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.
import sys, os, subprocess
if subprocess.call(['python', '%s/hghave' % os.environ['TESTDIR'],
'cacheable']):
sys.exit(80)
from mercurial import util, scmutil, extensions
filecache = scmutil.filecache
class fakerepo(object):
def __init__(self):
self._filecache = {}
def join(self, p):
return p
def sjoin(self, p):
return p
@filecache('x')
def cached(self):
print 'creating'
def invalidate(self):
for k in self._filecache:
try:
delattr(self, k)
except AttributeError:
pass
def basic(repo):
# file doesn't exist, calls function
repo.cached
repo.invalidate()
# file still doesn't exist, uses cache
repo.cached
# create empty file
f = open('x', 'w')
f.close()
repo.invalidate()
# should recreate the object
repo.cached
f = open('x', 'w')
f.write('a')
f.close()
repo.invalidate()
# should recreate the object
repo.cached
repo.invalidate()
# stats file again, nothing changed, reuses object
repo.cached
# atomic replace file, size doesn't change
# hopefully st_mtime doesn't change as well so this doesn't use the cache
# because of inode change
f = scmutil.opener('.')('x', 'w', atomictemp=True)
f.write('b')
f.close()
repo.invalidate()
repo.cached
def fakeuncacheable():
def wrapcacheable(orig, *args, **kwargs):
return False
def wrapinit(orig, *args, **kwargs):
pass
originit = extensions.wrapfunction(util.cachestat, '__init__', wrapinit)
origcacheable = extensions.wrapfunction(util.cachestat, 'cacheable',
wrapcacheable)
try:
os.remove('x')
except OSError:
pass
basic(fakerepo())
util.cachestat.cacheable = origcacheable
util.cachestat.__init__ = originit
print 'basic:'
print
basic(fakerepo())
print
print 'fakeuncacheable:'
print
fakeuncacheable()