[
https://issues.apache.org/jira/browse/IMPALA-8818?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16897333#comment-16897333
]
Sahil Takiar commented on IMPALA-8818:
--------------------------------------
CC: [[email protected]], [~kwho]
One thing I don't fully understand is if a max reservation needs to be set in
{{ResourceProfile}} for the {{PlanRootSink}}. As far as I can tell, most
{{PlanNodes}} leave this value as {{Long.MAX_VALUE}}
([link|https://github.com/apache/impala/blob/b587751e72c6d1ac1836edbae7d82e9110ca6574/fe/src/main/java/org/apache/impala/planner/ResourceProfileBuilder.java#L53]).
Or does the max reservation already get constrained by the query mem limit?
Just want to make sure that result spooling does not somehow take up more
resources than necessary.
> Replace deque queue with spillable queue in BufferedPlanRootSink
> ----------------------------------------------------------------
>
> Key: IMPALA-8818
> URL: https://issues.apache.org/jira/browse/IMPALA-8818
> Project: IMPALA
> Issue Type: Sub-task
> Components: Backend
> Reporter: Sahil Takiar
> Assignee: Sahil Takiar
> Priority: Major
>
> Add a {{SpillableRowBatchQueue}} to replace the {{DequeRowBatchQueue}} in
> {{BufferedPlanRootSink}}. The {{SpillableRowBatchQueue}} will wrap a
> {{BufferedTupleStream}} and take in a {{TBackendResourceProfile}} created by
> {{PlanRootSink#computeResourceProfile}}.
> *BufferedTupleStream Usage*:
> The wrapped {{BufferedTupleStream}} should be created in 'attach_on_read'
> mode so that pages are attached to the output {{RowBatch}} in
> {{BufferedTupleStream::GetNext}}. The BTS should start off as pinned (e.g.
> all pages are pinned). If a call to {{BufferedTupleStream::AddRow}} returns
> false (it returns false if "the unused reservation was not sufficient to add
> a new page to the stream large enough to fit 'row' and the stream could not
> increase the reservation to get enough unused reservation"), it should unpin
> the stream ({{BufferedTupleStream::UnpinStream}}) and then add the row (if
> the row still could not be added, then an error must have occurred, perhaps
> an IO error, in which case return the error and fail the query).
> *Constraining Resources*:
> When result spooling is disabled, a user can run a {{select * from
> [massive-fact-table]}} and scroll through the results without affecting the
> health of the Impala cluster (assuming they close they query promptly).
> Impala will stream the results one batch at a time to the user.
> With result spooling, a naive implementation might try and buffer the enter
> fact table, and end up spilling all the contents to disk, which can
> potentially take up a large amount of space. So there needs to be
> restrictions on the memory and disk space used by the {{BufferedTupleStream}}
> in order to ensure a scan of a massive table does not consume all the memory
> or disk space of the Impala coordinator.
> This problem can be solved by placing a max size on the amount of unpinned
> memory (perhaps through a new config option
> {{MAX_PINNED_RESULT_SPOOLING_MEMORY}} (maybe set to a few GBs by default).
> The max amount of pinned memory should already be constrained by the
> reservation (see next paragraph). NUM_ROWS_PRODUCED_LIMIT already limits the
> number of rows returned by a query, and so it should limit the number of rows
> buffered by the BTS as well (although it is set to 0 by default).
> SCRATCH_LIMIT already limits the amount of disk space used for spilling
> (although it is set to -1 by default).
> The {{PlanRootSink}} should attempt to accurately estimate how much memory it
> needs to buffer all results in memory. This requires setting an accurate
> value of {{ResourceProfile#memEstimateBytes_}} in
> {{PlanRootSink#computeResourceProfile}}. If statistics are available, the
> estimate can be based on the number of estimated rows returned multiplied by
> the size of the rows returned. The min reservation should account for a read
> and write page for the {{BufferedTupleStream}}.
--
This message was sent by Atlassian JIRA
(v7.6.14#76016)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
