Performance comparison with in-place serde + possible improvements

[ncpenke]

Awesome work as usual @jorgecarleitao.

I was curious what the performance would be with serde_json's in-place deserialization so I created a custom deserializer for parser::Value. I had to duplicate parser::Value to use converted numbers since it wasn't possible to use this library's lazy numeric parser without replicating all of serde_json's parsing code.

The code for the custom deserializer is here. I updated the benchmarks to include this implementation, and also added a test for escaped strings.

Results from my macbook are below. serde_json_custom is the benchmark for the custom deserializer. string_escaped_chars is the benchmark for escaped strings.

string json_deserializer 2^10 time:   [63.949 us 64.339 us 64.739 us]
string serde_json 2^10  time:   [100.51 us 101.21 us 101.94 us]
string serde_json_custom 2^10 time:   [41.162 us 41.491 us 41.923 us]
string simd_json 2^10   time:   [25.250 us 25.574 us 25.932 us]

string_escaped_chars json_deserializer 2^10 time:   [193.93 us 195.39 us 196.92 us]
string_escaped_chars serde_json 2^10 time:   [130.26 us 131.22 us 132.27 us]
string_escaped_chars serde_json_custom 2^10 time:   [137.42 us 138.47 us 139.55 us]
string_escaped_chars simd_json 2^10 time:   [46.427 us 46.821 us 47.252 us]

I was surprised at the results, especially since nothing stuck out from inspecting the code. So I dug in further and it seems to come to the relatively large match expression in compute_length. Reworking it improved up the numbers:

string json_deserializer 2^10 time:   [39.574 us 39.719 us 39.868 us]
string serde_json 2^10  time:   [103.74 us 104.84 us 105.99 us]
string serde_json_custom 2^10 time:   [46.926 us 48.188 us 49.584 us]
string simd_json 2^10   time:   [27.517 us 27.786 us 28.105 us]

string_escaped_chars json_deserializer 2^10 time:   [144.19 us 145.30 us 146.43 us]
string_escaped_chars serde_json 2^10 time:   [143.75 us 144.64 us 145.52 us]
string_escaped_chars serde_json_custom 2^10 time:   [145.71 us 147.09 us 148.73 us]
string_escaped_chars simd_json 2^10 time:   [47.152 us 47.652 us 48.162 us]

If you're able to replicate the results and interested in taking the changes can open a PR. Changes are in this branch. Thanks!

[jorgecarleitao]

Thanks!

Great comparison and analysis, @ncpenke , very well though through.

interested in taking the changes can open a PR

Yes! That would be awesome!

I think that it would also be valuable to have the code from the branch implementing Deserialize under a serde feature flag. I am thinking doing the same for the trait Value as discussed in #3

[ncpenke]

I think that it would also be valuable to have the code from the branch implementing Deserialize under a serde feature flag

Sounds great! I can make those as part of another pass.