(sys_stats["cpu.user"] + sys_stats["cpu.system"])
-            run_memory_stats.append(sys_stats["mem.rss_kb"])
+            run_memory_stats.append(sys_stats["mem.rss_diff_kb"])
             run_latencies.append(float(avg_latency))
             received_packet_pcts.append(received_packet_pct)
 
         result_msg = "\nTotal for {} run(s), {} threads, {} points/thread via {}:\n"
         result_msg += "\tDuration:\t\t{:.4f}s\n"
         result_msg += "\tLatency:\t\t{:.2f}μs\n"
         result_msg += "\tCPU:\t\t\t{:.4f}\n"
-        result_msg += "\tMemory:\t\t\t{:.0f}kb\n"
+        result_msg += "\tMemory (rss) diff:\t{:.0f}kb\n"
         result_msg += "\tReceived packets:\t{:.2f}%"
         print(
             result_msg.format(

tests/util/system_info_observer.py

@@ -39,8 +39,11 @@ def __enter__(self):
         pid = os.getpid()
         self.proc_info = psutil.Process(pid)
 
+        # Record baselines
         self.initial_cpu_user = self.proc_info.cpu_times().user
         self.initial_cpu_system = self.proc_info.cpu_times().system
+        self.initial_mem_rss = self.proc_info.memory_full_info().rss
+        self.initial_mem_vms = self.proc_info.memory_full_info().vms
 
         self.observer_thread = Thread(
             name=self.__class__.__name__,
@@ -53,6 +56,8 @@ def __enter__(self):
         self.observer_thread.daemon = True
         self.observer_thread.start()
 
+        return self
+
     def __exit__(self, exception_type, exception_value, exception_traceback):
         self.exit.set()
         self.observer_thread.join()
@@ -64,8 +69,8 @@ def poll_system_info(self, proc_info, interval):
             mem_info = proc_info.memory_full_info()
             datapoint = {
                 "interval": interval,
-                "mem.rss_kb": mem_info.rss / 1024,
-                "mem.vms_kb": mem_info.vms / 1024,
+                "mem.rss_diff_kb": (mem_info.rss - self.initial_mem_rss) / 1024,
+                "mem.vms_diff_kb": (mem_info.vms- self.initial_mem_vms)  / 1024,
             }
 
             self._stats.append(datapoint)