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https://issues.apache.org/jira/browse/IMPALA-8677?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Joe McDonnell resolved IMPALA-8677.
-----------------------------------
Resolution: Fixed
Fix Version/s: Impala 3.3.0
> Removing an unused node does not leave consistent remote scheduling unchanged
> -----------------------------------------------------------------------------
>
> Key: IMPALA-8677
> URL: https://issues.apache.org/jira/browse/IMPALA-8677
> Project: IMPALA
> Issue Type: Bug
> Components: Backend
> Affects Versions: Impala 3.2.0
> Reporter: Joe McDonnell
> Assignee: Joe McDonnell
> Priority: Major
> Fix For: Impala 3.3.0
>
>
> When working on IMPALA-8630, I discovered that
> SchedulerTest::RemoteExecutorCandidateConsistency works mostly by
> happenstance.
> The root of the issue is that in Scheduler::GetRemotExecutorCandidates() we
> want to avoid returning duplicates and put all the IpAddrs in a set:
> {code:java}
> set<IpAddr> distinct_backends;
> ...
> distinct_backends.insert(*executor_addr);
> ...
> for (const IpAddr& addr : distinct_backends) {
> remote_executor_candidates->push_back(addr);
> }{code}
> This sorts the IpAddrs, and the remote_executor_candidates does not return
> elements in the order in which they are encountered.
> Suppose that we are running with num_remote_executor_candidates=2 and random
> replicas is false. There is exactly one file. GetRemoteExecutorCandidates()
> returns these executor candidates (IpAddrs):
> {192.168.1.2, 192.168.1.3}
> The first entry is chosen because it is first. Nothing was scheduled on
> 192.168.1.3, but removing it may change the scheduling outcome. This is
> because of the sort. Suppose 192.168.1.3 is gone, but the next closest
> executor is 192.168.1.1 (or some node less than 192.168.1.2). Even though it
> is farther in the context of the hashring, GetRemoteExecutorCandidates()
> would return:
> {192.168.1.1, 192.168.1.2}
> and the first entry would be chosen.
> To eliminate this inconsistency, it might be useful to retain the order in
> which elements match via the hashring.
> In terms of impact, this would increase the number of files that would
> potentially change scheduling when a node leaves. It might have unnecessary
> changes. If using random replica set to true, it doesn't matter. It is
> unclear how much this would impact otherwise.
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