Compiling XPath to Ruby

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The process of evaluating a programming or query language is typically broken up in 3 steps:

  1. The lexing phase, which turns raw text into a sequence of "tokens". Tokens are usually a pair (e.g. an array or tuple) of a type and a value.
  2. The parsing phase, which turns a sequence of tokens into an Abstract Syntax Tree (AST).
  3. An evaluation phase, producing a set of instructions a machine should execute based on an AST.

For the third step there are two ways of doing things:

  1. Instructions are executed on the fly.
  2. Instructions are generated and executed separately.

Both options have their benefits and drawbacks. A system that executes instructions on the fly is typically easier to implement. However, these systems tend to be slower as there's very little to no room for optimizations as execution depends directly on the input AST. Directly evaluating ASTs also makes it very hard (if not downright impossible) to perform Just In Time (JIT) compilation.

A system that first generates instructions and then executes them can be harder to implement, at the benefit of allowing for better optimizations.

An example of the first method would be Ruby 1.8, while an example of the second method is your average C compiler (e.g. gcc).

XPath Evaluation in Oga

Up until version 1.3.0, Oga used to evaluate XPath queries on the fly. While the code was fairly easy to work with, performance left a lot to be desired. The setup of this evaluator was as following:

Every type of AST node would have a corresponding handler method called on_X where X would be the type of the AST node. For example, an int AST node would be handled by on_int. Each of these handlers would take their input, operate on it, and return the result. The usual return value would be an instance of Oga::XML::NodeSet, an Array-like object used for storing XML nodes.

The performance impact of this setup depends on two things: the size of the input document, and the size and complexity of the given XPath query. For small documents the performance wasn't too bad, but for larger documents (e.g. the 10 MB test file used for benchmarks) this could result in even simple queries taking seconds to complete.

In short, if I wanted to improve performance I would need to come up with a radically different way of evaluating XPath queries.

Compiling XPath

The alternative I started looking into was compiling XPath to some kind of format that could be executed in a more efficient way. One option would be to compile to some custom bytecode format and evaluate that. However, ideally the target format would be something that could take advantage of optimizations already provided by Ruby implementations. That way I wouldn't have to write my own optimization passes or maybe even some sort of JIT compiler.

Compiling to Ruby bytecode would be an option, if it weren't for every implementation using its own bytecode format. Also, no implementation to date actually considers the bytecode part of their public API (as far as I'm aware), meaning it could change at any given point.

Ruby source code on the other hand works across implementations, is stable, and can take advantage of all performance optimizations a Ruby implementation might have to offer.

Starting with version 1.3.0, Oga compiles XPath expressions to Ruby source code. The result is a Proc that takes an input document (or element) and returns the result of the XPath expression it was compiled from. The compiled Procs are cached on a per expression basis. This means that if you run the same query in a loop, Oga only has to compile it once.

Code wise the setup is fairly similar to the old evaluator. There are still AST node type specific handlers (on_int, on_axis_following_sibling, etc). However, instead of returning Oga::XML::NodeSet instances they return AST nodes used to produce Ruby source code.

Performance Improvements

The new compiler setup yields significant performance improvements over the old evaluator setup. In certain cases performance is even better than Nokogiri, which uses C for its XPath evaluation.

Of course any performance claim is meaningless without a benchmark to back it up. Oga has several benchmarks for the new compiler, these resides in the benchmark/xpath/compiler directory of the repository.

Benchmarks were run on a Thinkpad T520 running Linux 4.1 with a bunch of applications in the background, while listening to the Metal Gear Solid 5: The Phantom Pain soundtrack on YouTube. In other words, treat these numbers with a grain of salt. For best results you should run these benchmarks yourself. To do so, clone the Git repository of Oga, run rake generate fixtures and then run one of the benchmark files like any other Ruby script.

First, lets look at the benchmark big_xml_average_bench.rb. This benchmark takes a 10 MB test file and runs the query descendant-or-self::location 10 times, measuring the execution time for every iteration. Using Oga 1.2.3 we get the following output:

Iteration: 1: 3.493
Iteration: 2: 2.868
Iteration: 3: 2.934
Iteration: 4: 2.965
Iteration: 5: 2.926
Iteration: 6: 2.928
Iteration: 7: 3.008
Iteration: 8: 2.977
Iteration: 9: 2.938
Iteration: 10: 2.993

Iterations: 10
Average:    3.003 sec

Using Oga 1.3.0 the output is as following instead:

Iteration: 1: 0.432
Iteration: 2: 0.448
Iteration: 3: 0.522
Iteration: 4: 0.453
Iteration: 5: 0.44
Iteration: 6: 0.494
Iteration: 7: 0.448
Iteration: 8: 0.431
Iteration: 9: 0.432
Iteration: 10: 0.437

Iterations: 10
Average:    0.454 sec

Here Oga 1.3.0 is about 6.6 times faster.

Next, lets look at the benchmark concurrent_time_bench.rb. This benchmark uses the XML file kaf.xml and runs the query KAF/terms/term 10 times in parallel using 5 threads. The idea of this benchmark is to measure performance as the number of threads increase. A higher number of threads can result in more pressure on the garbage collector (GC), depending on the code being benchmarked. More pressure on the GC can in turn result in poorer performance due to the GC having to stop all threads more often.

Using Oga 1.2.3 the results of this benchmark are as following:

Starting threads...
Samples: 50
Average: 0.2316 seconds

Using Oga 1.3.0:

Starting threads...
Samples: 50
Average: 0.0342 seconds

Here Oga 1.3.0 is also around 6.6 times faster.

Finally, lets look at the benchmark comparing_gems_bench.rb. This benchmark uses the XML document <root><number>10</number></root> and retrieves all text nodes of all <number> nodes. This benchmark uses benchmark-ips.

The benchmark runs this query for the following libraries:

Note that Ox doesn't actually support XPath, it instead offers its own querying language. As a result it's not entirely fair to compare it with the other libraries. However, for the sake of showing the performance difference of Ox' query language versus the rest I've included it any way.

Using these Gems and Oga 1.2.3, the results are as following:

                  Ox    14.548k i/100ms
            Nokogiri     3.879k i/100ms
                 Oga     2.681k i/100ms
               REXML     1.114k i/100ms
                  Ox    197.284k (± 3.9%) i/s -    989.264k
            Nokogiri     46.701k (± 9.7%) i/s -    232.740k
                 Oga     28.293k (± 2.0%) i/s -    142.093k
               REXML     11.901k (± 2.8%) i/s -     60.156k

                  Ox:   197284.2 i/s
            Nokogiri:    46701.1 i/s - 4.22x slower
                 Oga:    28292.6 i/s - 6.97x slower
               REXML:    11900.5 i/s - 16.58x slower

And using Oga 1.3.0:

                  Ox    15.227k i/100ms
            Nokogiri     3.966k i/100ms
                 Oga    13.874k i/100ms
               REXML     1.168k i/100ms
                  Ox    201.044k (± 1.5%) i/s -      1.005M
            Nokogiri     47.338k (± 8.6%) i/s -    237.960k
                 Oga    166.485k (± 9.8%) i/s -    832.440k
               REXML     11.693k (± 5.3%) i/s -     58.400k

                  Ox:   201044.3 i/s
                 Oga:   166485.5 i/s - 1.21x slower
            Nokogiri:    47338.3 i/s - 4.25x slower
               REXML:    11692.7 i/s - 17.19x slower

Here Oga 1.3.0 is about 5.8 times faster compared to version 1.2.3. Using 1.3.0 Oga outperforms not only REXML but also Nokogiri.

Please keep in mind that performance will vary depending on the size of the input document and the query being used. There will be cases where Oga outperforms others, but there will (probably) also be cases where it performs worse.

Wrapping Up

The source code for the compiler can be found in lib/oga/xpath/compiler.rb. The source code used for the Ruby AST and code generation can be found in lib/oga/ruby. There are still plenty of parts in the compiler that could be optimized further as the current code is largely ported from the old evaluator.

Those who wish to take advantage of the new compiler can simply update to Oga 1.3.0. A full list of changes can be found in the changelog.