[New-bugs-announce] [issue37587] JSON loads performance improvement for long strings

[Marco Paolini]

New submission from Marco Paolini <markopaolini at gmail.com>:

I analysed the performance of json.loads in one production workload we have.

Spy-py tells me the majority of time is spent into C json module (see events.svg)

Digging deeper, linux perf tells me hottest loop (where 20%+ of the time is spent) in _json.scanstring_unicode, in this loop:

189:   movzx  eax,BYTE PTR [rbp+rbx*1+0x0]
       mov    DWORD PTR [rsp+0x44],eax
       cmp    eax,0x22
       je     22f
       cmp    eax,0x5c
       je     22f
       test   r13d,r13d
       je     180
       cmp    eax,0x1f

which is related to this code in Modules/_json.c


        /* Find the end of the string or the next escape */
        Py_UCS4 c = 0;
        for (next = end; next < len; next++) {
            c = PyUnicode_READ(kind, buf, next);
            if (c == '"' || c == '\\') {
                break;
            }
            else if (strict && c <= 0x1f) {
                raise_errmsg("Invalid control character at", pystr, next);
                goto bail;
            }
        }

Two optimisations can be done:

1. remove the mov entirely. It is not needed inside the loop and it is only needed later, outside the loop to access the variable
2. switch around the strict check (in the second if) because strict defaults to 1 so it will likely pass the test, while the likelyness of finding an invalid character is lower

Running the pyperformance json_loads benchmark I get:

+------------+----------------------+-----------------------------+
| Benchmark  | vanilla-pyperf-pgo38 | patched-pyperf-pgo38        |
+============+======================+=============================+
| json_loads | 54.9 us              | 53.9 us: 1.02x faster (-2%) |
+------------+----------------------+-----------------------------+


A micro benchmark on a 1MB long json string gives better results:

python -m pyperf timeit -s "import json; x = json.dumps({'k': '1' * 2 ** 20})" "json.loads(x)"

+-----------+------------+-----------------------------+
| Benchmark | vanilla-1m | patched-1m                  |
+===========+============+=============================+
| timeit    | 2.62 ms    | 2.39 ms: 1.10x faster (-9%) |
+-----------+------------+-----------------------------+

----------
components: Library (Lib)
files: events.svg
messages: 347832
nosy: christian.heimes, mpaolini, pablogsal
priority: normal
severity: normal
status: open
title: JSON loads performance improvement for long strings
type: performance
versions: Python 3.6, Python 3.7, Python 3.8, Python 3.9
Added file: https://bugs.python.org/file48476/events.svg

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37587>
_______________________________________