How to (#686)

* First draft of how Oj got faster

* First editorial pass

* Update readme for release

CHANGELOG.md

@@ -1,10 +1,16 @@
 # CHANGELOG
 
+## 3.13.0 - 2021-08-08
+
+- Added `Oj::Parser`, a faster parser with better option management.
+
+- Watson1978 increasd dump performance ever more and is now a collaborator on Oj!
+
 ## 3.12.3 - 2021-08-02
 
 - Update documents for the `:cache_keys` and `:cache_strings`.
 
-- Watson1978 increasd dump performance for rails mode.
+- Watson1978 increased dump performance for rails mode.
 
 ## 3.12.2 - 2021-07-25
 

README.md

@@ -8,8 +8,7 @@
 
 A *fast* JSON parser and Object marshaller as a Ruby gem.
 
-Version 3.0 is out! 3.0 provides better json gem and Rails compatibility. It
-also provides additional optimization options.
+Version 3.13 is out with a much faster parser (`Oj::Parser`) and option isolation.
 
 ## Using
 

lib/oj/version.rb

@@ -1,5 +1,5 @@
 
 module Oj
   # Current version of the module.
-  VERSION = '3.12.3'
+  VERSION = '3.13.0'
 end

pages/Parser.md

@@ -0,0 +1,309 @@
+# How Oj Just Got Faster
+
+The original Oj parser is a performant parser that supports several
+modes. As of this writing Oj is almost 10 years old. A dinosaur by
+coding standards. It was time for an upgrade. Dealing with issues over
+the years it became clear that a few things could have been done
+better. The new `Oj::Parser` is a response that not only attempts to
+address some of the issues but also give the Oj parser a significant
+boost in performance. `Oj::Parser` takes a different approach to JSON
+parsing than the now legacy Oj parser. Not really a legacy parser yet
+since the `Oj::Parser` is not a drop-in replacement for the JSON gem
+but it is as much 3 times or more faster than the previous parser in
+some modes.
+
+## Address Issues
+
+There are a few features of the`Oj.load` parser that continue to be
+the reason for many of the issue on the project. The most significant
+area is compatibility with both Rails and the JSON gem as they battle
+it out for which behavior will win out in any particular
+situation. Most of the issues are on the writing or dumping side of
+the JSON packages but some are present on the parsing as
+well. Conversion of decimals is one area where the Rails and the JSON
+gem vary. The `Oj::Parser` addresses this by allowing for completely
+separate parser instances. Create a parser and configure it for the
+situation and leave the others parsers on their own.
+
+The `Oj::Parser` is mostly compatible with the JSON gem and Rails but
+no claims are made that the behavior will be the same as either.
+
+The most frequent issues that can addressed with the new parser are
+around the handling of options. For `Oj.load` there is a set of
+default options that can be set and the same options can be specified
+for each call to parse or load. This approach as a couple of
+downsides. One the defaults are shared across all calls to parse no
+matter what the desire mode is. The second is that having to provide
+all the options on each parse call incurs a performance penalty and is
+just annoying to repeat the same set of options over may calls.
+
+By localizing options to a specific parser instance there is never any
+bleed over to other instances.
+
+## How
+
+It's wonderfull to wish for a faster parser that solves all the
+annoyances of the previous parser but how was it done is a much more
+interesting question to answer.
+
+At the core, the API for parsing was changed. Instead of a sinle
+global parser any number of parsers can be created and each is seprate
+from the others. The parser itself is able to rip through a JSON
+string, stream, or file and then make calls to a delegate to process
+the JSON elements according to the delegate behavior. This is similar
+to the `Oj.load` parser but the new parser takes advantage of
+character maps, reduced conditional branching, and calling function
+pointers.
+
+### Options
+
+As mentioned, one way to change the options issues was to change the
+API. Instead of having a shared set of default options a separate
+parser is created and configured for each use case. Options are set
+with methods on the parser so no more guessing what options are
+available. With options isolated to individual parsers there is no
+unintended leakage to other parse use cases.
+
+### Structure
+
+A relative small amount of time is spent in the actual parsing of JSON
+in `Oj.load`. Most of the time is spent building the Ruby
+Objects. Even cutting the parsing time in half only gives a 10%
+improvement in performance but 10% is still an improvement.
+
+The `Oj::Parser` is designed to reduce conditional branching. To do
+that it uses character maps for the various states that the parser
+goes through when parsing. There is no recursion as the JSON elements
+are parsed. The use of a character maps for each parser state means
+the parser function can and is re-entrant so partial blocks of JSON
+can be parsed and the results combined.
+
+There are no Ruby calls in the parser itself. Instead delegates are
+used to implement the various behaviors of the parser which are
+currently validation (validate), callbacks (SAJ), or building Ruby
+objects (usual). The delegates are where all the Ruby calls and
+related optimizations take place.
+
+Considering JSON file parsing, `Oj.load_file` is able to read a file a
+block at a time and the new `Oj::Parser` does the same. There was a
+change in how that is done though. `Oj.load_file` sets up a reader
+that must be called for each character. Basically a buffered
+reader. `Oj::Parser` drops down a level and uses a re-entrant parser
+that takes a block of bytes at a time so there is no call needed for
+each character but rather just iterating over the block read from the
+file.
+
+Reading a block at a time also allows for an efficient second thread
+to be used for reading blocks. That feature is not in the first
+iteration of the `Oj::Parser` but the stage is set for it in the
+future. The same approach was used successfully in
+[OjC](https://github.com/ohler55/ojc) which is where the code for the
+parser was taken from.
+
+### Delegates
+
+There are three delegates; validate, SAJ, and usual.
+
+#### Validate
+
+The validate delegate is trivial in that does nothing other than let
+the parser complete. There are no options for the validate
+delegate. By not making any Ruby calls other than to start the parsing
+the validate delegate is no surprise that the validate delegate is the
+best performer.
+
+#### SAJ (Simple API for JSON)
+
+The SAJ delegate is compatible with the SAJ handlers used with
+`Oj.saj_parse` so it needs to keep track of keys for the
+callbacks. Two optimizations are used. The first is a reuseable key
+stack while the second is a string cache similar to the Ruby intern
+function.
+
+When parsing a Hash (JSON object) element the key is passed to the
+callback function if the SAJ handler responds to the method. The key
+is also provided when closing an Array or Hash that is part of a
+parent Hash. A key stack supports this.
+
+If the option is turned on a lookup is made and previously cached key
+VALUEs are used. This avoids creating the string for the key and
+setting the encoding on it. The cache used is a auto expanding hash
+implementation that is limited to strings less than 35 characters
+which covers most keys. Larger strings use the slower string creation
+approach. The use of the cache reduces object creation which save on
+both memory allocation and time. It is not appropriate for one time
+parsing of say all the keys in a dictionary but is ideally suited for
+loading similar JSON multiple times.
+
+#### Usual
+
+By far the more complex of the delegates is the 'usual' delegate. The
+usual delegate builds Ruby Objects when parsing JSON. It incorporates
+many options for configuration and makes use of a number of
+optimizations.
+
+##### Reduce Branching
+
+In keeping with the goal of reducing conditional branching most of the
+delegate options are implemented by changing a function pointer
+according to the option selected. For example when turning on or off
+`:symbol_keys` the function to calculate the key is changed so no
+decision needs to be made during parsing. Using this approach option
+branching happens when the option is set and not each time when
+parsing.
+
+##### Cache
+
+Creating Ruby Objects whether Strings, Array, or some other class is
+expensive. Well expensive when running at the speeds Oj runs at. One
+way to reduce Object creation is to cache those objects on the
+assumption that they will most likely be used again. This is
+especially true of Hash keys and Object attribute IDs. When creating
+Objects from a class name in the JSON a class cache saves resolving
+the string to a class each time. Of course there are times when
+caching is not preferred so caching can be turned on or off with
+option methods on the parser which are passed down to the delegate..
+
+The Oj cache implementation is an auto expanding hash. When certain
+limits are reached the hash is expanded and rehashed. Rehashing can
+take some time as the number of items cached increases so there is
+also an option to start with a larger cache size to avoid or reduce
+the likelihood of a rehash.
+
+The Oj cache has an advantage over the Ruby intern function
+(`rb_intern()`) in that several steps are needed for some cached
+items. As an example Object attribute IDs are created by adding an `@`
+character prefix to a string and then converting to a ID. This is done
+once when inserting into the cache and after that only a lookup is
+needed.
+
+##### Bulk Insert
+
+The Ruby functions available for C extension functions are extensive
+and offer many options across the board. The bulk insert functions for
+both Arrays and Hashes are much faster than appending or setting
+functions that set one value at a time. The Array bulk insert is
+around 15 times faster and for Hash it is about 3 times faster.
+
+To take advantage of the bulk inserts arrays of VALUEs are
+needed. With a little planning there VALUE arrays can be reused which
+leads into another optimization, the use of stacks.
+
+##### Stacks
+
+Parsing requires memory to keep track of values when parsing nested
+JSON elements. That can be done on the call stack making use of
+recursive calls or it can be done with a stack managed by the
+parser. The `Oj.load` method maintains a stack for Ruby object and
+builds the output as the parsing progresses.
+
+`Oj::Parser` uses three different stacks. One stack for values, one
+for keys, and one for collections (Array and Hash). By postponing the
+creation of the collection elements the bulk insertions for Array and
+Hash can be used. For arrays the use of a value stack and creating the
+array after all elements have been identified gives a 15x improvement
+in array creation.
+
+For Hash the story is a little different. The bulk insert for Hash
+alternates keys and values but there is a wrinkle to consider. Since
+Ruby Object creation is triggered by the occurance of an element that
+matches a creation identifier the creation of a collection is not just
+for Array and Hash but also Object. Setting Object attributes uses an
+ID and not a VALUE. For that reason the keys should not be created as
+String or Symbol types as they would be ignored and the VALUE creation
+wasted when setting Object attributes. Using the bulk insert for Hash
+gives a 3x improvement for that part of the object building.
+
+Looking at the Object creation the JSON gem expects a class method of
+`#json_create(arg)`. The single argument is the Hash resulting from
+the parsing assuming that the parser parsed to a Hash first. This is
+less than ideal from a performance perspective so `Oj::Parser`
+provides an option to take that approach or to use the much more
+efficient approach of never creating the Hash but instead creating the
+Object and then setting the attributes directly.
+
+To further improve performance and reduce the amount of memory
+allocations and frees the stacks are reused from one call to `#parse`
+to another.
+
+## Results
+
+The results are even better than expected. Running the
+[perf_parser.rb](https://github.com/ohler55/oj/blob/develop/test/perf_parser.rb)
+file shows the improvements. There are four comparisons all run on a
+MacBook Pro with Intel processor.
+
+### Validation
+
+Without a comparible parser that just validates a JSON document the
+`Oj.saj_parse` callback parser with a nil handler is used for
+comparison to the new `Oj::Parser.new(:validate)`. In that case the
+comparison is:
+
+```
+             System  time (secs)  rate (ops/sec)
+-------------------  -----------  --------------
+Oj::Parser.validate       0.101      494369.136
+       Oj::Saj.none       0.205      244122.745
+```
+
+The `Oj::Parser.new(:validate)` is **2.03** times faster!
+
+### Callback
+
+Oj has two callback parsers. One is SCP and the other SAJ. Both are
+similar in that a handler is provided that implements methods for
+processing the various element types in a JSON document. Comparing
+`Oj.saj_parse` to `Oj::Parser.new(:saj)` with a all callback methods
+implemented handler gives the following raw results:
+
+```
+        System  time (secs)  rate (ops/sec)
+--------------  -----------  --------------
+Oj::Parser.saj       0.783       63836.986
+   Oj::Saj.all       1.182       42315.397
+```
+
+The `Oj::Parser.new(:saj)` is **1.51** times faster.
+
+### Parse to Ruby primitives
+
+Parsing to Ruby primitives and Array and Hash is possible with most
+parsers including the JSON gem parser. The raw results comparing
+`Oj.strict_load`, `Oj::Parser.new(:usual)`, and the JSON gem are:
+
+```
+          System  time (secs)  rate (ops/sec)
+----------------  -----------  --------------
+Oj::Parser.usual       0.452      110544.876
+ Oj::strict_load       0.699       71490.257
+       JSON::Ext       1.009       49555.094
+```
+
+The `Oj::Parser.new(:saj)` is **1.55** times faster than `Oj.load` and
+**2.23** times faster than the JSON gem.
+
+### Object
+
+Oj supports two modes for Object serialization and
+deserialization. Comparing to the JSON gem compatible mode
+`Oj.compat_load`, `Oj::Parser.new(:usual)`, and the JSON gem yields
+the following raw results:
+
+```
+          System  time (secs)  rate (ops/sec)
+----------------  -----------  --------------
+Oj::Parser.usual       0.071      703502.033
+ Oj::compat_load       0.225      221762.927
+       JSON::Ext       0.401      124638.859
+```
+
+The `Oj::Parser.new(:saj)` is **3.17** times faster than
+`Oj.compat_load` and **5.64** times faster than the JSON gem.
+
+## Summary
+
+With a performance boost of from 1.5x to over 3x over the `Oj.load`
+parser the new `Oj::Parser` is a big win in the performance arena. The
+isolation of options is another feature that should make life easier
+for developers.

test/perf_parser.rb

@@ -8,6 +8,7 @@
 require 'optparse'
 require 'perf'
 require 'oj'
+require 'json'
 
 $verbose = false
 $iter = 50_000
@@ -54,6 +55,7 @@
 else
   Oj.default_options = {cache_keys: false, cache_str: 0, symbol_keys: $symbol_keys}
 end
+JSON.parser = JSON::Ext::Parser
 
 class AllSaj
   def initialize()
@@ -121,6 +123,7 @@ def initialize()
 perf = Perf.new()
 perf.add('Oj::Parser.usual', '') { p_usual.parse($json) }
 perf.add('Oj::strict_load', '') { Oj.strict_load($json) }
+perf.add('JSON::Ext', 'parse') { JSON.load($json) }
 perf.run($iter)
 
 ### Usual Objects ######################
@@ -165,11 +168,14 @@ def self.json_create(arg)
 p_usual.class_cache = true
 p_usual.ignore_json_create = true
 
+JSON.create_id = '^'
+
 puts '-' * 80
 puts "Parse Usual Object Performance"
 perf = Perf.new()
 perf.add('Oj::Parser.usual', '') { p_usual.parse($obj_json) }
 perf.add('Oj::compat_load', '') { Oj.compat_load($obj_json) }
+perf.add('JSON::Ext', 'parse') { JSON.load($obj_json) }
 perf.run($iter)
 
 unless $failed.empty?