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From "Arun Suresh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-7783) Add validation step to ensure constraints are not violated due to order in which a request is processed
Date Wed, 24 Jan 2018 01:03:00 GMT

    [ https://issues.apache.org/jira/browse/YARN-7783?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16336695#comment-16336695

Arun Suresh commented on YARN-7783:

Yes, [~kkaranasos] also suggested validating implied constraints, at the time of placement.
But inferring the reverse cardinality constraint is not straight forward. "foo" max cardinality
of 3 with "bar" does not imply "bar" max cardinality of 3 with "foo" - Also it would be pretty
non-trivial to implement.

bq. The problem here is a circular dependency,
Can you provide an example / counter-example where the validation scheme proposed in this
patch will not work ?

Like I said, I am open to a better approach, but for the short term:
This patch handles most common cases, and given that - even though it will not give optimal
placements, it DOES guarantee constraints are NOT violated (which, without this patch does
not happen - which would be a bad first time user experience). And if it cant place, it will
notify the AM by rejecting the request.

Thoughts ?

> Add validation step to ensure constraints are not violated due to order in which a request
is processed
> -------------------------------------------------------------------------------------------------------
>                 Key: YARN-7783
>                 URL: https://issues.apache.org/jira/browse/YARN-7783
>             Project: Hadoop YARN
>          Issue Type: Sub-task
>            Reporter: Arun Suresh
>            Assignee: Arun Suresh
>            Priority: Blocker
>         Attachments: YARN-7783-YARN-6592.001.patch, YARN-7783-YARN-6592.002.patch, YARN-7783-YARN-6592.003.patch,
> When the algorithm has placed a container on a node, allocation tags are added to the
node if the constraint is satisfied, But depending on the order in which the algorithm sees
the request, it is possible that a constraint that happen to be valid during placement of
an earlier-seen request, might not be valid after all subsequent requests have been placed.
> For eg:
> Assume nodes n1, n2, n3, n4 and n5
> Consider the 2 constraints:
> # *foo* -> anti-affinity with *foo*
> # *bar* -> anti-affinity with *foo*
> And 2 requests
> # req1: NumAllocations = 4, allocTags = [foo]
> # req2: NumAllocations = 1, allocTags = [bar]
> If *req1* is seen first, the algorithm can place the 4 containers in n1, n2, n3 and n4.
And when it gets to *req2*, it will see that 4 nodes have the *foo* tag and will place it
on n5. But if *req2* is seen first, then *bar* tag will be placed on any node, since no node
will at that point have *foo*, and then when it gets to *req1*, since *foo* has no anti-affinity
with *bar*, the algorithm can end up placing *foo* on a node with *bar* violating the second
> To prevent the above, we need a validation step: after the placements for a batch of
requests are made, then for each req, we remove its tags from the node and try to see of constraints
are still satisfied if the tag were to be added back on the node.
> When applied to the example above, after the algorithm has run through *req2* and then
*req1*, we remove the *bar* tag from the node and try to add it back on the node. This time,
constraint satisfaction will fail, since there is now a *foo* tag on the node and *bar* cannot
be added. The algorithm will then retry placing *req2* on another node.

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