[polymer] Merge of master into polymer10-migration

devil/docs/persistent_device_list.md

+<!-- Copyright 2016 The Chromium Authors. All rights reserved.
+     Use of this source code is governed by a BSD-style license that can be
+     found in the LICENSE file.
+-->
+
+# Devil: Persistent Device List
+
+## What is it?
+
+A persistent device list that stores all expected devices between builds. It
+is used by the perf test runner in order to properly shard tests by device
+affinity. This is important because the same performance test can yield
+meaningfully different results when run on different devices.
+
+## Bots
+
+The list is usually located at one of these locations:
+
+  - `/b/build/site_config/.known_devices`.
+  - `~/.android`.
+
+Look at recipes listed below in order to find more up to date location.
+
+## Local Runs
+
+The persistent device list is unnecessary for local runs. It is only used on the
+bots that upload data to the perf dashboard.
+
+## Where it is used
+
+The persistent device list is used in performance test recipes via
+[api.chromium\_tests.steps.DynamicPerfTests](https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/chromium_tests/steps.py?q=DynamicPerfTests).
+For example, the [android/perf](https://cs.chromium.org/chromium/build/scripts/slave/recipes/android/perf.py) recipe uses it like this:
+
+```python
+dynamic_perf_tests = api.chromium_tests.steps.DynamicPerfTests(
+    builder['perf_id'], 'android', None,
+    known_devices_file=builder.get('known_devices_file', None))
+dynamic_perf_tests.run(api, None)
+```
+