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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AVRO-1428) Schema.computeHash() to add if check to avoid unnecessary hashcode computation
Date Wed, 15 Jan 2014 22:20:19 GMT

    [ https://issues.apache.org/jira/browse/AVRO-1428?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13872689#comment-13872689
] 

Doug Cutting commented on AVRO-1428:
------------------------------------

It seems you're arguing that Field should cache its hashcode, which might help cases where
Field instances are reused in multiple schemas.  In that case, we should do this by adding
a hashcode cache to Field, no?

> Schema.computeHash() to add if check to avoid unnecessary hashcode computation
> ------------------------------------------------------------------------------
>
>                 Key: AVRO-1428
>                 URL: https://issues.apache.org/jira/browse/AVRO-1428
>             Project: Avro
>          Issue Type: Improvement
>          Components: java
>            Reporter: Tie Liu
>         Attachments: AVRO-1428.patch, AVRO-1428.patch-V2, TestAvro.java
>
>
> In current Schma.java we have following implementation:
>   public final int hashCode() {
>     if (hashCode == NO_HASHCODE)
>       hashCode = computeHash();
>     return hashCode;
>   }
>   int computeHash() { return getType().hashCode() + props.hashCode(); }
>   While hashCode is doing the checking of "if (hashCode == NO_HASHCODE)", the computeHash
method is not. But the computeHash method is being called from Schema$Field.hashCode and the
subclasses hashCode implementation like following:
> public int hashCode() { return name.hashCode() + schema.computeHash(); }  //this is from
Schema$Field class
>   This is causing the the calculation of hashCode getting called unnecessarily extensively.
The proposed changed is to add the "if" check inside the computeHash method instead:
>     int computeHash()
>     {
>         if (hashCode == NO_HASHCODE)
>         {
>             hashCode = getType().hashCode() + props.hashCode();
>         }
>         return hashCode;
> }
> We did a simple test to compare the performance difference, below is a summary of the
heap snapshot of comparing the difference:
> As a test I wrote a small program that creates a HashMap<Schema.Field, Integer>()
and enters a loop simply identifying whether various Schema.Field instances are keys in the
map. Obviously this is a pathological test case, but when running with the current implementation
of Schema.Field it has (in about 30 seconds) used up nearly 8 GBytes of heap in instantiating
intermediate objects associated with calling Schema.computeHash():
> Heap
> PSYoungGen      total 17432576K, used 8666481K [0x0000000340000000, 0x0000000800000000,
0x0000000800000000)
>   eden space 14942208K, 58% used [0x0000000340000000,0x0000000550f5c650,0x00000006d0000000)
>   from space 2490368K, 0% used [0x0000000768000000,0x0000000768000000,0x0000000800000000)
>   to   space 2490368K, 0% used [0x00000006d0000000,0x00000006d0000000,0x0000000768000000)
> ParOldGen       total 1048576K, used 0K [0x0000000300000000, 0x0000000340000000, 0x0000000340000000)
>   object space 1048576K, 0% used [0x0000000300000000,0x0000000300000000,0x0000000340000000)
> PSPermGen       total 21504K, used 5782K [0x00000002fae00000, 0x00000002fc300000, 0x0000000300000000)
>   object space 21504K, 26% used [0x00000002fae00000,0x00000002fb3a5818,0x00000002fc300000)
> When running with the modified implementation (and no other change) all the object allocation
vanishes:
> Heap
> PSYoungGen      total 17432576K, used 896532K [0x0000000340000000, 0x0000000800000000,
0x0000000800000000)
>   eden space 14942208K, 6% used [0x0000000340000000,0x0000000376b852d0,0x00000006d0000000)
>   from space 2490368K, 0% used [0x0000000768000000,0x0000000768000000,0x0000000800000000)
>   to   space 2490368K, 0% used [0x00000006d0000000,0x00000006d0000000,0x0000000768000000)
> ParOldGen       total 1048576K, used 0K [0x0000000300000000, 0x0000000340000000, 0x0000000340000000)
>   object space 1048576K, 0% used [0x0000000300000000,0x0000000300000000,0x0000000340000000)
> PSPermGen       total 21504K, used 5768K [0x00000002fae00000, 0x00000002fc300000, 0x0000000300000000)
>   object space 21504K, 26% used [0x00000002fae00000,0x00000002fb3a2240,0x00000002fc300000)
> As a side-effect the test runs x3 faster with the modified hashCode() implementation.



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