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From "Michael Ho (Code Review)" <>
Subject [Impala-ASF-CR] IMPALA-2831: Bound the number of scanner threads per scan node.
Date Wed, 31 Aug 2016 02:18:56 GMT
Hello Dan Hecht,

I'd like you to reexamine a change.  Please visit

to look at the new patch set (#5).

Change subject: IMPALA-2831: Bound the number of scanner threads per scan node.

IMPALA-2831: Bound the number of scanner threads per scan node.

Our current code base allows a scan node to spin up as many as
3x the number of logical cpu cores of scanner threads. However,
the scanner threads are cpu bound so there is diminishing return
for starting more scanner threads than the number of logical cores.
In fact, it may be detrimental due to context switching overhead.

This change bounds the number of scanner threads spun up by a scan
node to the number of logical cpu cores unless the query option
'num_scanner_threads' is set. The total number of available thread
tokens is unchanged. With this change, the peak memory usage of the
following query on a single node Impala cluster running on a machine
with 8 logical cores reduces from 287MB to 101MB.

select count(*) from tpch100_parquet.lineitem where l_orderkey > 20

The reduction comes mostly from the fewer outstanding IO buffers.
The IO for scan ranges will be scheduled by the scanner threads
which pick them up. There will be at least an IO buffer of 8 to 16MB
associated with each scan range. So, more threads we start up,
more memory will be consumed by the IO buffers, leading to the
higher peak memory usages.

Change-Id: I191988ad18d6b4caf892fc967258823edcf9681f
M be/src/exec/
M be/src/exec/hdfs-scan-node.h
2 files changed, 23 insertions(+), 12 deletions(-)

  git pull ssh:// refs/changes/74/4174/5
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Gerrit-MessageType: newpatchset
Gerrit-Change-Id: I191988ad18d6b4caf892fc967258823edcf9681f
Gerrit-PatchSet: 5
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Michael Ho <>
Gerrit-Reviewer: Dan Hecht <>
Gerrit-Reviewer: Michael Ho <>
Gerrit-Reviewer: Tim Armstrong <>

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