json.dump(x,f) is much slower than f.write(json.dumps(x))

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

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

• gh-130076

Start by reviewing linked PR gh-130076 and the CPython JSON code around sequential writes from iterencode. Reproduce the reported dump-versus-dumps timings, then verify the change improves dump performance for larger cases without changing output.