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https://issues.apache.org/jira/browse/SPARK-18392?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15832404#comment-15832404
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Yun Ni commented on SPARK-18392:
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Hi David,
Thanks for the question. I did not group the records by their hash signatures
for 2 reasons:
(1) A large number of instances could have the same hash signature. In that
case, the group-by operations could cause OOM or spill. See
http://stackoverflow.com/questions/25136064/spark-groupby-outofmemory-woes
(2) Since Spark Datasets are lazy evaluated, tagging every instance with hash
signature and comparing with every instance will run in the same staging. (Even
more steps may run the same stage.) Thus it should not hurt the running time.
If you see any performance issues, please let us know.
> LSH API, algorithm, and documentation follow-ups
> ------------------------------------------------
>
> Key: SPARK-18392
> URL: https://issues.apache.org/jira/browse/SPARK-18392
> Project: Spark
> Issue Type: Improvement
> Components: ML
> Reporter: Joseph K. Bradley
>
> This JIRA summarizes discussions from the initial LSH PR
> [https://github.com/apache/spark/pull/15148] as well as the follow-up for
> hash distance [https://github.com/apache/spark/pull/15800]. This will be
> broken into subtasks:
> * API changes (targeted for 2.1)
> * algorithmic fixes (targeted for 2.1)
> * documentation improvements (ideally 2.1, but could slip)
> The major issues we have mentioned are as follows:
> * OR vs AND amplification
> ** Need to make API flexible enough to support both types of amplification in
> the future
> ** Need to clarify which we support, including in each model function
> (transform, similarity join, neighbors)
> * Need to clarify which algorithms we have implemented, improve docs and
> references, and fix the algorithms if needed.
> These major issues are broken down into detailed issues below.
> h3. LSH abstraction
> * Rename {{outputDim}} to something indicative of OR-amplification.
> ** My current top pick is {{numHashTables}}, with {{numHashFunctions}} used
> in the future for AND amplification (Thanks [~mlnick]!)
> * transform
> ** Update output schema to {{Array of Vector}} instead of {{Vector}}. This
> is the "raw" output of all hash functions, i.e., with no aggregation for
> amplification.
> ** Clarify meaning of output in terms of multiple hash functions and
> amplification.
> ** Note: We will _not_ worry about users using this output for dimensionality
> reduction; if anything, that use case can be explained in the User Guide.
> * Documentation
> ** Clarify terminology used everywhere
> *** hash function {{h_i}}: basic hash function without amplification
> *** hash value {{h_i(key)}}: output of a hash function
> *** compound hash function {{g = (h_0,h_1,...h_{K-1})}}: hash function with
> AND-amplification using K base hash functions
> *** compound hash function value {{g(key)}}: vector-valued output
> *** hash table {{H = (g_0,g_1,...g_{L-1})}}: hash function with
> OR-amplification using L compound hash functions
> *** hash table value {{H(key)}}: output of array of vectors
> *** This terminology is largely pulled from Wang et al.'s survey and the
> multi-probe LSH paper.
> ** Link clearly to documentation (Wikipedia or papers) which matches our
> terminology and what we implemented
> h3. RandomProjection (or P-Stable Distributions)
> * Rename {{RandomProjection}}
> ** Options include: {{ScalarRandomProjectionLSH}},
> {{BucketedRandomProjectionLSH}}, {{PStableLSH}}
> * API privacy
> ** Make randUnitVectors private
> * hashFunction
> ** Currently, this uses OR-amplification for single probing, as we intended.
> ** It does *not* do multiple probing, at least not in the sense of the
> original MPLSH paper. We should fix that or at least document its behavior.
> * Documentation
> ** Clarify this is the P-Stable Distribution LSH method listed in Wikipedia
> ** Also link to the multi-probe LSH paper since that explains this method
> very clearly.
> ** Clarify hash function and distance metric
> h3. MinHash
> * Rename {{MinHash}} -> {{MinHashLSH}}
> * API privacy
> ** Make randCoefficients, numEntries private
> * hashDistance (used in approxNearestNeighbors)
> ** Update to use average of indicators of hash collisions [SPARK-18334]
> ** See [Wikipedia |
> https://en.wikipedia.org/wiki/MinHash#Variant_with_many_hash_functions] for a
> reference
> h3. All references
> I'm just listing references I looked at.
> Papers
> * [http://cseweb.ucsd.edu/~dasgupta/254-embeddings/lawrence.pdf]
> * [https://people.csail.mit.edu/indyk/p117-andoni.pdf]
> * [http://web.stanford.edu/class/cs345a/slides/05-LSH.pdf]
> * [http://www.cs.princeton.edu/cass/papers/mplsh_vldb07.pdf] - Multi-probe
> LSH paper
> Wikipedia
> *
> [https://en.wikipedia.org/wiki/Locality-sensitive_hashing#LSH_algorithm_for_nearest_neighbor_search]
> * [https://en.wikipedia.org/wiki/Locality-sensitive_hashing#Amplification]
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