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From "Steven Rand (JIRA)" <j...@apache.org>
Subject [jira] [Created] (YARN-7391) Consider square root instead of natural log for size-based weight
Date Wed, 25 Oct 2017 08:12:02 GMT
Steven Rand created YARN-7391:
---------------------------------

             Summary: 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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