Bug 1633866 - [taskgraph] Truncate runtimes that exceed DYNAMIC_CHUNK_DURATION so they don't artificially increase chunks, r=gbrown

For example, assume we have three manifests:

foo -> 98 minutes
bar -> 1 minute
baz -> 1 minute

And let's say we want average chunk lengths of ~20 minutes. The calculation
would currently say we need 5 chunks ((98 + 1 + 1) / 20). Even if set the max
chunks to the # of manifests, it would still return 3. Clearly this only needs
at most 2 chunks to run.

By truncating the runtime, we'd have:

foo -> 20 minutes (truncated)
bar -> 1 minute
baz -> 1 minute

So we'd actually only assign one chunk (round((20 + 1 + 1) / 20)). Despite
having a single 100 minute chunk, this outcome is desirable as we now avoid the
overhead of a second chunk that only runs 2 minutes worth of tests.

Depends on D77871

Differential Revision: https://phabricator.services.mozilla.com/D77873

taskcluster/taskgraph/transforms/tests.py

@@ -1442,7 +1442,10 @@ def resolve_dynamic_chunks(config, tasks):
         runtimes = {m: r for m, r in get_runtimes(task['test-platform'], task['suite']).items()
                     if m in task['test-manifests']['active']}
 
-        times = list(runtimes.values())
+        # Truncate runtimes that are above the desired chunk duration. They
+        # will be assigned to a chunk on their own and the excess duration
+        # shouldn't cause additional chunks to be needed.
+        times = [min(DYNAMIC_CHUNK_DURATION, r) for r in runtimes.values()]
         avg = round(sum(times) / len(times), 2) if times else 0
         total = sum(times)
 
@@ -1451,7 +1454,11 @@ def resolve_dynamic_chunks(config, tasks):
         missing = [m for m in task['test-manifests']['active'] if m not in runtimes]
         total += avg * len(missing)
 
-        task['chunks'] = int(round(total / DYNAMIC_CHUNK_DURATION)) or 1
+        chunks = int(round(total / DYNAMIC_CHUNK_DURATION))
+
+        # Make sure we never exceed the number of manifests, nor have a chunk
+        # length of 0.
+        task['chunks'] = min(chunks, len(task['test-manifests']['active'])) or 1
         yield task