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