Github user jerryshao commented on a diff in the pull request:
https://github.com/apache/spark/pull/18711#discussion_r129245415
--- Diff: core/src/main/scala/org/apache/spark/deploy/master/Master.scala
---
@@ -580,7 +580,13 @@ private[deploy] class Master(
* The number of cores assigned to each executor is configurable. When
this is explicitly set,
* multiple executors from the same application may be launched on the
same worker if the worker
* has enough cores and memory. Otherwise, each executor grabs all the
cores available on the
- * worker by default, in which case only one executor may be launched on
each worker.
+ * worker by default, in which case only one executor may be launched on
each worker during one
+ * single schedule iteration.
+ * Note that when `spark.executor.cores` is not set, we may still launch
multiple executors from
+ * the same application on the same worker. Consider appA and appB both
have one executor running
+ * on worker1, and appA.coresLeft > 0, then appB is finished and release
all its cores on worker1,
+ * thus for the next schedule iteration, appA launchs a new executor
that grabs all the free cores
+ * on worker1, therefore we get mulfiple executors from appA running on
worker1.
--- End diff --
nit: multiple.
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