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[Björn Tillenius <mp+435189@xxxxxxxxxxxxxxxxxx>]

Björn Tillenius has proposed merging ~bjornt/maas:perftests-no-gc-collect-noise into maas:master.

Commit message:
Control the garbage collection for the performance tests.

Before, one test could create a lot of garbage that then got
collected while running another test. That made the second test
look slower than it actually was.

Now we collect the garbage before a test being run, and also
collect it after the test is being run, including the last collection
in the timing of the test.



Requested reviews:
  Alberto Donato (ack)

For more details, see:
https://code.launchpad.net/~bjornt/maas/+git/maas/+merge/435189
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diff --git a/src/maastesting/pytest/perftest.py b/src/maastesting/pytest/perftest.py
index 7a9ce9b..491f889 100644
--- a/src/maastesting/pytest/perftest.py
+++ b/src/maastesting/pytest/perftest.py
@@ -6,6 +6,7 @@
 
 from contextlib import contextmanager, ExitStack
 from cProfile import Profile
+import gc
 import json
 import os
 import sys
@@ -77,8 +78,18 @@ class Timing:
 
 @contextmanager
 def measure_time():
+    # Collect all the garbage before the timing begins,
+    # so that collection of unrelated garbage won't slow
+    # things down.
+    gc.collect()
     timing = Timing()
     yield timing
+    # Collect the garbage that was created by the code that is
+    # being timed, so that we get a more consistent timing.
+    # Otherwise, a small change to the code we time could cause
+    # a big change in time due to a new garbage collection being
+    # triggered.
+    gc.collect()
     timing.stop()