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From "Wangda Tan (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 Mon, 22 Jan 2018 03:59:01 GMT

    [ https://issues.apache.org/jira/browse/YARN-7783?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16333846#comment-16333846

Wangda Tan commented on YARN-7783:

[~asuresh] ,

I think this is a known issue and we knew this from the beginning. This is bad for inter-application
affinity. Intra-application is better since it's easier to control requests within the same
app. A simpler way to fix this problem is restricting anti-affinity only to its own allocation
tags. To me, this is acceptable since we're implementing a subset of PlacementConstraint in
any case. 

Just looked at the patch, I'm a little worried about the change, this change looks pretty
intrusive. Changes like temp tag for the internal algorithm (including AppPlacementAllocator
should not be a part of AllocationTagsManager. AllocationTagsManager should restrict to allocated

> 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
> 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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