Hong Zhiguo created YARN-2768:

             Summary: optimize FSAppAttempt.updateDemand by avoid clone of 
Resource which takes 85% of computing time of update thread
                 Key: YARN-2768
                 URL: https://issues.apache.org/jira/browse/YARN-2768
             Project: Hadoop YARN
          Issue Type: Improvement
          Components: fairscheduler
            Reporter: Hong Zhiguo
            Assignee: Hong Zhiguo
            Priority: Minor

See the attached picture of profiling result. The clone of Resource object 
within Resources.multiply() takes up **85%** (19.2 / 22.6) CPU time of the 
function FairScheduler.update().

The code of FSAppAttempt.updateDemand:
public void updateDemand() {
    demand = Resources.createResource(0);
    // Demand is current consumption plus outstanding requests
    Resources.addTo(demand, app.getCurrentConsumption());

    // Add up outstanding resource requests
    synchronized (app) {
      for (Priority p : app.getPriorities()) {
        for (ResourceRequest r : app.getResourceRequests(p).values()) {
          Resource total = Resources.**multiply**(r.getCapability(), 
          Resources.addTo(demand, total);

The code of Resources.multiply:
public static Resource multiply(Resource lhs, double by) {
    return multiplyTo(**clone**(lhs), by);

The clone could be skipped by directly update the value of this.demand.

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