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From "Robert Joseph Evans (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-371) Resource-centric compression in AM-RM protocol limits scheduling
Date Wed, 06 Feb 2013 15:45:44 GMT

    [ https://issues.apache.org/jira/browse/YARN-371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13572512#comment-13572512
] 

Robert Joseph Evans commented on YARN-371:
------------------------------------------

I don't see any change like this happening in the 2.0 time frame.  Especially not a change
that is going to break backwards compatibility. I also agree with Tom completely that how
well YARN is adopted outside of Map/Reduce will be determined by how well it can *easily*
accommodate other computing/scheduling needs.  The current API can be used to support all
of the use cases I mentioned, but in a highly inefficient manor.  For gang scheduling I can
request all of the containers and just have them sitting waiting until the final one shows
up.  The others are similar, if not more wasteful.

I would encourage people interested in the scheduler or any other part of Hadoop to make a
prototype with a specific set of problems in mind.  The original problem on this JIRA was
delay scheduling was not having the desired impact on data locality, which may be a good enough
reason to make a protocol change.  After making the prototype measure the impact of your changes
and present those findings with the patch.  This will give people hard numbers to look at
and code that they can use to benchmark on other clusters.  The performance impact of a change
is rarely predictable, especially in very large distributed systems like Hadoop. Even on small
programs running on a single computer the rule is always measure change measure again.  Try
to stress your changes.  A change that speeds up a 100 node cluster is great, but I personally
don't run any 100 node clusters and if the change makes it worse for me to run a 4000 node
cluster churning through 90,000 apps a day I am likely to give it a -1 unless it can be fixed.
 I am already dealing with too many issues with YARN at that scale. But, if it solves a true
problem and does not make anything else worse I would be honored to be the one to check it
in.
                
> 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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