ScheduledThreadPoolExecutor$DelayedWorkQueue are filling with cancelled FutureTasks

Environment

• Java

Issue

• We see a large amount of cancelled tasks or runnables sitting in a ScheduledThreadPoolExecutor$DelayedWorkQueue

Resolution

• On Java 7, you can use setRemoveOnCancelPolicy. Enabling this will let cancelled tasks be removed immediately.
• Otherwise, call purge periodically on the scheduled executor to force cancelled tasks to be cleaned up.

Root Cause

• Cancelled tasks are only removed from the executor's queue after their scheduled delay passes

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