The simdjson library uses three-quarters less instructions than state-of-the-art parser RapidJSON and fifty percent less than sajson. To our knowledge, simdjson is the first fully-validating JSON parser to run at gigabytes per second on commodity processors. It can parse millions of JSON documents per second on a single core. It takes advantage of modern microarchitectures, parallelizing with SIMD vector instructions, reducing branch misprediction, and reducing data dependency to take advantage of each CPU’s multiple execution cores.

You may use simdjson for more than just parsing JSON. It offers functions to minify JSON documents at 6 GB/s or validate UTF-8 strings at 13 GB/s.

On a Skylake processor, the parsing speeds (in GB/s) of various processors on the twitter.json file are as follows, using again GNU GCC 9 (with the -O3 flag). The popular JSON for Modern C++ library is particularly slow: it obviously trades parsing speed for other desirable features.

parser GB/s
simdjson 2.5
RapidJSON UTF8-validation 0.29
RapidJSON UTF8-valid., exact numbers 0.28
RapidJSON insitu, UTF8-validation 0.41
RapidJSON insitu, UTF8-valid., exact 0.39
sajson (insitu, dynamic) 0.62
sajson (insitu, static) 0.88
dropbox 0.13
fastjson 0.27
gason 0.59
ultrajson 0.34
jsmn 0.25
cJSON 0.31
JSON for Modern C++ (nlohmann/json) 0.11

The simdjson library offers high speed whether it processes tiny files (e.g., 300 bytes) or larger files (e.g., 3MB).

All our experiments are reproducible.

You can go beyond 4 GB/s with our new On Demand API. For NDJSON files, we can exceed 3 GB/s with our multithreaded parsing functions.


The work is supported by the Natural Sciences and Engineering Research Council of Canada under grant number RGPIN-2017-03910.


This code is made available under the Apache License 2.0.

Some of our command-line tools use the cxxopts library: it is under the liberal (business-friendly) MIT license.

Under Windows, we build some tools using the windows/dirent_portable.h file (which is outside our library code): it is under the liberal (business-friendly) MIT license.

For some benchmarking tasks, we use Google Benchmark. It is under the Apache License 2.0.