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From Piyush Narang <>
Subject Debugging slow Impala hdfs-scans
Date Wed, 06 Dec 2017 22:01:22 GMT
Hi folks,

I’m trying to debug why the performance of our new Impala setup is performing a bit worse
on the same queries as Presto. We’re running Impala 2.10.0 and starting it up with 225G
of memory (-mem-limit). It runs on the same set of nodes as Presto (4 nodes, 48 cores each,
10G ethernet). We noticed a couple of queries in Impala were fairly slow in the hdfs-scan
stage so we tried to isolate the behavior with a slightly simpler query:
select max(hour + nb_display) from my_large_parquet_table where day = '2017-10-04';

This query takes around 28-30 mins on Impala and seems to run in around 8.5 mins on Presto.
The input table is 11.64TB and uses Parquet (snappy compressed).

Looking at the performance profile (attached in case anyone’s interested), I noticed that
during the query Impala seems to be averaging around 1.1 - 1.2 MB/s per thread and the total
read throughput is around 2.13 MB/s. I tried bumping up the number of scanner threads (from
48 to 96) but that seemed to only help marginally (improved runtimes by ~10s). Running “sar
–n DEV 1” on some of our hosts while the query is running, seems to show that Impala is
reading at a rate of 30-50 MB/s (whereas we see this go up to 60-125 MB/s on our other runs).

We haven’t tweaked our Impala setup much beyond the defaults that come out of the box. I’m
wondering if I’m missing some tuning settings that help improve read rates from HDFS when
Impala is running outside of the datanodes. If anyone has any ideas / suggestions, they’d
be welcome. I am happy to provide more details if needed as well.


-- Piyush

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