Simeon Simeonov created SPARK-10574:
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Summary: HashingTF should use MurmurHash3
Key: SPARK-10574
URL: https://issues.apache.org/jira/browse/SPARK-10574
Project: Spark
Issue Type: Improvement
Components: MLlib
Affects Versions: 1.5.0
Reporter: Simeon Simeonov
Priority: Critical
{{HashingTF}} uses the Scala native hashing {{##}} implementation. There are
two significant problems with this.
First, per the [Scala
documentation|http://www.scala-lang.org/api/2.10.4/index.html#scala.Any] for
{{hashCode}}, the implementation is platform specific. This means that feature
vectors created on one platform may be different than vectors created on
another platform. This can create significant problems when a model trained
offline is used in another environment for online prediction. The problem is
made harder by the fact that following a hashing transform features lose
human-tractable meaning and a problem such as this may be extremely difficult
to track down.
Second, the native Scala hashing function performs badly on longer strings,
exhibiting [200-500% higher collision
rates|https://gist.github.com/ssimeonov/eb12fcda75615e4a8d46] than, for
example,
[MurmurHash3|http://www.scala-lang.org/api/2.10.4/#scala.util.hashing.MurmurHash3$]
which is also included in the standard Scala libraries and is the hashing
choice of fast learners such as Vowpal Wabbit and others. If Spark users apply
{{HashingTF}} only to very short, dictionary-like strings the hashing function
choice will not be a big problem but why have an implementation in MLlib with
this limitation when there is a better implementation readily available in the
standard Scala library?
Switching to MurmurHash3 solves both problems. If there is agreement that this
is a good change, I can prepare a PR.
Note that changing the hash function would mean that models saved with a
previous version would have to be re-trained. This introduces a problem that's
orthogonal to breaking changes in APIs: breaking changes related to artifacts,
e.g., a saved model, produced by a previous version. Is there a policy or best
practice currently in effect about this? If not, perhaps we should come up with
a few simple rules about how we communicate these in release notes, etc.
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