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From "David Allsopp (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-2975) Upgrade MurmurHash to version 3
Date Wed, 16 Nov 2011 09:36:52 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-2975?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

David Allsopp updated CASSANDRA-2975:
-------------------------------------

    Comment: was deleted

(was: Having slept on it, I realized that the {{switch-case}} is slow because of the {{switch-case}}
fallthrough in Java - _i.e. all 15 cases get tested every time_ (http://download.oracle.com/javase/tutorial/java/nutsandbolts/switch.html)

Adding a {{break}} to the end of each case fixes this without the verbosity of the binary
search approach, i.e:

{noformat}
		switch (length & 15)
		{
		case 0: break;
		case 1:
			k1 ^= ((long) key.get(offset));
			k1 *= c1;
			k1 = rotl64(k1, 31);
			k1 *= c2;
			h1 ^= k1;
			break;
		case 2:
			k1 ^= ((long) key.get(offset + 1)) << 8;
			break;
                ...etc...
{noformat}

Combining this with the inlining almost doubles the speed of hashing for keys with lengths
from 1-32 bytes!

UPDATE: Argh, this breaks something - the hash output is different. Can't see why yet...)
    
> Upgrade MurmurHash to version 3
> -------------------------------
>
>                 Key: CASSANDRA-2975
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-2975
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Brian Lindauer
>            Assignee: Brian Lindauer
>            Priority: Trivial
>              Labels: lhf
>             Fix For: 1.1
>
>         Attachments: 0001-Convert-BloomFilter-to-use-MurmurHash-v3-instead-of-.patch,
0002-Backwards-compatibility-with-files-using-Murmur2-blo.patch
>
>
> MurmurHash version 3 was finalized on June 3. It provides an enormous speedup and increased
robustness over version 2, which is implemented in Cassandra. Information here:
> http://code.google.com/p/smhasher/
> The reference implementation is here:
> http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp?spec=svn136&r=136
> I have already done the work to port the (public domain) reference implementation to
Java in the MurmurHash class and updated the BloomFilter class to use the new implementation:
> https://github.com/lindauer/cassandra/commit/cea6068a4a3e5d7d9509335394f9ef3350d37e93
> Apart from the faster hash time, the new version only requires one call to hash() rather
than 2, since it returns 128 bits of hash instead of 64.

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