Sunday, December 13, 2015

Binary delta compression and seed dictionaries as universal OS services

Some late Saturday night compression ideas:

Brotli's inclusion of a seed dictionary that helps it gain higher ratios on textual data, and our recent work on offline and real-time binary delta compression (patching), has given me this idea:

First, Brotli's static dictionary isn't "breaking" any rules at all. What they've done makes a lot of sense. They've tailored their codec for a very common use case (text), a place where it will have a lot of value. The seed dictionary is a legitimate part of the decompressor, except instead of pure executable code they also tacked on some interesting "universal" data to it.

Which got me thinking, the OS itself should just include a universal seed dictionary, or a set of them for different data types. A small set of seed data, like even just 4-16MB, would be fine for starters. This data can be automatically distributed to clients as part of an OS update or upgrade. It could contain natural language phrases, common x86 opcode sequences, XML, common JPEG quantization tables/markers/headers, common GLSL/HLSL keywords, etc. Once Linux does it, everybody will do it sooner or later.

Normally, when you want to compress some data for either real-time transmission, or archival purposes, you would use plain non-delta based compression. You can't assume the remote side has your seed dictionary right? But this is a weak if not flawed argument, because the remote side already has a compatible decompressor. They can surely download a little more data to have a good, stable universal seed dictionary too. And if they only download it once (through OS updates), then it's not a big cost.

Alternately, we can adopt a new codec that has a good built-in selection of universal seed data, and embed that into the OS. But the seed data should be available to OS processes as well, so they can compress/decompress data against it without having to carry around and distribute their own private seed dictionaries.

Other related thoughts:

Obviously, static dictionaries don't help in all cases. No harm done, just don't use them in those cases.

This method can be applied to networking too. The seed dictionary can be made part of the actual protocol. Packets can be delta compressed against app specific or universal seed dictionaries.

New codecs should just support seed dictionaries as required features. New LZ algorithms should be competitive against bsdiff in binary patching scenarios.

In the open source world, we need something better than bsdiff, which requires huge amounts of RAM on large files (and is just unreliable). Another interesting area of work would be to figure out the "best" universal seed dictionary to use for a OS service like this.

Plain lossless data compression and binary delta compression should not be separate concepts or tools: let's unify them. LZ-Sub shows one potential way forward.

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