[YARN-4526] Make SystemClock singleton so AppSchedulingInfo could use it

• Type: Sub-task
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.8.0
• Fix Version/s: 2.9.0, 3.0.0-alpha1
• Component/s: scheduler
• Target Version/s: 2.9.0
• Hadoop Flags: Reviewed
• Assignee: Karthik Kambatla
• Reporter: Karthik Kambatla
• Created: 30/Dec/15 00:31
• Updated: 05/Oct/16 18:07
• Resolved: 18/Jan/16 10:01

Description

To track the time a request is received, we need to get current system time. For better testability of this, we are likely better off using a Clock instance that uses SystemClock by default. Instead of creating umpteen instances of SystemClock, we should just reuse the same instance.