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https://issues.apache.org/jira/browse/IMPALA-8685?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16869711#comment-16869711
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Michael Ho commented on IMPALA-8685:
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Yes, I agree that setting NUM_REMOTE_EXECUTOR_CANDIDATES to anything other than
1 will definitely reduce the effectiveness of the cache in general. I believe I
need to understand the issue of skew and scheduling in general better and see
if there are workarounds than setting it to 3.
> Evaluate default configuration of NUM_REMOTE_EXECUTOR_CANDIDATES
> ----------------------------------------------------------------
>
> Key: IMPALA-8685
> URL: https://issues.apache.org/jira/browse/IMPALA-8685
> Project: IMPALA
> Issue Type: Improvement
> Components: Backend
> Reporter: Michael Ho
> Priority: Critical
>
> The query option {{NUM_REMOTE_EXECUTOR_CANDIDATES}} is set to 3 by default.
> This means that there are potentially 3 different executors which can process
> a remote scan range. Over time, the data of a given remote scan range will be
> spread across these 3 executors. My understanding of why this is not set to 1
> is to avoid hot spots in pathological cases. On the other hand, this may mean
> that we may not maximize the utilization of the file handle cache and data
> cache. Also, for small clusters (e.g. a 3 node cluster), the default value
> may render deterministic remote scan range scheduling ineffective. We may
> want to re-evaluate the default value of {{NUM_REMOTE_EXECUTOR_CANDIDATES}}.
> One idea is to set it to min(3, half of cluster size) so it works okay with
> small cluster, which may be rather common for demo purposes. There may also
> be other criteria for evaluating the default value.
> cc'ing [~joemcdonnell], [~tlipcon] and [~drorke]
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