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Avro >> mail # dev >> propose change to java Schema.computeHash implementation to avoid unnessary object allocation/hashcode calculation


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Re: propose change to java Schema.computeHash implementation to avoid unnessary object allocation/hashcode calculation
Tony,

This sounds like it could be a useful change.  I have some concerns,
but these would better be addressed in Jira once a patch has been
posted.

https://cwiki.apache.org/confluence/display/AVRO/How+To+Contribute

Thanks,

Doug

On Thu, Jan 2, 2014 at 11:21 AM, Liu, Tie <[EMAIL PROTECTED]> wrote:
> Hi,
>   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.
>
> Please let us know how you think of this proposal. If it's ok, I can create a jira and submit a patch for the change.
>
> Thanks,
>
> Tony
>
>
>
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