[issue3440] largefiles: cachelfiles makes updates unusably slow

[Ben Challenor]

New submission from Ben Challenor <ben at challenor.org>:

Updating between revision n and n+1, where n+1 adds a single 100MB 
largefile, takes 20 seconds. The same update in the original repo without 
largefiles enabled takes just 4 seconds.

The bottleneck appears to be in cachelfiles in lfcommands.py (I have 
290850e7aa43 from three weeks ago). It loops through every file, despite 
only the one largefile changing.

Surely it must be possible to do skip some of this work, as vanilla HG does 
it much faster without compromising correctness.

Some repo stats:
- ~10000 commits
- ~10000 files
- ~140 largefiles
- total largefile size: ~7 GB
- max largefile size ~800 MB
- avg largefile size: ~50 MB

----------
messages: 20030
nosy: bchallenor
priority: urgent
status: unread
title: largefiles: cachelfiles makes updates unusably slow

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