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https://issues.apache.org/jira/browse/IMPALA-8685?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16869757#comment-16869757
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Michael Ho edited comment on IMPALA-8685 at 6/21/19 7:26 PM:
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Yes, we can definitely see that in the profile if one were to look at the bytes
scanned in the scan node instances and we can treat the query option as a
workaround instead of penalizing the cache effectiveness by default. Also, the
metrics {{impala-server.io-mgr.bytes-read}} can definitely show IO skew among
executors.
I guess what I am trying to understand is in a workload in which all scan
ranges are remote, will the hash ring provide enough randomness / fairness that
the skew is not an issue ? Would there be unexpected side effect if we mix
remote / local scan ranges and can they be avoided ? Just thinking out aloud
here but I should probably go look at the code more :-).
was (Author: kwho):
Yes, we can definitely see that in the profile if one were to look at the bytes
scanned in the scan node instances and we can treat the query option as a
workaround instead of penalizing the cache effectiveness by default.
I guess what I am trying to understand is in a workload in which all scan
ranges are remote, will the hash ring provide enough randomness / fairness that
the skew is not an issue ? Would there be unexpected side effect if we mix
remote / local scan ranges and can they be avoided ? Just thinking out aloud
here but I should probably go look at the code more :-).
> 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
> Assignee: Joe McDonnell
> 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 possible 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.
> However, it doesn't address the problem of cache effectiveness in larger
> clusters as the footprint of the cache is still amplified by
> {{NUM_REMOTE_EXECUTOR_CANDIDATES}}. There may also be other criteria for
> evaluating the default value.
> cc'ing [~joemcdonnell], [~tlipcon] and [~drorke]
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