Description

wjmelements

opened on Feb 6, 2025

Issue body actions

Bug report

Bug description:

Experimentally I measured a huge performance improvement when I switched my code from

json.dump(x, f, **)

to

f.write(json.dumps(x, **))

Method

I essentially wrote the same contents to different files sequentially and measured the total amount of time taken. The json contents had 1, 300, and 400 entries per level, and 1, 5, and 6 levels of depth. There's quite a level of variance here but this wasn't what I was trying to measure in the first place. I discovered this by chance, so forgive the lack of precision. I also don't have the source code anymore because I wasn't originally planning to report this discovery.

Results

File Size	Consecutive Files	dump µs	dumps µs
74	1	508	581
74	2	520	541
74	4	1153	1151
74	8	1930	1750
39184	1	6363	1086
39184	2	11261	1821
39184	4	38126	3521
39184	8	80411	6466
468218	1	82821	11921
468218	2	150234	38017
468218	4	302357	42137
468218	8	573450	78545

Conclusion

A cursory investigation into the cpython code suggests that the slow part is the sequential writing of the iterencode yield. The chunks are quite small.

CPython versions tested on:

3.10

Operating systems tested on:

macOS

Linked PRs

• Draft: gh-129711: Reduce number of yield statements in json encoder #130076

Activity

1. wjmelements

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Feb 6, 2025
2. tomasr8

   added this to JSON issueson Feb 6, 2025
3. pochmann3 commented on Feb 6, 2025

   

   pochmann3

   on Feb 6, 2025

   Contributor

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   Old issue that I think partially covers this (wasn't about depth, though):

   #56343
4. pochmann3 commented on Feb 6, 2025

   

   pochmann3

   on Feb 6, 2025

   Contributor

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   Demo with deep nesting:

    37.3 ms  json.dump(x, f)
     0.3 ms  f.write(json.dumps(x))
    36.2 ms  json.dump(x, f)
     0.3 ms  f.write(json.dumps(x))
    36.2 ms  json.dump(x, f)
     0.3 ms  f.write(json.dumps(x))
   

   from timeit import timeit
   
   setup = '''
   import io, json
   f = io.StringIO()
   x = [0] * 1000
   for i in range(900):
       x = [x]
   '''
   
   for code in [
       'json.dump(x, f)',
       'f.write(json.dumps(x))',
   ] * 3:
       t = timeit(code, setup, number=10) * 100
       print(f'{t:5.1f} ms  {code}')

   Attempt This Online!

   Reacted by Tomas R.
5. eendebakpt

   added

   performancePerformance or resource usagePerformance or resource usage

   on Feb 7, 2025
6. picnixz

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   on Feb 7, 2025
7. pganssle commented on Feb 7, 2025

   

   pganssle

   on Feb 7, 2025

   Member

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   I think the main difference between these code paths is that when you are incrementally encoding the JSON, CPython uses the Python version of make_encoder, whereas when the encoding is done all at once, CPython uses the C implementation from _json.c. If you modify @pochmann3's example to disable the C encoder:

   from timeit import timeit
   import json.encoder
   json.encoder.c_make_encoder = None
   
   setup = '''
   ...

   The two functions perform the same:

    37.9 ms  json.dump(x, f)
    37.3 ms  f.write(json.dumps(x))
    36.7 ms  json.dump(x, f)
    38.0 ms  f.write(json.dumps(x))
    35.0 ms  json.dump(x, f)
    35.4 ms  f.write(json.dumps(x))
   

   I'm guessing that the performance would be a lot closer by impementing an incremental encoder in C.
10. pochmann3 commented on Feb 9, 2025

    

    pochmann3

    on Feb 9, 2025

    Contributor

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    the primary difference stems from json.dump performing incremental writes using Python's iterencode, whereas json.dumps utilizes the C-optimized encoder

    I haven't looked at the C version, but dump was that slow in my demo mostly because of the recursion, not because it uses Python.
11. eendebakpt commented on Feb 13, 2025

    

    eendebakpt

    on Feb 13, 2025

    Contributor

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    @Ghost4 Performance related improvements are often a trade-off between performance and added code complexity. Here the gain is significant, but it is not yet clear how complex a C implementation would be. Also it is not entirely clear what the reason is for the performance differences: C vs. Pytho, recursion, or something else.

    Guidelines for making PRs can be found at https://devguide.python.org/
12. eendebakpt

    added a commit that references this issue on Jun 15, 2026

    pythongh-129711: Add a streaming C implementation of the JSON encoder

    baf0b93
13. eendebakpt

    added 2 commits that reference this issue on Aug 11, 2026

    pythongh-129711: Add a streaming C implementation of the JSON encoder

    7eb7a4d

    pythongh-129711: Factor out circular-marker and dict-key helpers in _…

    72e86c4
14. 5 remaining items
15. eendebakpt

    added 11 commits that reference this issue on Aug 12, 2026

    pythongh-129711: Add tests for the streaming JSON encoder

    58a11f5

    pythongh-129711: Add a streaming C implementation of the JSON encoder

    0a53763

    pythongh-129711: Add tests for the streaming JSON encoder

    f293cd3

    pythongh-129711: Factor out circular-marker and dict-key helpers in _…

    40ed013

    pythongh-129711: Add a streaming C implementation of the JSON encoder

    d0ad68e

    pythongh-129711: Add tests for the streaming JSON encoder

    2541a17

    pythongh-129711: Refactor _json encoder helpers for reuse

    4aafd9e

    pythongh-129711: Add a streaming C implementation of the JSON encoder

    c676a7c

    pythongh-129711: Add tests for the streaming JSON encoder

    1a3d569

    pythongh-129711: Add a streaming C implementation of the JSON encoder

    0e44342

    pythongh-129711: Add tests for the streaming JSON encoder

    4ce308e
16. eendebakpt

    added 4 commits that reference this issue on Aug 22, 2026

    pythongh-129711: Add a no-escape fast path to the _json str escapers

    1903996

    pythongh-129711: Add a no-escape fast path to the _json str escapers

    c131104

    pythongh-129711: Add a no-escape fast path to the _json str escapers

    9a9172b

    pythongh-129711: Add a no-escape fast path to the _json str escapers

    e32789e

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