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From "Karthik Kambatla (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-4039) New AM instances waste resource by waiting only for resource availability when all available resources are already used
Date Sat, 15 Aug 2015 08:42:46 GMT

    [ https://issues.apache.org/jira/browse/YARN-4039?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14698183#comment-14698183

Karthik Kambatla commented on YARN-4039:

maxRunningApps and maxAMShare are configs only to prevent the AMs from occupying the entire

Yarn is susceptible to the problem described in the description. On most clusters, this isn't
a huge problem because the resources used by the AMs are significantly smaller than the total
resources available on the cluster. However, this could be an issue on smaller clusters running
many applications. 

One could try using two queues, one each for the larger and smaller apps. The queue for the
smaller apps could use a FIFO policy so we run a single AM at any point in time. 

> New AM instances waste resource by waiting only for resource availability when all available
resources are already used
> -----------------------------------------------------------------------------------------------------------------------
>                 Key: YARN-4039
>                 URL: https://issues.apache.org/jira/browse/YARN-4039
>             Project: Hadoop YARN
>          Issue Type: Improvement
>          Components: fairscheduler
>    Affects Versions: 2.4.0, 2.5.0, 2.6.0, 2.7.0
>            Reporter: Sadayuki Furuhashi
>            Assignee: Sadayuki Furuhashi
>         Attachments: YARN-4039.1.patch, YARN-4039.2.patch
> 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
> 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
> Therefore, this patch is enabled only if new option "demand-blocks-am-enabled" is true.

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