working_copy: avoid "empty" fsmonitor saves - #10049
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I've been looking at jj's performance in a large (for me) repository:
Firefox.
It only became bearable with fsmonitor/watchman but I discovered that
even with no files changing advancing the fsmonitor clock triggered the
complete tree state rewrite.
I figured we could just keep the old clock if there are no changes.
Here are some "jj st" numbers from my system (macOS 26.5.2, M1 Pro,
fast NVMe, hardcoded WATCHMAN_SOCK to avoid watchman socket discovery):
jj version Runs Median ms Mean ms Min ms Max ms
stable 0.44.0 20 210.990 257.952 190.111 836.981
7190eda + this patch 20 135.723 136.843 127.764 153.652
I expect this to be safe as querying again from the older clock can
replay paths, but it cannot omit future file updates.
Stable minus custom is 75.267 ms median, 121.109 ms mean, 62.347 ms
minimum, and 683.329 ms maximum.
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Aug 25, 2026
| // old clock after that scan even if it found untracked paths. | ||
| if should_update_watchman_clock | ||
| && ((stats.untracked_paths.is_empty() && stats.invalid_utf8_paths.is_empty()) | ||
| || watchman_clock.is_none()) |
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nit: It seems slightly easier to follow nested ifs rather than combining multiple && and || conditions.
| // Reusing the older clock may make a later query do more work, but it | ||
| // cannot omit changes. | ||
| let should_update_watchman_clock = | ||
| !matches!(&changed_files, Some(files) if files.is_empty()); |
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nit: .as_ref().is_some_and() or .is_none_or() also works.
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I've been looking at jj's performance in a large (for me) repository: Firefox.
It only became bearable with fsmonitor/watchman but I discovered that even with no files changing advancing the fsmonitor clock triggered the complete tree state rewrite.
I figured we could just keep the old clock if there are no changes.
Here are some "jj st" numbers from my system (macOS 26.5.2, M1 Pro,
fast NVMe, hardcoded WATCHMAN_SOCK to avoid watchman socket discovery):
I expect this to be safe as querying again from the older clock can replay paths, but it cannot omit future file updates.
Stable minus custom is 75.267 ms median, 121.109 ms mean, 62.347 ms minimum, and 683.329 ms maximum.
Checklist
If applicable:
CHANGELOG.mdREADME.md,docs/,demos/)cli/src/config-schema.json)how it works, how it's organized), including any code drafted by an LLM.
an eye towards deleting anything that is irrelevant, clarifying anything
that is confusing, and adding details that are relevant. This includes,
for example, commit descriptions, PR descriptions, and code comments.