[MAPREDUCE-4714] Historyserver retrieves job from jobID more often than necessary

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 0.23.3, 2.0.0-alpha
• Component/s: jobhistoryserver, mrv2
• Assignee: Unassigned
• Reporter: Jason Darrell Lowe
• Created: 08/Oct/12 20:18
• Updated: 08/Oct/12 20:18

Description

When serving up a web page for job history, AppController.requireJob() is converting from a jobID to a job twice when it only needs to do it once. The resulting job is stored into the app object, which we could, in turn, use in historyserver web page render() methods to simply retrieve the job rather than performing yet another jobID->job lookup. That would cut the job lookups down from 3 to 1.

jobID->job lookups aren't necessarily cheap. If the job isn't in the historyserver joblist cache then the historyserver needs to scan directories looking for it, adding an unnecessary extra load onto the namenode.