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data from a preliminary benchmark of mmJSON parsing
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Which of the C++ JSON parsers is the fastest one for your JSON files?

I got curious because different benchmarks show different results. So, which of the DOM-building parsers can be the fastest one as of August 2017?

Note: this was written before simdjson.

  • RapidJSON by Milo Yip. It is a well-known fastest C++ parser. When there is a news on r/rust that a library in Rust is faster than the fastest C++ JSON library -- they mean RapidJSON. RapidJSON may also be the most flexible one. It has full-precision mode for slow but pedantic parsing of numbers, in-situ mode for fast parsing that modifies the input buffer (like sajson and gason), and so on.

  • sajson by Chad Austin. I learnt about it from the author's recent blog post. Interesting read. Recently sajson got a boost by borrowing a few tricks from Rich Geldreich's pjson. After this boost sajson is faster than RapidJSON, but still not as fast as pjson. Unfortunately, pjson itself was just dumped into pastebin with no intention to maintain it.

  • gason by Ivan Vashchaev. It is a tiny library, only about 450 LOC. Benchmarks from the project's page show performance similar to RapidJSON. Unlike other parsers listed here gason simplifies numeric parsing by having only a single numeric type (integers are stored as double).

    Apparently, in the last years the author has been working on gason2 which aims to be both faster and more accurate in number parsing. But it is not clear to me what is the current status of gason2.

I'll skip Jsmn, a parser/tokenizer by Serge Zaitsev, because it doesn't create a DOM-like structure. Jsmn wins in one of the RapidJSON's benchmarks (Statistics).

On the other hand I'll include taocpp/json (by Daniel Frey and Colin Hirsch), which doesn't aim to be that fast. On the contrary, it uses standards C++ containers, avoids manual optimisations (actually, the parser is "generated" by the excellent PEGTL library), and is as correct as possible. Despite of this it has a decent performance.

I'm including taocpp as a baseline, because it is a neat project that I may use at some point -- if not for JSON then for CBOR (good alternative to MessagePack and UBJSON). Unlike most of the other JSON parsers taocpp has bus factor 2 not 1.

The easiest way to compare the parsers is to run RapidJSON's nativejson-benchmark with own data files.


This benchmark has an impressive collection of more than 40 C and C++ parsers. It does not have pjson and gason2 (it has a pjson but a different one), and I won't bother with adding them, although they could be the fastest ones.

To run it, you need premake5 for build and php for report generation. And more than 0.5 GB of the disk space for cloning all the tested parsers.

Pick data files

To change the data files used in the benchmark just drop your files into the data/ directory and edit data/data.txt.

I'm here to benchmark parsing of mmJSON files. As an example I'll use 4un4 converted from mmCIF to mmJSON with gemmi-convert.

And I'll also add apache_builds.min.json from sajson benchmark -- to check if I get the same result (sajson faster than RapidJSON).

jq -c . <thirdparty/sajson/testdata/apache_builds.json >data/apache_builds.min.json

Pick parsers

Maintaining a code that includes 40+ third-party libraries is an ungrateful task and as can be seen from the project issues, it is not easy to compile. But it will be quite easy if you want to check only a few libraries: just remove most of the files from src/tests/ leaving only tests for selected parsers (gasontest.cpp, rapidjsonfullprectest.cpp, rapidjsoninsitutest.cpp, rapidjsontest.cpp, sajsontest.cpp, taocpptest.cpp).

You may also want to comment out building of problematic static libraries in build/premake5.lua (they are built for individual parsers).

I also updated each of the tested parsers in thirdparty/ to its latest version, just to be sure.

Pick compilers and options

To use GCC 5 just do:

CXX=g++-5 CC=gcc-5 PATH="path/to/premake5/dir:$PATH" make

Unfortunately, the benchmark doesn't build with GCC 7 and Clang 4. As a workaround I disabled memory statistic (USE_MEMORYSTAT), which in turn confused the report generator. So I got full reports for GCC 5 and somewhat broken reports for GCC 7 and Clang 4. Anyway, it's awesome that that the benchmark generates reports with plots in addition to printing the numbers.

Compilation options can be changed in build/premake5.lua.

Conformance reports

Nativejson-benchmark produces also a conformance report for each parser. The simplest parsers, such as gason, get a poor score. But looking closely at it the failing conformance tests are not necessarily bad things.

The first category has strictness tests. Gason and many others are intentionally more liberal in accepting input than the JSON standard. This may actually be a good thing.

Then, numerical accuracy. For example, gason stores 1.234e-10 as 1.2340000000000014e-10 instead of 1.2340000000000001e-10. A trade-off between speed and accuracy.

Finally, gason does not handle correctly the null byte (\0) in the middle of a string, and fails to translate \u surrogate pairs into UTF-8. This is good to know, even if it is not relevant to my input files.

Sajson and RapidJSON without the full-precision mode fail only on the numerical tests. Taocpp/json and RapidJSON-full-prec are 100% conformant.


I ran the tests on Ubuntu 16.04 on a laptop with Intel Broadwell processor. By default, the benchmark is compiled with -O3 -march=native. I tested it also without -march=native, but the results were approximately the same. Both sets of results are in this repo.

Here is what I got with GCC 7:

GCC7 results

(Yes, it'd be better to show MB/s).

Three of the benchmarks are in-situ: RapidJSON_Insitu, sajson, gason. Two are not in-situ and with full numeric precision: taocpp and RapidJSON_FullPrec.

The results are consistent with both gason and sajson benchmarks. Each of RapidJSON, sajson and gason wins at least one test and can be rightfully called the fastest parser.

As Chad noted in #1, sajson in the dynamic allocation mode (that was used here) trades a bit of CPU performance for much lower peak memory usage when parsing.

Here are results I got with Clang 4:

Clang 4 results

Chad's results were showing very low performance of rapidjson/clang (the graph for Ubuntu 16.04 on Intel Broadwell). Here Clang is almost as good as GCC.

And if you wonder about memory usage:

Memory usage

Note that benchmarks of the in-situ parsers first copy the original data, so if you already have a mutable buffer with the data it is gason that will have the smallest memory footprint.

Finally, parsing is only part of the job and one should also benchmark access to the resulting data structure. But this would require more work...

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