json/tests/benchmarks/json_view/README.md

json_view compared with other libraries

The in-tree benchmarks in tests/benchmarks measure json_document against json::parse only. The programs here compare it with yyjson, simdjson, and Boost.JSON: the question users ask when they pick a library. They are not built by CMake or run by CI.

Reproducing the numbers

compare.py builds the programs against include/ of this checkout, runs them, and writes the results together with everything needed to reproduce them to results/<date>-<host>.md (and .csv): the date, the commit, the CPU, the OS, the compiler, the flags, and the versions of all libraries.

python3 tests/benchmarks/json_view/compare.py --data <json_test_data directory> [--native] [--rounds 30]

• --data is the downloaded test data, e.g. the test_files directory of a CMake build directory. It needs nativejson-benchmark/{twitter,citm_catalog,canada}.json and jeopardy/jeopardy.json.
• The other libraries come from the system: pkg-config, or Homebrew (brew install yyjson simdjson boost). With --download, pinned releases are downloaded instead and checked against their SHA-256. Without Boost headers (or with --no-boost), the Boost.JSON columns are skipped, and the results say so.
• --aligned additionally builds and runs everything with all functions aligned to 64 bytes (-mllvm -align-all-functions=6 for Clang, -falign-functions=64 -falign-loops=64 for GCC), and reports both result tables as "default build" and "aligned build". Code layout alone can move a result by a few percent; a difference that appears in only one of the two tables is a layout artifact. Off by default, which doubles the run time.
• --corpus file... adds files to the corpus benchmark, e.g. those of simdjson-data or the yyjson benchmark.
• Only the Python 3 standard library is used; a C++17 compiler is needed (CXX and CC are honored).

For numbers worth publishing, use a quiet machine (see Getting stable numbers), the default 30 rounds or more, and --native only if the other libraries were built for the same CPU.

On GitHub-hosted runners

The workflow json_view benchmarks runs compare.py --download on demand: by hand (Actions → "json_view benchmarks" → "Run workflow"), on an x86-64 or AArch64 Ubuntu runner with GCC or Clang, or when a pull request gets the label benchmark, on both architectures with GCC. The results appear as the job summary and as an artifact. Shared runners are noisy, so these numbers show where json_view stands on another architecture; they are not meant for publication.

What is measured

bench_view.cpp runs these workloads on twitter, citm_catalog, canada, jeopardy, a single tweet (status), and a JSON-RPC request (rpc):

workload	what it does
parse	build and free a document
traverse	parse, then visit every value, convert every number, touch every string and key
select	parse, then read a few fields per record (e.g. id, user name, and retweet count of each tweet)
dump	serialize a parsed document (compact)
parse+dump	build a fresh document, then serialize it (compact), in one timed call

bench_corpus.cpp runs parse, traverse, and dump on any list of files, so that no library is tuned to a handful of documents. Its dump also writes the numbers as they are in the input: json_view with number_format::source, and yyjson with numbers read as raw text (YYJSON_READ_NUMBER_AS_RAW), without converting them.

bench_edit.cpp measures read-modify-write: parse, apply the same logical edits with each library's own API, and serialize (compact). Workloads: patch (a handful of edits at fixed places) and update (edits in every record). An editable json_document edits in place; yyjson copies its immutable document into a mutable one first (yyjson_doc_mut_copy); Boost.JSON and json::parse build mutable DOMs; simdjson cannot edit a document. All outputs are checked to describe the same value.

Before anything is timed, all engines must accept each document and agree on the traversal: the number of values, the bytes of all strings and keys, and the sum of all numbers. All engines run interleaved in every round, and the best round is reported, as time and as a factor of the json_view time (below 1 means faster than json_view). Each timed call follows an untimed call of the same engine: otherwise the engine after json::parse pays for the allocator cleaning up the tens of thousands of nodes json::parse just freed (with glibc, this made json_view look 1.7 times slower on citm_catalog traverse).

json_view is an index into the source text, not a tree of owning nodes. Its like-for-like peers are yyjson and simdjson DOM (immutable, indexed, no per-node allocation). Boost.JSON and json::parse build mutable, owning DOMs and are reference points for what that costs, not equals; simdjson On-Demand is lazy and forward-only, so it is the other way round: it has no random access to pay for. All engines validate UTF-8 and syntax by default.

Number conversion differs: json_view converts integers while parsing but converts floating-point numbers lazily, when they are read or dumped; yyjson (default flags) and simdjson DOM convert every number while parsing. The cost of float conversion therefore appears in the traverse and dump rows for json_view, and in the parse row for the others. For float-heavy documents (canada), the fair reading is a sum of parse and use: the traverse workload already includes the parse, and parse+dump measures a fresh document and its dump in one timed call; the dump row alone leaves the conversion in json_view and out of the others. Eager conversion at parse time was prototyped as an opt-in and measured: canada traverse (with its parse) was 4-7 % faster, but parse+dump 11-30 % slower and parse 36-90 % slower. It was dropped, so lazy conversion is the only mode.

json_view can also be edited: json_editable_document has the same read API, and the (editable) rows in bench_view measure it on the same workloads. They are the fair counterpart of Boost.JSON and json::parse, which are mutable too; the plain json_view rows are the counterpart of yyjson and simdjson DOM, which are not. bench_edit adds the read-modify-write workloads. Read the tables with that in mind: an immutable-only library is not compared with an editable json_view unless it says so.

The engines do not all offer the same features, which the numbers should be read with:

engine	document	random access	editable	notes
json_view	immutable index into the text	yes	no	a fresh document per parse; "reused" parses into the same document
json_view (editable)	index into the text plus edits	yes	yes, in place	json_editable_document; keeps member order and number spellings
yyjson	immutable (yyjson_read)	yes	via a mutable copy
simdjson DOM	immutable, parser reused	yes	no	"fresh" uses a new parser per parse
simdjson On-Demand	none: forward-only, lazy	no	no	only traverse and select
Boost.JSON	owning, mutable DOM	yes	yes	monotonic resource
json::parse	owning, mutable DOM	yes	yes

Reusing memory matters as much as the parser. simdjson DOM reuses its parser, so it writes into memory it already touched; a fresh json_view document or yyjson document gets new memory for every parse. On Linux, glibc returns large blocks to the system when they are freed, so every fresh parse of a large document pays a page fault per 4 KiB page: on x86-64 Linux, a fresh json_view parse of jeopardy took about twice as long as a reused one. On macOS on Apple silicon, with 16 KiB pages, the difference is much smaller. Compare "json_view (reused)" with "simdjson DOM", and the fresh json_view with "simdjson DOM (fresh)" and yyjson.

Published results

Results are only published with the file compare.py wrote, which names the machine and the versions; see results/. Numbers from one machine and compiler do not carry over to another: rerun the script.