Performance improvements when encoding very large hashes with symbol keys

[MatmaRex]

I'm currently working on a project which involves processing a huge hash (a couple gigabytes in memory) and occasionally dumping it as JSON (about 1 GB of it). (The project is MatmaRex/commons-media-views, please don't ask why I didn't do something saner, this seemed like a good idea at the time and now it's an interesting mental exercise.) I switched from built-in JSON parser/encode to YAJL for streamed encoding, but the performance seemed not quite as good as I expected. I did some digging and here are the results.

This set of patches should improve encoding performance across the board, but particularly when encoding hashes, and particularly when they have symbolic keys, and especially when they're really large. It looks like most of it is thanks to reduced number of object allocations, and thus fewer GC pauses while encoding. The only potential drawback is that some monkey-patched methods on builtin classes that previously were respected will no longer be. I don't think that's something you're aiming to support.

Testing with this large file: https://dl.dropboxusercontent.com/u/10983006/tmp/big.json (~110 MB) parsed with Yajl::Parser.new(symbolize_keys: true), I get a 2x performance improvement when encoding the parsed data back into JSON.

[MatmaRex]

Some benchmarks:

a) Without these patches:

benchmark/encode.rb benchmark/subjects/ohai.json 10000

                                         user     system      total        real
Yajl::Encoder#encode (to an IO)      5.881000   0.406000   6.287000 (  6.795389)
Yajl::Encoder#encode (to a String)   5.148000   0.047000   5.195000 (  5.460313)

benchmark/encode.rb benchmark/subjects/big.json 5

                                         user     system      total        real
Yajl::Encoder#encode (to an IO)     17.129000   0.406000  17.535000 ( 18.351049)
Yajl::Encoder#encode (to a String)  16.084000   0.140000  16.224000 ( 16.613950)

benchmark/encode.rb benchmark/subjects/big.json 5 (with symbolize_keys: true)

                                         user     system      total        real
Yajl::Encoder#encode (to an IO)    113.179000   0.297000 113.476000 (117.212704)
Yajl::Encoder#encode (to a String) 114.801000   0.171000 114.972000 (118.737791)

b) With these patches:

benchmark/encode.rb benchmark/subjects/ohai.json 10000

                                         user     system      total        real
Yajl::Encoder#encode (to an IO)      5.367000   0.234000   5.601000 (  5.910338)
Yajl::Encoder#encode (to a String)   4.555000   0.015000   4.570000 (  4.727270)

benchmark/encode.rb benchmark/subjects/big.json 5

                                         user     system      total        real
Yajl::Encoder#encode (to an IO)     13.775000   0.140000  13.915000 ( 14.898852)
Yajl::Encoder#encode (to a String)  12.308000   0.031000  12.339000 ( 13.032745)

benchmark/encode.rb benchmark/subjects/big.json 5 (with symbolize_keys: true)

                                         user     system      total        real
Yajl::Encoder#encode (to an IO)     60.856000   0.328000  61.184000 ( 62.784591)
Yajl::Encoder#encode (to a String)  59.717000   0.202000  59.919000 ( 61.520518)