master.chromium.perf.fyi: document buildbucket usage

masters/master.chromium.perf.fyi/master_site_config.py

Index: masters/master.chromium.perf.fyi/master_site_config.py
diff --git a/masters/master.chromium.perf.fyi/master_site_config.py b/masters/master.chromium.perf.fyi/master_site_config.py
index 735a83ea169f5282b111b2d5cb904d320f959746..6c308eb840ddd57b30b23ed68035aebc4eaa8580 100644
--- a/masters/master.chromium.perf.fyi/master_site_config.py
+++ b/masters/master.chromium.perf.fyi/master_site_config.py
@@ -13,5 +13,34 @@ class ChromiumPerfFyi(Master.Master1):
   master_port_alt = 8261
   buildbot_url = 'http://build.chromium.org/p/chromium.perf.fyi/'
   service_account_file = 'service-account-chromium.json'
+
+  # master.chromium.perf.fyi uses bulidbucket in an interesting way:
+  #
+  # master.chromium.perf has builders and testers, but master.chromium.perf.fyi
+  # has only testers. The former builds chromium and triggers testers on the FYI
+  #
+  # Some builders on master.chromium.perf run for each commit, and thus trigger
+  # builds on FYI for each commit. FYI does not have capacity to run a build
+  # for each commit, so it uses buildbot "merge requests" feature that collapses
+  # multiple build requests into a single build.
+  #
+  # The chromium.perf -> chromium.perf.fyi build triggering is implemented using
+  # buildbucket which does not support build request merging. As a result,
+  # buildbucket builds are converted to buildbot build requests, one buildbot
+  # build is run (with a proper blamelist) and then, due to the issue,
+  # only one buildbucket build is updated with the build result and the rest
+  # are marked cancelled.
+  #
+  # However, nothing consumes buildbucket build results, including the builds
+  # that trigger them. Only humans care about the triggered builds and they
+  # consume them by looking at the buildbot pages which don't have the issue,
+  # because the build requests are correctly scheduled, and the build with a
+  # correct blamelist is run. So, this is fine.
   buildbucket_bucket = 'master.chromium.perf.fyi'
+  # Buildbucket creates a "changes" table row for each change in each
+  # buildbucket build (unless global change id is specified; it is not), and
+  # buildbot console view does not like it. We can it avoid by telling
+  # buildbucket that all changes have unique URLs (because they are chromium
+  # commit URLs, see long comment above), so buildbucket reuses "changes" table
+  # rows created by gitiles poller.
   buildbucket_unique_change_urls = True