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From "Daniel Templeton (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (YARN-7391) Consider square root instead of natural log for size-based weight
Date Wed, 25 Oct 2017 17:58:00 GMT

    [ https://issues.apache.org/jira/browse/YARN-7391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16219191#comment-16219191
] 

Daniel Templeton edited comment on YARN-7391 at 10/25/17 5:57 PM:
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Do you have a particular scenario where the log-based weight isn't given reasonable results?
 My concern would be that a sqrt-based weight would allow a big-memory app to starve out smaller
apps.  Without a motivating issue, I'm not super excited about changing this part of the code.

On a tangentially related note, I was profiling the scheduler yesterday and noticed that we
spend a ton of our scheduling time waiting for the lock in this method.  Looks like a good
candidate for caching in {{FSAppAttempt}}.


was (Author: templedf):
Do you have a particular scenario where the log-based weight isn't given reasonable results?
 My concern would be that a sqrt-based weight would allow a big-memory app to starve out smaller
apps.  Without a motivating issue, I'm not super excited about changing this part of the code.

On a tangentially related note, I was profiling the scheduler yesterday and noticed that we
spend a ton on our scheduling time waiting for the lock in this method.  Looks like a good
candidate for caching in {{FSAppAttempt}}.

> Consider square root instead of natural log for size-based weight
> -----------------------------------------------------------------
>
>                 Key: YARN-7391
>                 URL: https://issues.apache.org/jira/browse/YARN-7391
>             Project: Hadoop YARN
>          Issue Type: Improvement
>          Components: fairscheduler
>    Affects Versions: 3.0.0-beta1
>            Reporter: Steven Rand
>
> Currently for size-based weight, we compute the weight of an app using this code from
https://github.com/apache/hadoop/blob/trunk/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FairScheduler.java#L377:
> {code}
>       if (sizeBasedWeight) {
>         // Set weight based on current memory demand
>         weight = Math.log1p(app.getDemand().getMemorySize()) / Math.log(2);
>       }
> {code}
> Because the natural log function grows slowly, the weights of two apps with hugely different
memory demands can be quite similar. For example, {{weight}} evaluates to 14.3 for an app
with a demand of 20 GB, and evaluates to 19.9 for an app with a demand of 1000 GB. The app
with the much larger demand will still have a higher weight, but not by a large amount relative
to the sum of those weights.
> I think it's worth considering a switch to a square root function, which will grow more
quickly. In the above example, the app with a demand of 20 GB now has a weight of 143, while
the app with a demand of 1000 GB now has a weight of 1012. These weights seem more reasonable
relative to each other given the difference in demand between the two apps.
> The above example is admittedly a bit extreme, but I believe that a square root function
would also produce reasonable results in general.
> The code I have in mind would look something like:
> {code}
>       if (sizeBasedWeight) {
>         // Set weight based on current memory demand
>         weight = Math.sqrt(app.getDemand().getMemorySize());
>       }
> {code}
> Would people be comfortable with this change?



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