jm + sketching   6

Caffeine cache adopts Window TinyLfu eviction policy
'Caffeine is a Java 8 rewrite of Guava's cache. In this version we focused on improving the hit rate by evaluating alternatives to the classic least-recenty-used (LRU) eviction policy. In collaboration with researchers at Israel's Technion, we developed a new algorithm that matches or exceeds the hit rate of the best alternatives (ARC, LIRS). A paper of our work is being prepared for publication.'

Specifically:
W-TinyLfu uses a small admission LRU that evicts to a large Segmented LRU if accepted by the TinyLfu admission policy. TinyLfu relies on a frequency sketch to probabilistically estimate the historic usage of an entry. The window allows the policy to have a high hit rate when entries exhibit a high temporal / low frequency access pattern which would otherwise be rejected. The configuration enables the cache to estimate the frequency and recency of an entry with low overhead. This implementation uses a 4-bit CountMinSketch, growing at 8 bytes per cache entry to be accurate. Unlike ARC and LIRS, this policy does not retain non-resident keys.
tinylfu  caches  caching  cache-eviction  java8  guava  caffeine  lru  count-min  sketching  algorithms 
november 2015 by jm
Philippe Flajolet’s contribution to streaming algorithms [preso]
Nice deck covering HyperLogLog and its origins, plus a slide at the end covering the Flajolet/Wegman Adaptive Sampling algorithm ("how do you count the number of elements which appear only once in stream using constant size memory?")
algorithms  sketching  hyperloglog  flajolet  wegman  adaptive-sampling  sampling  presentations  slides 
november 2013 by jm
Recordinality
a new, and interesting, sketching algorithm, with a Java implementation:
Recordinality is unique in that it provides cardinality estimation like HLL, but also offers "distinct value sampling." This means that Recordinality can allow us to fetch a random sample of distinct elements in a stream, invariant to cardinality. Put more succinctly, given a stream of elements containing 1,000,000 occurrences of 'A' and one occurrence each of 'B' - 'Z', the probability of any letter appearing in our sample is equal. Moreover, we can also efficiently store the number of times elements in our distinct sample have been observed. This can help us to understand the distribution of occurrences of elements in our stream. With it, we can answer questions like "do the elements we've sampled present in a power law-like pattern, or is the distribution of occurrences relatively even across the set?"
sketching  coding  algorithms  recordinality  cardinality  estimation  hll  hashing  murmurhash  java 
august 2013 by jm
Sketch of the Day: K-Minimum Values
Another sketching algorithm -- this one supports set union and intersection operations more easily than HyperLogLog when there are more than 2 sets
algorithms  coding  space-saving  cardinality  streams  stream-processing  estimation  sets  sketching 
june 2013 by jm
js-hll
Good UI for exploration of HyperLogLog set intersections and unions.
One of the first things that we wanted to do with HyperLogLog when we first started playing with it was to support and expose it natively in the browser. The thought of allowing users to directly interact with these structures -- perform arbitrary unions and intersections on effectively unbounded sets all on the client -- was exhilarating to us. [...] we are pleased to announce the open-source release of AK’s HyperLogLog implementation for JavaScript, js-hll. We are releasing this code under the Apache License, Version 2.0.

We knew that we couldn’t just release a bunch of JavaScript code without allowing you to see it in action — that would be a crime. We passed a few ideas around and the one that kept bubbling to the top was a way to kill two birds with one stone. We wanted something that would showcase what you can do with HLL in the browser and give us a tool for explaining HLLs. It is typical for us to explain how HLL intersections work using a Venn diagram. You draw some overlapping circles with a border that represents the error and you talk about how if that border is close to or larger than the intersection then you can’t say much about the size of that intersection. This works just ok on a whiteboard but what you really want is to just build a visualization that allows you to select from some sets and see the overlap. Maybe even play with the precision a little bit to see how that changes the result. Well, we did just that!
javascript  ui  hll  hyperloglog  algorithms  sketching  js  sets  intersection  union  apache  open-source 
june 2013 by jm
hlld
a high-performance C server which is used to expose HyperLogLog sets and operations over them to networked clients. It uses a simple ASCII protocol which is human readable, and similar to memcached.

HyperLogLog's are a relatively new sketching data structure. They are used to estimate cardinality, i.e. the unique number of items in a set. They are based on the observation that any bit in a "good" hash function is indepedenent of any other bit and that the probability of getting a string of N bits all set to the same value is 1/(2^N). There is a lot more in the math, but that is the basic intuition. What is even more incredible is that the storage required to do the counting is log(log(N)). So with a 6 bit register, we can count well into the trillions. For more information, its best to read the papers referenced at the end. TL;DR: HyperLogLogs enable you to have a set with about 1.6% variance, using 3280 bytes, and estimate sizes in the trillions.


(via:cscotta)
hyper-log-log  hlld  hll  data-structures  memcached  daemons  sketching  estimation  big-data  cardinality  algorithms  via:cscotta 
june 2013 by jm

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