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https://issues.apache.org/jira/browse/YARN-1198?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14063548#comment-14063548
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Jason Lowe commented on YARN-1198:
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bq. I think it would be worthwhile to do a min of the calculated headroom 
against "cluster headroom" as a sanity check, cluster headroom being the total 
cluster resource - utilized resources.

There are a couple of min's that need to be added to the current headroom 
calculation to catch some unsolved deadlock scenarios (again ignoring 
blacklisting effects):

- Need to min against the available resources in the current queue, otherwise 
we don't account for the resources consumed by other users in the queue.
- Need to min against the available resources in the parent queues (all the way 
up to the root queue), otherwise we don't account for the resources consumed 
elsewhere in the cluster.

The first min above solves the deadlock where two apps from different users but 
in the same queue completely exhaust the queue's limits but there is still 
available resources in the cluster for other queues.  I believe the patch for 
YARN-1857 is intended to correct that scenario.

For the second min, a min against the cluster avail resources is equivalent if 
there aren't any hierarchical queues, but it fails to prevent some deadlocks if 
those are employed.  We could have a parent queue that's not allowed to use all 
of the cluster resources and two leaf queues underneath that queue whose max 
cap each can be the entire parent queue.  Apps competing between those two leaf 
queues could completely saturate the parent queue and deadlock but there are 
still resources available at the cluster level.  I think YARN-2008 is related 
there.

In summary, I think we need to add in a min against the queue avail resources 
for all queues from the current leaf queue up to and including the root queue 
(i.e.: the whole cluster).

> Capacity Scheduler headroom calculation does not work as expected
> -----------------------------------------------------------------
>
>                 Key: YARN-1198
>                 URL: https://issues.apache.org/jira/browse/YARN-1198
>             Project: Hadoop YARN
>          Issue Type: Bug
>            Reporter: Omkar Vinit Joshi
>            Assignee: Omkar Vinit Joshi
>         Attachments: YARN-1198.1.patch
>
>
> Today headroom calculation (for the app) takes place only when
> * New node is added/removed from the cluster
> * New container is getting assigned to the application.
> However there are potentially lot of situations which are not considered for 
> this calculation
> * If a container finishes then headroom for that application will change and 
> should be notified to the AM accordingly.
> * If a single user has submitted multiple applications (app1 and app2) to the 
> same queue then
> ** If app1's container finishes then not only app1's but also app2's AM 
> should be notified about the change in headroom.
> ** Similarly if a container is assigned to any applications app1/app2 then 
> both AM should be notified about their headroom.
> ** To simplify the whole communication process it is ideal to keep headroom 
> per User per LeafQueue so that everyone gets the same picture (apps belonging 
> to same user and submitted in same queue).
> * If a new user submits an application to the queue then all applications 
> submitted by all users in that queue should be notified of the headroom 
> change.
> * Also today headroom is an absolute number ( I think it should be normalized 
> but then this is going to be not backward compatible..)
> * Also  when admin user refreshes queue headroom has to be updated.
> These all are the potential bugs in headroom calculations



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