Document our policy about performance testing environment and epochs

Author: qinsoon

docs/team/ci.md

@@ -1,6 +1,6 @@
 # Continuous Integration
 
-## Testing
+## Correctness Testing
 
 MMTK core runs CI tests *before* a pull request is merged.
 
@@ -11,3 +11,13 @@ MMTk core sets up two sets of tests, the *minimal tests* and the *extended tests
 * Extended tests only run for a pull request if the pull request is tagged with the label `PR-extended-testing`. This set of tests
   may take hours, and usually include integration tests with bindings which run the language implementation's standard test suite
   as much as possible.
+
+## Performance Testing
+
+We conduct performance testing for each MMTk core commit after it has been merged.
+
+### Testing Environment and Epochs
+
+We track the performance of MMTk over years. Naturally, changes in the testing environment (hardware or software) and methodology are sometimes necessary. Each time we make such a change, it marks the start of a new *epoch* in our performance evaluation.
+
+Since changes in the testing environment can significantly impact performance, we do not directly compare performance results across different epochs. Within an epoch, we ensure that **MMTk does not experience performance regressions**, and **we only update the testing environment when there is no performance regression in the current epoch**.