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From "Tom White (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-371) Resource-centric compression in AM-RM protocol limits scheduling
Date Wed, 06 Feb 2013 12:51:13 GMT

    [ https://issues.apache.org/jira/browse/YARN-371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13572374#comment-13572374
] 

Tom White commented on YARN-371:
--------------------------------

Glad this turned into a good debate :)

On the task-centric vs resource-centric approach - I agree with Arun and Bobby's points. Supporting
two protocols is not to be done lightly, and we'd need a strong motivating use case. Perhaps
there is one, but the change may not be something we need to do before 2.0 goes GA.

The restrictions on how you specify locations for resource requests are interesting. You have
to say "node 1, rack 1, *" - it's not sufficient to say "node 1" or "rack 1" or even "node
1, rack 1". The schedulers use the * (ALL) request to calculate the total resources for an
application, but could they support the ones without ALL with no protocol changes? E.g. calculate
the total resources for an app by summing the lower-level requests? Perhaps this could be
explored in another JIRA.

More generally, the request styles that Bobby mentions like gang scheduling, any rack placement
(where containers are on the same rack for locality, but it doesn't matter which one), one-per
node placement (for e.g. HBase) - are all currently beyond the current system. How well YARN
supports non-MR workloads will be determined to some extent by how well it can support these
request styles. Perhaps we should simply make sure that the API is flexible enough to accommodate
changes. I see ResourceRequest is an abstract class so it would be possible to add a method
in the future in a compatible way to support optional extra information that the scheduler
might be able to use. Is that sufficient, or should some of the YARN APIs be downgraded from
@Stable to provide an option to change them to support alternative request styles in the 2.x
timeframe?
                
> Resource-centric compression in AM-RM protocol limits scheduling
> ----------------------------------------------------------------
>
>                 Key: YARN-371
>                 URL: https://issues.apache.org/jira/browse/YARN-371
>             Project: Hadoop YARN
>          Issue Type: Improvement
>          Components: api, resourcemanager, scheduler
>    Affects Versions: 2.0.2-alpha
>            Reporter: Sandy Ryza
>            Assignee: Sandy Ryza
>
> Each AMRM heartbeat consists of a list of resource requests. Currently, each resource
request consists of a container count, a resource vector, and a location, which may be a node,
a rack, or "*". When an application wishes to request a task run in multiple localtions, it
must issue a request for each location.  This means that for a node-local task, it must issue
three requests, one at the node-level, one at the rack-level, and one with * (any). These
requests are not linked with each other, so when a container is allocated for one of them,
the RM has no way of knowing which others to get rid of. When a node-local container is allocated,
this is handled by decrementing the number of requests on that node's rack and in *. But when
the scheduler allocates a task with a node-local request on its rack, the request on the node
is left there.  This can cause delay-scheduling to try to assign a container on a node that
nobody cares about anymore.
> Additionally, unless I am missing something, the current model does not allow requests
for containers only on a specific node or specific rack. While this is not a use case for
MapReduce currently, it is conceivable that it might be something useful to support in the
future, for example to schedule long-running services that persist state in a particular location,
or for applications that generally care less about latency than data-locality.
> Lastly, the ability to understand which requests are for the same task will possibly
allow future schedulers to make more intelligent scheduling decisions, as well as permit a
more exact understanding of request load.
> I would propose the tweak of allowing a single ResourceRequest to encapsulate all the
location information for a task.  So instead of just a single location, a ResourceRequest
would contain an array of locations, including nodes that it would be happy with, racks that
it would be happy with, and possibly *.  Side effects of this change would be a reduction
in the amount of data that needs to be transferred in a heartbeat, as well in as the RM's
memory footprint, becaused what used to be different requests for the same task are now able
to share some common data.
> While this change breaks compatibility, if it is going to happen, it makes sense to do
it now, before YARN becomes beta.

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