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SIMD Json for Rust   Build Status Build Status ARM Quality Latest Version Code Coverage

Rust port of extremely fast simdjson JSON parser with serde compatibility.

readme (for real!)

simdjson version

Currently tracking version 0.2.x of simdjson upstream (work in progress, feedback welcome!).

CPU target

To be able to take advantage of simd-json your system needs to be SIMD capable. This means that it needs to compile with native cpu support and the given features. This also requires that projects using simd-json also need to be configured with native cpu support. Look at The cargo config in this repository to get an example of how to configure this in your project.

simd-json supports AVX2, SSE4.2 and NEON.

Unless the allow-non-simd feature is passed to your simd-json dependency in your Cargo.toml simd-json will fail to compile, this is to prevent unexpected slowness in fallback mode that can be hard to understand and hard to debug.


For best performance we highly suggest using mimalloc or jemalloc instead of the system allocator used by default. Another recent allocator that works well ( but we have yet to test in production a setting ) is snmalloc.


simd-json is compatible with serde and serde-json. The Value types provided implement serializers and deserializers. In addition to that simd-json implements the Deserializer trait for the parser so it can deserialize anything that implements the serde Deserialize trait. Note, that serde provides both a Deserializer and a Deserialize trait.

That said the serde support is contained in the serde_impl feature which is part of the default feature set of simd-json, but it can be disabled.


The known-key feature changes the hash mechanism for the DOM representation of the underlying JSON object, from ahash to fxhash. The ahash hasher is faster at hashing and provides protection against DOS attacks by forcing multiple keys into a single hashing bucket. The fxhash hasher on the other hand allows for repeatable hashing results, which in turn allows memoizing hashes for well known keys and saving time on lookups. In workloads that are heavy at accessing some well known keys this can be a performance advantage.

The known-key feature is optional and disabled by default and should be explicitly configured.


This flag has no effect on simd-json itself but purely affets the Value structs.

The value-no-dup-keys feature flag toggles stricter behaviour for objects when deserializing into a Value. When enabled, the Value deserializer will remove duplicate keys in a JSON object and only keep the last one. If not set duplicate keys are considered undefined behaviour and Value will not make guarantees on it's behaviour.


simd-json uses a lot of unsafe code.

There are a few reasons for this:

  • SIMD intrinsics are inherently unsafe. These uses of unsafe are inescapable in a library such as simd-json.
  • We work around some performance bottlenecks imposed by safe rust. These are avoidable, but at a cost to performance. This is a more considered path in simd-json.

simd-json goes through extra scrutiny for unsafe code. These steps are:

  • Unit tests - to test 'the obvious' cases, edge cases, and regression cases
  • Structural constructive property based testing - We generate random valid JSON objects to exercise the full simd-json codebase stochastically. Floats are currently excluded since slighty different parsing algorithms lead to slighty different results here. In short "is simd-json correct".
  • Data-oriented property based testing of string-like data - to assert that sequences of legal printable characters don't panic or crash the parser (they might and often error so - they are not valid json!)
  • Destructive Property based testing - make sure that no illegal byte sequences crash the parser in any way
  • Fuzzing - fuzz based on upstream & jsonorg simd pass/fail cases
  • Miri testing for UB

This doesn't ensure complete safety nor is at a bullet proof guarantee, but it does go a long way to asserting that the library is production quality and fit for purpose for practical industrial applications.

Other interesting things

There are also bindings for upstream simdjson available here


simd-json itself is licensed under either of

However it ports a lot of code from simdjson so their work and copyright on that should be respected along side.

The serde integration is based on their example and serde-json so again, their copyright should as well be respected.