store: implement fncache basic path encoding in C
(This is not yet enabled; it will be turned on in a followup patch.)
The path encoding performed by fncache is complex and (perhaps
surprisingly) slow enough to negatively affect the overall performance
of Mercurial.
For a short path (< 120 bytes), the Python code can be reduced to a fairly
tractable state machine that either determines that nothing needs to be
done in a single pass, or performs the encoding in a second pass.
For longer paths, we avoid the more complicated hashed encoding scheme
for now, and fall back to Python.
Raw performance: I measured in a repo containing 150,000 files in its tip
manifest, with a median path name length of 57 bytes, and 95th percentile
of 96 bytes.
In this repo, the Python code takes 3.1 seconds to encode all path
names, while the hybrid C-and-Python code (called from Python) takes
0.21 seconds, for a speedup of about 14.
Across several other large repositories, I've measured the speedup from
the C code at between 26x and 40x.
For path names above 120 bytes where we must fall back to Python for
hashed encoding, the speedup is about 1.7x. Thus absolute performance
will depend strongly on the characteristics of a particular repository.
$ hg init rep; cd rep
$ touch empty-file
$ python -c 'for x in range(10000): print x' > large-file
$ hg addremove
adding empty-file
adding large-file
$ hg commit -m A
$ rm large-file empty-file
$ python -c 'for x in range(10,10000): print x' > another-file
$ hg addremove -s50
adding another-file
removing empty-file
removing large-file
recording removal of large-file as rename to another-file (99% similar)
$ hg commit -m B
comparing two empty files caused ZeroDivisionError in the past
$ hg update -C 0
2 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ rm empty-file
$ touch another-empty-file
$ hg addremove -s50
adding another-empty-file
removing empty-file
$ cd ..
$ hg init rep2; cd rep2
$ python -c 'for x in range(10000): print x' > large-file
$ python -c 'for x in range(50): print x' > tiny-file
$ hg addremove
adding large-file
adding tiny-file
$ hg commit -m A
$ python -c 'for x in range(70): print x' > small-file
$ rm tiny-file
$ rm large-file
$ hg addremove -s50
removing large-file
adding small-file
removing tiny-file
recording removal of tiny-file as rename to small-file (82% similar)
$ hg commit -m B
should all fail
$ hg addremove -s foo
abort: similarity must be a number
[255]
$ hg addremove -s -1
abort: similarity must be between 0 and 100
[255]
$ hg addremove -s 1e6
abort: similarity must be between 0 and 100
[255]
$ cd ..
Issue1527: repeated addremove causes util.Abort
$ hg init rep3; cd rep3
$ mkdir d
$ echo a > d/a
$ hg add d/a
$ hg commit -m 1
$ mv d/a d/b
$ hg addremove -s80
removing d/a
adding d/b
recording removal of d/a as rename to d/b (100% similar) (glob)
$ hg debugstate
r 0 0 1970-01-01 00:00:00 d/a
a 0 -1 unset d/b
copy: d/a -> d/b
$ mv d/b c
no copies found here (since the target isn't in d
$ hg addremove -s80 d
removing d/b (glob)
copies here
$ hg addremove -s80
adding c
recording removal of d/a as rename to c (100% similar) (glob)
$ cd ..