[ https://issues.apache.org/jira/browse/YARN-6163?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15862041#comment-15862041 ] 

ASF GitHub Bot commented on YARN-6163:
--------------------------------------

Github user templedf commented on a diff in the pull request:

    https://github.com/apache/hadoop/pull/192#discussion_r100640868
  
    --- Diff: hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSAppAttempt.java ---
    @@ -1106,6 +1111,97 @@ boolean isStarvedForFairShare() {
         return !Resources.isNone(fairshareStarvation);
       }
     
    +  /**
    +   * Helper method for {@link #getStarvedResourceRequests()}:
    +   * Given a map of visited {@link ResourceRequest}s, it checks if
    +   * {@link ResourceRequest} 'rr' has already been visited. The map is updated
    +   * to reflect visiting 'rr'.
    +   */
    +  private static boolean checkAndMarkRRVisited(
    +      Map<Priority, List<Resource>> visitedRRs, ResourceRequest rr) {
    +    Priority priority = rr.getPriority();
    +    Resource capability = rr.getCapability();
    +    if (visitedRRs.containsKey(priority)) {
    +      List<Resource> rrList = visitedRRs.get(priority);
    +      if (rrList.contains(capability)) {
    +        return true;
    +      } else {
    +        rrList.add(capability);
    +        return false;
    +      }
    +    } else {
    +      List<Resource> newRRList = new ArrayList<>();
    +      newRRList.add(capability);
    +      visitedRRs.put(priority, newRRList);
    +      return false;
    +    }
    +  }
    +
    +  /**
    +   * Fetch a list of RRs corresponding to the extent the app is starved
    +   * (fairshare and minshare). This method considers the number of containers
    +   * in a RR and also only one locality-level (the first encountered
    +   * resourceName).
    +   *
    +   * @return list of {@link ResourceRequest}s corresponding to the amount of
    +   * starvation.
    +   */
    +  List<ResourceRequest> getStarvedResourceRequests() {
    +    List<ResourceRequest> ret = new ArrayList<>();
    +    Map<Priority, List<Resource>> visitedRRs= new HashMap<>();
    +
    +    Resource pending = getStarvation();
    +    for (ResourceRequest rr : appSchedulingInfo.getAllResourceRequests()) {
    +      if (Resources.isNone(pending)) {
    +        break;
    +      }
    +      if (checkAndMarkRRVisited(visitedRRs, rr)) {
    +        continue;
    +      }
    +
    +      // Compute the number of containers of this capability that fit in the
    +      // pending amount
    +      int ratio = (int) Math.floor(
    --- End diff --
    
    This is super obtuse logic.  Please document it thoroughly.


> FS Preemption is a trickle for severely starved applications
> ------------------------------------------------------------
>
>                 Key: YARN-6163
>                 URL: https://issues.apache.org/jira/browse/YARN-6163
>             Project: Hadoop YARN
>          Issue Type: Sub-task
>          Components: fairscheduler
>    Affects Versions: 2.9.0
>            Reporter: Karthik Kambatla
>            Assignee: Karthik Kambatla
>         Attachments: yarn-6163-1.patch
>
>
> With current logic, only one RR is considered per each instance of marking an application starved. This marking happens only on the update call that runs every 500ms.  Due to this, an application that is severely starved takes forever to reach fairshare based on preemptions.



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