Hashing in Fashionable Recommender Techniques: A Primer | by Samuel Flender | Mar, 2023


Understanding essentially the most underrated trick in utilized Machine Studying


Hashing is likely one of the most typical “methods” utilized in industrial Machine Studying purposes, but it doesn’t get almost as a lot consideration because it deserves.

The largest benefit of hashing, particularly in trendy recommender systems, is its finite-memory assure: with out hashing, it might be extremely impractical to study the relevance of billions of movies, information articles, images, or net pages for billions of customers with out working out of reminiscence.

However we’re getting forward of ourselves right here. This submit is a primer, so let’s return to the place it began: the well-known 2009 “hashing trick” paper.

The paper that began all of it

The concept of utilizing hashing as a option to course of options, in addition to the time period “hashing trick”, have been first launched in a 2009 paper by a staff of researchers from Yahoo, led by Kilian Weinberger, within the context of E-mail spam detection. An electronic mail, in spite of everything, is a sequence of phrases, and every phrase may be considered a characteristic. With hashing, the authors clarify, we will characterize emails as vectors (the place every index corresponds to a selected phrase), and use these vectors in a spam collaborative filtering mannequin.

For instance, the phrase “Your prescription is prepared” in a 20-dimensional hash area could also be equal to


and so forth, the place the place of those is dependent upon the actual alternative of hash operate used.

Nevertheless, totally different phrases might have totally different relevance to totally different individuals. The phrase “prescription” might sign spam to 1 person, however to not one other. With a view to account for this distinction in relevance, the authors introduce “customized” tokens, wherein they mix the person id with the phrases themselves. For the phrase above, they’d not simply hash the tokens

"your", "prescription",  "is", "prepared",

but in addition

"user123_your", "user123_prescription", "user123_is", "user123_ready",

and so forth. Given their dataset of 40 Million distinctive phrases and 400,000 customers, this customized tokenization ends in a complete of 16 Trillion attainable options.

By hashing these customized options right into a hash measurement of 2²², or roughly 4.2 Million (a discount of seven orders of magnitude!), the authors obtain a complete spam discount of 30%, in comparison with a baseline mannequin with no hashing-powered personalization, one of many first clear demonstrations of the usefulness of hashing in ML issues.

Hashing in trendy recommender programs

Quick ahead from 2009 to at present, and we see that loads of the ML expertise has modified (deep neural nets have largely changed linear collaborative filters), however hashing remains to be there.

Fashionable recommender programs are normally some variant of a two-tower neural community, the place one tower learns person embeddings from person ids, and the opposite tower learns, say, video embedding from video ids (for a video recommender system). At coaching time the mannequin is given person/video pairs from historic engagement knowledge, corresponding to video impressions with clicks as positives, and people with out clicks as negatives, leading to a shared embedding area for customers and movies. We are able to then retailer the embeddings for all of our customers and movies in a set of embedding tables, and use these tables to make advice at serving time, for instance utilizing a kNN algorithm.

Thus far so good, however the place does hashing slot in right here?

Effectively, take into account a mannequin that shops modest 100-dimensional embeddings of 1B movies and 1B customers: that’s already 800GB of reminiscence. This can be a enormous reminiscence footprint, and the mannequin will probably be very impractical (if not unattainable) and costly to serve. With hashing, we will first scale back the “vocabulary” of movies and customers right down to, say, 10M every, leading to a extra manageable reminiscence footprint of 8GB.

Hashing, in different phrases, permits us to scale trendy recommender programs to Billions of customers and Billions of things. With out hashing, the dimensions of recommender programs could be basically restricted by the quantity of reminiscence we will afford.

In direction of collision-free hashing

The draw back of hashing is the existence of hash collisions, the truth that a number of totally different ids will finally be mapped into the identical hash, leading to a number of customers or gadgets sharing the identical embedding within the embedding desk. Clearly, this “cramping” of knowledge can degrade the ensuing suggestions. One of the vital essential analysis questions in recommender programs at present is subsequently find out how to make them collision-free.

One concept is the usage of “Deep Hash Embeddings” (DHE), proposed in 2021 by a staff from Google Mind led by Wang-Cheng Kang. As an alternative of a single hash operate, they use numerous hash features, and concatenate all hashes for a single id right into a dense vector, which is then fed right into a deep neural web which learns higher-oder representations of the ids. The important thing concept is that, if the variety of hash features is giant sufficient, then hash collisions develop into statistically unattainable.

And certainly, their method reveals promise: utilizing DHE with 1024 hash features, the authors see an enchancment of 0.25% in AUC over common hashing on a set of public benchmark datasets. One caveat with this method although is that’s doesn’t work for multivalent options, just for ids instantly. (For instance, DHE, as proposed right here, couldn’t encode an inventory of flicks you’ve watched on Netflix up to now 30 days, which might be a multivalent characteristic.)

One other promising collision-free hashing method is “Cuckoo hashing”, first launched in 2001 by Rasmus Pagh and Flemming Rodler at Arhus College, Denmark. Cuckoo hashing is utilized in Monolith, ByteDance’s on-line advice system, which is launched of their 2022 paper, led by Zhuoran Liu.

In Cuckoo hashing we keep not one however a number of hash tables: in ByteDance’s paper, they use 2. After we encounter a brand new id, by default we hash it into the primary hash desk. If the primary hash desk is already occupied with one other id, we evict that id (therefore the title of the algorithm) and re-hash it right into a spot within the second hash desk. This course of is repeated till there aren’t any extra evictions, and the set of hash tables stabilizes with out collisions. In comparison with common hashing (with collisions) the authors discover an enchancment of 0.5% AUC on a public benchmark dataset with their collision-free hashing method.

Coda: essentially the most underrated trick in utilized Machine Studying

Recap for reminiscence:

  • Hashing permits us to scale trendy recommender programs to Billions of customers and Billions of things with a finite-memory assure.
  • The concept of utilizing hashing in ML software dates again to a 2009 paper from Yahoo, which demonstrated its usefulness in an E-mail spam detection mannequin.
  • Hashing nevertheless introduces hash collisions, which might degrade suggestions as a result of a number of customers and gadgets sharing the identical embedding.
  • Because of this, current analysis in recommender programs focuses on find out how to make hashing collision-free. Notable examples are Deep Hash Embeddings (Google Mind) and Cuckoo Hashing (ByteDance).

For a few years, hashing has been among the many most underrated methods within the utilized ML literature, with a lot of the eye focusing as a substitute on mannequin structure, knowledge choice, or characteristic engineering.

This may increasingly start to alter. Because the current papers from Google Mind, ByteDance, and others have proven, optimizing recommender programs for hash collisions can allow notable features in efficiency. The stunning rise in reputation of TikTok (owned by ByteDance) might, a minimum of partially, be defined by higher hashing.

Watch this area: new breakthroughs are actually on the horizon.


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