rkyv is faster than {bincode, capnp, cbor, flatbuffers, postcard, prost, serde_json}
Mar 11, 2021 – 9 min read
Rust, Rkyv, Serialization

I've been working on rkyv, a zero-copy deserialization library, since November of 2020. rkyv is similar to Cap'n Proto and FlatBuffers, but has a handful of different design choices that make it stand out:

But just having design goals isn't good enough, you need results to back them up. With that in mind, I can't disclaim enough that I am the creator and maintainer of rkyv. However, the last thing I want is to be biased, so I made some benchmarks to hopefully convince you on their own merits.


There are a couple different benchmarks already available, but in general they fail in a couple different ways:

They test with too little data

This leads to highly variable results and can make it difficult to see whether one library is really faster than another

They test only with simple data

The library may perform completely differently with complex and highly structured data

They test only serialization and deserialization

For most serialization formats, all you can do is serialize and deserialize data. But zero-copy deserialization libraries can access and traverse data without deserializing it first. Knowing how these operations compare with each other is essential to evaluating their relative performance.


With these shortcomings in mind, I set off to make my own benchmarks. The goal was to be thorough and complete, and I think I did a pretty good job.

You can run the benchmarks yourself or look over the raw data from the github repo. I'll summarize the results.


Each library got tested on three different data sets:

Each data set is randomly generated from an RNG seeded with the first 20 digits of pi, so the data tested is identical for every run. For each data set, a library was measured for the following:

Additionally, zero-copy deserialization libraries were tested for:

There are a couple footnotes that need explaining:

  1. Abomonation requires a mutable backing to access and read serialized data. This means that it's not viable for some use cases.
  2. Abomonation does not support buffer mutation, so this wasn't tested.
  3. While Flatbuffers and Cap'n Proto support buffer mutation in the main (usually C++) libraries, the rust counterparts do not and they couldn't be tested for this.
  4. None of the other zero-copy deserialization frameworks provided deserialization capabilities by default. Writing and benchmarking my own deserialization code is somewhat meaningless for these. You can get an idea of what sort of deserialization performance you'd get by looking at the read benchmark.
  5. Abomonation's decode qualified as access not deserialize because it yields an immutable reference instead of a mutable object. In order to deserialize this object, a simple Clone would suffice but I'm not here to write and benchmark my own deserialization code.


These results are directly from the benchmark repo.


This data set is composed of HTTP request logs that are small and contain many strings.

Raw data

For operations, time per iteration; for size, bytes. Lower is better.

Format / Lib Serialize Access Read Update Deserialize Size Size (zlib)
abomonation 315.13 us 36.773 us* 58.999 us* 1705800 507971
bincode 640.51 us 4.2787 ms 1045784 374305
capnp 1.8558 ms 259.95 ns 711.84 us § 1843240 537966
cbor 1.9698 ms 8.9702 ms 1407835 407372
flatbuffers 2.6780 ms 2.9815 ns 162.95 us § 1276368 469962
postcard 714.70 us 4.4387 ms 765778 312739
prost 5.4927 ms 5.1024 ms 764951 269811
rkyv 422.92 us 1.3616 ns 18.962 us 71.321 us 3.2492 ms 1065784 333895
serde_json 4.4054 ms 10.148 ms 1827461 474358


Relative to best. Higher is better.

Format / Lib Serialize Access Read Update Deserialize Size Size (zlib)
abomonation 100.00% 0.00%* 32.14%* 44.84% 53.12%
bincode 49.20% 75.94% 73.15% 72.08%
capnp 16.98% 0.52% 2.66% § 41.50% 50.15%
cbor 16.00% 36.22% 54.34% 66.23%
flatbuffers 11.77% 45.67% 11.64% § 59.93% 57.41%
postcard 44.09% 73.20% 99.89% 86.27%
prost 5.74% 63.68% 100.00% 100.00%
rkyv 74.51% 100.00% 100.00% 100.00% 100.00% 71.77% 80.81%
serde_json 7.15% 32.02% 41.86% 56.88%


The data set is a single mesh. The mesh contains an array of triangles, each of which has three vertices and a normal vector.

Raw data

For operations, time per iteration; for size, bytes. Lower is better.

Format / Lib Serialize Access Read Update Deserialize Size Size (zlib)
abomonation 430.61 us 2.4135 ns* 177.87 us* 6000024 5380836
bincode 7.0288 ms 12.294 ms 6000008 5380823
capnp 15.854 ms 247.35 ns 8.9442 ms § 16000056 6780527
cbor 43.109 ms 70.247 ms 13122324 7527423
flatbuffers 1.9518 ms 2.9588 ns 152.39 us § 6000024 5380800
postcard 6.6844 ms 8.9408 ms 6000003 5380817
prost 34.037 ms 20.232 ms 8750000 6683814
rkyv 1.1217 ms 1.4006 ns 172.20 us 649.18 us 1.9594 ms 6000008 4263104
serde_json 105.86 ms 83.016 ms 26192883 9612105


Relative to best. Higher is better.

Format / Lib Serialize Access Read Update Deserialize Size Size (zlib)
abomonation 100.00% 58.03%* 85.67%* 100.00% 79.23%
bincode 6.13% 15.94% 100.00% 79.23%
capnp 2.72% 0.57% 1.70% § 37.50% 62.87%
cbor 1.00% 2.79% 45.72% 56.63%
flatbuffers 22.06% 47.34% 100.00% § 100.00% 79.23%
postcard 6.44% 21.92% 100.00% 79.23%
prost 1.27% 9.68% 68.57% 63.78%
rkyv 38.39% 100.00% 88.50% 100.00% 100.00% 100.00% 100.00%
serde_json 0.41% 2.36% 22.91% 44.35%


The data set is composed of Minecraft player saves that contain highly-structured data.

Raw data

For operations, time per iteration; for size, bytes. Lower is better.

Format / Lib Serialize Access Read Update Deserialize Size Size (zlib)
abomonation 368.23 us 40.823 us* 41.413 us* 1290592 393696
bincode 806.73 us 3.4132 ms 569975 240897
capnp 863.41 us 256.55 ns 5.3431 us § 835784 342099
cbor 2.4356 ms 8.8797 ms 1109821 347562
flatbuffers 38.683 ms 2.9212 ns 3.9676 us § 849472 349208
postcard 774.37 us 3.7533 ms 356311 213270
prost 5.8678 ms 5.4083 ms 596811 306728
rkyv 843.80 us 1.3837 ns 282.88 ns 6.5422 us 2.4810 ms 725176 334238
serde_json 4.3501 ms 10.699 ms 1623197 472162


Relative to best. Higher is better.

Format / Lib Serialize Access Read Update Deserialize Size Size (zlib)
abomonation 100.00% 0.00%* 0.68%* 27.61% 54.17%
bincode 45.64% 72.69% 62.51% 88.53%
capnp 42.65% 0.54% 5.29% § 42.63% 62.34%
cbor 15.12% 27.94% 32.11% 61.36%
flatbuffers 0.95% 47.37% 7.13% § 41.94% 61.07%
postcard 47.55% 66.10% 100.00% 100.00%
prost 6.28% 45.87% 59.70% 69.53%
rkyv 43.64% 100.00% 100.00% 100.00% 100.00% 49.13% 63.81%
serde_json 8.46% 23.19% 21.95% 45.17%


* abomonation requires a mutable backing to access data

abomonation does not support buffer mutation

do not provide deserialization capabilities, but the user can write their own

§ supports buffer mutation, but not in the rust implementation


CBOR / serde_json

Unsurprisingly, these two had very similar performance because they're almost the same format. CBOR did a bit better than serde_json in every benchmark, but these two consistently trailed behind all the other frameworks (in some cases, very considerably behind).


Prost was the chosen representative for protobuf-style serialization. Its performance was average-to-lackluster on every benchmark, with the exception of the log size benchmark. It beat out postcard, which consistently performed extremely well in the size/zlib categories. This shows just how much the format was optimized for stringy data and minimizing wire size.

bincode / postcard

Despite being completely different libraries, bincode and postcard had very similar benchmark results. Serialize and deserialize speed were very close for both of them, and the main difference between the two was usually the final size. Postcard consistently beat bincode on size and zlib. I suspect that they are using very similar techniques, but that postcard has a few more tricks up its sleeve that don't cost much to perform but give it a sizeable advantage.

Cap'n Proto

Cap'n Proto had a good showing, and it proved its worth as a replacement for protobuf. Compared to prost, it was faster to serialize, and supported comparatively fast zero-copy deserialization. These two features are absolutely killer. Unfortunately, it didn't stack up nearly as well against the other zero-copy frameworks. It consistently had disapointing access and read times compared to its competitors, and failed pretty miserably on the mesh size benchmarks. This makes sense as it wasn't built to handle large amounts of raw data, but it was disappointing to see so much wasted space compared to FlatBuffers.


FlatBuffers is the comparison point for zero-copy deserialization. It's got a lot of usage, was built specifically for performance, and proves out the zero-copy concept. It did well in all categories on most of the tests, but had a major stumbling block. In the minecraft_savedata test, its serialization performance was by far the worst, even worse than serde_json (which had to write twice as much data!). This highlights a major weakness of FlatBuffers: its very poor serialization performance on highly-structured data. It's possible (even probable) that I wrote this bench more poorly than it could be, but it's enough that I wouldn't recommend its use for general-purpose data.


Abomonation was definitely a bright spot in the benchmarks. It proved out its insanely fast serialization on every bench, and didn't suffer from some of the size traps that its competitors fell into. It would be an easy library to recommend if it didn't come with so many caveats. It's very unsafe, non-portable, requires mutable backing to access its data, and doesn't support mutations. Nonetheless, abomonation was a really impressive contender in every benchmark.


I went into these benchmarks not knowing how rkyv would perform relative to its peers, but confident that it would make a good showing. It ended up doing much better than I expected. It won nearly every performance category, and was highly competitive with the winner when it didn't. It also did so without compromising on size, where it was also highly competitive. Finally it showed exceptional scalability, peforming equally well on all different kinds of data where its zero-copy competitors all hard shortcomings on one or more of the data sets. Unlike abomonation, it's also a safe, highly-portable format that doesn't need mutable backing and has more feature support than other competitors.


I welcome and encourage anyone to run the benchmarks for themselves and open pull requests to improve or clean up whatever they want. I am confident in the validity of these results, and will happily update the tables as changes are made. I will update my analyses if there are any major changes.

My hope is that this article not only convinced you that rkyv is one of the best-performing serializers available, but that it also helped you understand the relationships between the different serialization solutions available in rust today.

If you're interested in rkyv, I encourage you to contribute to the request for feedback for planning its future

Thanks to burntsushi for the article title inspiration