Compact Off-Heap Structures and Tuples In Java

http://mechanical-sympathy.blogspot.co.uk/2012/10/compact-off-heap-structurestuples-in.html

by pacoverdi • 14 years ago

30 12 14 years ago

mechanical-sympathy.blogspot.co.uk

chii• 14 years ago

That is both scary, and so very intriguingly interesting!

pacoverdi2• 14 years ago

Funny, I was actually manipulating C structures from Java, and after hitting a nasty issue, went procrastinating on twitter where I stumbled upon this blog post :)

For those interested, there are less low-level ways to access fields in structures, for example Javolution http://javolution.org/target/site/apidocs/javolution/io/Stru...

I'm working on a very similar lib but where Structs are stateless, allowing to use them concurrently from multiple threads.

jmount• 14 years ago

Just a quick question. For the flyweight idea in the article you get additional thread unsafeness even for immutable data in that you are changing where the flyweight points (which is how you want to do it to avoid creating and destroying a lot of flyweight objects). What is your idea to pool/control multiple flyweight accessors?

mjpt7774• 14 years ago

I like to have only one writer thread for any data. This can have its own flyweight. Other threads can be readers each having their own read only flyweight implementation.

pacoverdi2• 14 years ago

[Not sure whether the question is directed to the author or to my comment but I'll answer anyway :) ]

Basically, there are 3 notions

1) the buffer (byte[] or whatever) containing the data, immutable at least while the data is being accessed by a given thread

2) the Struct object: defines the structure of a message (field length, offset, arrays etc.)

3) the Message (aka the flyweight) that points to a given offset in buffer and is bound to a Struct instance

The buffer and the message are normally accessed by only one thread, but several threads can share Struct instances (i.e. the dictionary).

Each field (even deep in the field hierarchy) knows its offset relative to the beginning of the message, so values can be accessed like eg.

  int foo = struct.foo.getInt(msg);

The only complicated thing is when dealing with arrays (especially when nested in other arrays). The message does the bookkeeping necessary to safely access elements, ie. the mutable state is stored in the flyweight.

lmm3• 14 years ago

Pretty much as expected. I'd be interested to see a comparison with object pooling (obviously not appropriate to the algorithm as written, but in a real system you're more likely to be streaming in data in chunks rather than putting it all in a big buffer and then reading through it), which lets you get a lot of the performance advantages of avoiding GC without completely abandoning Java's safety guarantees.

mjpt7774• 14 years ago

How would it work with object pooling if I wanted to query a large table of data? This is often needed in real big data applications.

lmm3• 14 years ago

For that kind of problem I'd probably be using hadoop, which does object pooling internally with the objects it passes into your mappers/reducers.

For a non-hadoop datasource you could do the same thing by hand: stream in the data from the table, turning it into objects from your pool and passing them through to your reducer function in small batches.

jeffffff• 14 years ago

you can do this on heap with byte[] using Unsafe.arrayBaseOffset and getInt(Object, long), putInt(Object, long, int) and friends

Compact Off-Heap Structures and Tuples In Java

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