[YARN-4039] New AM instances waste resource by waiting only for resource availability when all available resources are already used

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.4.0, 2.5.0, 2.6.0, 2.7.0
• Component/s: fairscheduler
• Assignee: Tsuyoshi Ozawa
• Reporter: Sadayuki Furuhashi
• Created: 10/Aug/15 07:12
• Updated: 28/Oct/16 00:42

Description

Problem:
In FairScheduler, maxRunningApps doesn't work well if we can't predict size of an application in a queue because small maxRunningApps can't use all resources if many small applications are issued, while large maxRunningApps wastes resources if large applications run.

Background:
We're using FairScheduler. In following scenario, AM instances wastes resources significantly:

• A queue has X MB of capacity.
• An application requests 32 containers where a container requires (X / 32) MB of memory
  • In this case, a single application occupies entire resource of the queue.
• Many those applications are issued (10 applications)
• Ideal behavior is that applications run one by one to maximize throughput.
• However, all applications run simultaneously. As the result, AM instances occupy resources and prevent other tasks from starting. At worst case, most of resources are occupied by waiting AMs and applications progress very slowly.

A solution is setting maxRunningApps to 1 or 2. However, it doesn't work well if following workload exists at the same queue:

• An application requests 2 containers where a container requires (X / 32) MB of memory
• Many those applications are issued (say, 10 applications)
• Ideal behavior is that all applications run simultaneously to maximize concurrency and throughput.
• However, number of applications are limited by maxRunningApps. At worst case, most of resources are idling.

This problem happens especially with Hive because we can't estimate size of a MapReduce application.

Solution:
AM doesn't have to start if there are waiting resource requests because the AM can't grant resource requests even if it starts.

Patch:
I attached a patch that implements this behavior. But this implementation has this trade-off:

• When AM is registered to FairScheduler, its demand is 0 because even AM attempt is not created. Starting this AM doesn't change resource demand of a queue. So, if many AMs are issued to a queue at the same time, all AMs will be RUNNING. But we want to prevent it.
• When a AM starts, demand of the AM is only AM attempt. Then AM requires more resources. Until AM requires resources, demand of the queue is low. But starting AM during this time will start unnecessary AMs.
• So, this patch doesn't start immediately when AM is registered. Instead, it starts AM only every continuous-scheduling-sleep-ms.
• Setting large continuous-scheduling-sleep-ms will prevent wasting AMs. But increases latency.

Therefore, this patch is enabled only if new option "demand-blocks-am-enabled" is true.

Issue Links

• is related to: YARN-624 Support gang scheduling in the AM RM protocol