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Golang port of simdjson: parsing gigabytes of JSON per second

Golang port of simdjson: parsing gigabytes of JSON per second

simdjson-go

This is a Golang port of simdjson, a high performance JSON parser developed by Daniel Lemire and Geoff Langdale. It makes extensive use of SIMD instructions to achieve parsing performance of gigabytes of JSON per second.

Performance wise, simdjson-go runs on average at about 40% to 60% of the speed of simdjson. Compared to Golang's standard package encoding/json, simdjson-go is about 10x faster.

Features

simdjson-go is a validating parser, meaning that it amongst others validates and checks numerical values, booleans etc. Therefore these values are available as the appropriate int and float64 representations after parsing.

Additionally simdjson-go has the following features:

  • No 4 GB object limit
  • Support for ndjson (newline delimited json)
  • Proper memory management
  • Pure Go (no need for cgo)

Performance vs simdjson

Based on the same set of JSON test files, the graph below shows a comparison between simdjson and simdjson-go.

These numbers were measured on a MacBook Pro equipped with a 3.1 GHz Intel Core i7. Also, to make it a fair comparison, the constant GOLANG_NUMBER_PARSING was set to false (default is true) in order to use the same number parsing function (which is faster at the expense of some precision; see more below).

Performance vs encoding/json

Below is a performance comparison to Golang's standard package encoding/json based on the same set of JSON test files.

$ benchcmp                    encoding_json.txt      simdjson-go.txt
benchmark                     old MB/s               new MB/s         speedup
BenchmarkApache_builds-8      106.77                  948.75           8.89x
BenchmarkCanada-8              54.39                  519.85           9.56x
BenchmarkCitm_catalog-8       100.44                 1565.28          15.58x
BenchmarkGithub_events-8      159.49                  848.88           5.32x
BenchmarkGsoc_2018-8          152.93                 2515.59          16.45x
BenchmarkInstruments-8         82.82                  811.61           9.80x
BenchmarkMarine_ik-8           48.12                  422.43           8.78x
BenchmarkMesh-8                49.38                  371.39           7.52x
BenchmarkMesh_pretty-8         73.10                  784.89          10.74x
BenchmarkNumbers-8            160.69                  434.85           2.71x
BenchmarkRandom-8              66.56                  615.12           9.24x
BenchmarkTwitter-8             79.05                 1193.47          15.10x
BenchmarkTwitterescaped-8      83.96                  536.19           6.39x
BenchmarkUpdate_center-8       73.92                  860.52          11.64x

Also simdjson-go uses less additional memory and allocations.

Usage

Run the following command in order to install simdjson-go

$ go get github.com/minio/simdjson-go

In order to parse a JSON byte stream, you either call simdjson.Parse() or simdjson.ParseND() for newline delimited JSON files. Both of these functions return a ParsedJson object that can be used to navigate the JSON object by calling Iter().

Using the type Iter you can call Advance() to iterate over the tape, like so:

for {
    typ := iter.Advance()

    switch typ {
    case simdjson.TypeRoot:
        if typ, tmp, err = iter.Root(tmp); err != nil {
            return
        }

        if typ == simdjson.TypeObject {
            if obj, err = tmp.Object(obj); err != nil {
                return
            }

            e := obj.FindKey(key, &elem)
            if e != nil && elem.Type == simdjson.TypeString {
                v, _ := elem.Iter.StringBytes()
                fmt.Println(string(v))
            }
        }

    default:
        return
    }
}

More examples can be found in the examples subdirectory and further documentation can be found at godoc.

Design

simdjson-go follows the same two stage design as simdjson. During the first stage the structural elements ({, }, [, ], :, and ,) are detected and forwarded as offsets in the message buffer to the second stage. The second stage builds a tape format of the structure of the JSON document.

Note that in contrast to simdjson, simdjson-go outputs uint32 increments (as opposed to absolute values) to the second stage. This allows arbitrarily large JSON files to be parsed (as long as a single (string) element does not surpass 4 GB...).

Also, for better performance, both stages run concurrently as separate go routines and a go channel is used to communicate between the two stages.

Stage 1

Stage 1 has been converted from the original C code (containing the SIMD intrinsics) to Golang assembly using c2goasm. It essentially consists of five seperate steps, being:

  • find_odd_backslash_sequences: detect backslash characters used to escape quotes
  • find_quote_mask_and_bits: generate a mask with bits turned on for characters between quotes
  • find_whitespace_and_structurals: generate a mask for whitespace plus a mask for the structural characters
  • finalize_structurals: combine the masks computed above into a final mask where each active bit represents the position of a structural character in the input message.
  • flatten_bits_incremental: output the active bits in the final mask as incremental offsets.

For more details you can take a look at the various test cases in find_subroutines_amd64_test.go to see how the individual routines can be invoked (typically with a 64 byte input buffer that generates one or more 64-bit masks).

There is one final routine, find_structural_bits_in_slice, that ties it all together and is invoked with a slice of the message buffer in order to find the incremental offsets.

Stage 2

During Stage 2 the tape structure is constructed. It is essentially a single function that jumps around as it finds the various structural characters and builds the hierarchy of the JSON document that it processes. The values of the JSON elements such as strings, integers, booleans etc. are parsed and written to the tape.

Any errors (such as an array not being closed or a missing closing brace) are detected and reported back as errors to the client.

Tape format

Similarly to simdjson, simdjson-go parses the structure onto a 'tape' format. With this format it is possible to skip over arrays and (sub)objects as the sizes are recorded in the tape.

simdjson-go format is exactly the same as the simdjson tape format with the following 2 exceptions:

  • In order to support ndjson, it is possible to have many root elements on the tape. Also, to allow for fast navigation over root elements, a root points to the next root element (and as such the last root element points 1 index past the length of the tape).
  • Strings are handled differently, unlike simdjson the string size is not prepended in the String buffer but is added as an additional element to the tape itself (much like integers and floats). Only strings that contain special characters are copied to the String buffer in which case the payload from the tape is the offset into the String buffer. For string values without special characters the tape's payload points directly into the message buffer.

For more information, see TestStage2BuildTape in stage2_build_tape_test.go.

Requirements

simdjson-go has the following requirements:

  • A CPU with both AVX2 and CLMUL is required (Haswell from 2013 onwards should do for Intel, for AMD a Ryzen/EPIC CPU (Q1 2017) should be sufficient).

Minor number inprecisions

The number parser has minor inprecisions compared to Golang's standard number parsing. There is constant GOLANG_NUMBER_PARSING (on by default) that uses Golang's parsing functionality at the expense of giving up some performance. Note that the performance metrics mentioned above have been measured by setting the GOLANG_NUMBER_PARSING to false.

License

simdjson-go is released under the Apache License v2.0. You can find the complete text in the file LICENSE.

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