Wednesday, January 27, 2016

Java's Flat Collections - what's the deal? (Part I)

I've been moving my job to Java environment and as my previous interest in performance is still there, I thought why sometimes Java runs slow (even typically - after you wait your code to warm up)  and I described some solutions that are around Java's ecosystem.

There are 3 solutions for now which are competing to give high performance Java code by allowing quick performance or OS integration: 
- PackedObjects, a way to remove object headers to object collections and it works sadly for now only with IBM JVMs, It should be primarily used by JNI like code to speed it up and removing individual copying. It requires medium changes in compiler, garbage collectors but no language changes (or minimal ones)
- ObjectLayout, a way to give hints for JVM to allocate continuously arrays in a structured manner which may be implemented. It requires GC changes, very few annotations but no language changes 
- Array 2.0 (or Project Panama) is the project which basically plans to bring .Net's struct type to Java, This is the most extensive of all because it has to change: bytecodes, internal changes inside compiler, inside GC

So, I'm here to present a new solution which I found it handy, but it is in very early stage and requires no language changes (still, to take advantage of this, you require yourself some few code changes), it should work with any Java at least newer than 5.0 (maybe Java 1.2, but I'm not 100%) or if it is not fully possible to work with this solution, it will be very easy to patch.

Enter FlatCollection, a code generator which flattens your collections and can make it very easy to work with high(er) performance code for many common cases.

How does it work:
- you find any types it has the same type of fields (for now I think the coding supports only primitive types, as the fully working prototype works with Point of x,y integer fields, but very likely at the time you may read this code, it will work as a generator for any field type)
- you add all types with full namespace inside: input.flat file
- you run the project to create two flat classes out of it: an ArrayListOfYourType, and a CursorOfYourType
- copy all these files inside package you will add in your project: "flatcollections"

Look inside a "realistic" benchmark code to see the same code using an array of Point and this mapped ArrayList inside .

In this kind of real life, the memory consumption for this collection is in range of a half of a full array of points, and the performance of both populating it and reading it is at least 2x-4x in performance.

How does it work: it merges all fields in a continuous array of "primitive" types and removes basically one indirection and many allocation costs.

I will extend in future the samples to show parsing of CSV files and operations like it. If you reuse the collection using  .clear() call, no reallocation is needed, excluding  the new "per-row" code allocates more memory than previous implementations.

Why is important to flatten the data layout? Basically, you can reduce the GC hits or you can map naturally code that was ugly otherwise: let's say to have a Tuple class. Also, the full GC cost (which involves visiting all small objects in your heap) is very low on these collections. So I would assume at least for batch processing or maybe games written in Java it could be a really friendly tool of trade.

What should be done:
- it should be tested to work with collections of any type and to support specializations
- it should work with POJO which are not exposed as fields including Tuple classes
- not mandatory but it should support iterators or any other friendly structures

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