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https://issues.apache.org/jira/browse/SPARK-23310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16351071#comment-16351071
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Nicolas Poggi commented on SPARK-23310:
---------------------------------------
[~sitalke...@gmail.com] we have found around 18% higher time in
[Q95|https://github.com/databricks/spark-sql-perf/blob/master/src/main/resources/tpcds_2_4/q95.sql]
of TPC-DS at least at scale factor 1000 (1TB). From 348.3. to 422.6s with the
setting ON. The regression is in {{{color:#24292e}ReadAheadInputStream.read}},
and {color}seems to happen when reading a small amounts of data due to checks
and locks. [~juliuszsompolski] can provide more internal details.
Overall for TPC-DS SF 1000, we don't see any significat improvement (over 5%)
with the feature ON in the rest of the queries. But of course other workloads
can probably improve.
> Perf regression introduced by SPARK-21113
> -----------------------------------------
>
> Key: SPARK-23310
> URL: https://issues.apache.org/jira/browse/SPARK-23310
> Project: Spark
> Issue Type: Bug
> Components: Spark Core
> Affects Versions: 2.3.0
> Reporter: Yin Huai
> Priority: Blocker
>
> While running all TPC-DS queries with SF set to 1000, we noticed that Q95
> (https://github.com/databricks/spark-sql-perf/blob/master/src/main/resources/tpcds_2_4/q95.sql)
> has noticeable regression (11%). After looking into it, we found that the
> regression was introduced by SPARK-21113. Specially, ReadAheadInputStream
> gets lock congestion. After setting
> spark.unsafe.sorter.spill.read.ahead.enabled set to false, the regression
> disappear and the overall performance of all TPC-DS queries has improved.
>
> I am proposing that we set spark.unsafe.sorter.spill.read.ahead.enabled to
> false by default for Spark 2.3 and re-enable it after addressing the lock
> congestion issue.
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