I found a method in HashMapWrapper class . I think hive will use statistics
to adjust threshold automatically.
public static int calculateTableSize(
float keyCountAdj, int threshold, float loadFactor, long keyCount) {
if (keyCount >= 0 && keyCountAdj != 0) {
// We have statistics for the table. Size appropriately.
threshold = (int)Math.ceil(keyCount / (keyCountAdj * loadFactor));
}
LOG.info("Key count from statistics is " + keyCount + "; setting map size to "
+ threshold);
return threshold;
}
I have a question. I use hive 1.1.0 ,so hive.stats.dbclass default value is fs.
Mean store statistics in local filesystem. Any one can tell what is the file
path to store statistics ?
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From: [email protected]
Date: 2015-08-28 13:03
To: user
Subject: Re: Re: sql mapjoin very slow
I increase hive.hashtable.initialCapacity to 1000000 and decrease
hive.hashtable.loadfactor to 0.5 . The query run faster.
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From: Sergey Shelukhin
Date: 2015-08-28 09:56
To: user
Subject: Re: sql mapjoin very slow
Is the small-side table large, does it have a lot of rows for the same keys, or
does it have a lot of skew?
Are there lots of misses (where there’d be no value in the small table for the
large table value)?
If you have enough memory you can try increasing initial size and decreasing
load factor. Although without low-level debugging it’s hard to tell if the
issue is not obvious (I.e the above).
If there’s no obvious problem you might consider not using map join.
From: "[email protected]" <[email protected]>
Reply-To: user <[email protected]>
Date: Thursday, August 27, 2015 at 18:51
To: user <[email protected]>
Subject: Re: Re: sql mapjoin very slow
I use MR.
My mapjoin config as showed in follow picture:
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From: Sergey Shelukhin
Date: 2015-08-28 09:21
To: user
Subject: Re: sql mapjoin very slow
Are you using MR and Tez? You could try optimized hash table in case of Tez,
although it’s supposed to improve memory, not necessarily perf.
Can you also share characteristics of the query and data? It is surprising to
see so much time for HashMap.get.
From: "[email protected]" <[email protected]>
Reply-To: user <[email protected]>
Date: Thursday, August 27, 2015 at 18:03
To: user <[email protected]>
Subject: sql mapjoin very slow
When I enable mapjoin ,I see Mapjoin task run very slow. My envrioment is
hadoop 2.3.0 hive 1.1.0.
My attach is one map hive log and this map's xprof log.
In map xprof log ,I see
Compiled + native Method
92.3% 643527 + 0 java.util.HashMap.get
2.8% 19856 + 0 java.util.HashMap.put
1.2% 8623 + 0
org.apache.hadoop.hive.ql.exec.persistence.HashMapWrapper$GetAdaptor.setFromRow
0.1% 953 + 0 org.apache.hadoop.hive.ql.exec.ExprNodeColumnEvaluator._evaluate
0.1% 576 + 0
org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorUtils.copyToStandardObject
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