status: prefer relative paths in Rust code
… when the repository root is under the current directory,
so the kernel needs to traverse fewer directory in every call
to `read_dir` or `symlink_metadata`.
Better yet would be to use libc functions like `openat` and `fstatat`
to remove such repeated traversals entirely, but the standard library
does not provide APIs based on those.
Maybe with a crate like https://crates.io/crates/openat instead?
Benchmarks of `rhg status` show that this patch is neutral in some configurations,
and makes the command up to ~20% faster in others.
Below is semi-arbitrary subset of results. The four numeric columns are:
time (in seconds) with this changeset’s parent, time with this changeset,
time difference (negative is better), time ratio (less than 1 is better).
```
mercurial-dirstate-v1 | default-plain-clean.no-iu.pbr | 0.0061 -> 0.0059: -0.0002 (0.97)
mercurial-dirstate-v2 | default-plain-clean.no-iu.pbr | 0.0029 -> 0.0028: -0.0001 (0.97)
mozilla-dirstate-v1 | default-plain-clean.no-iu.pbr | 0.2110 -> 0.2102: -0.0007 (1.00)
mozilla-dirstate-v2 | default-copies-clean.ignored.pbr | 0.0489 -> 0.0401: -0.0088 (0.82)
mozilla-dirstate-v2 | default-copies-clean.no-iu.pbr | 0.0479 -> 0.0393: -0.0085 (0.82)
mozilla-dirstate-v2 | default-copies-large.all.pbr | 0.1262 -> 0.1210: -0.0051 (0.96)
mozilla-dirstate-v2 | default-copies-small.ignored-unknown.pbr | 0.1262 -> 0.1200: -0.0062 (0.95)
mozilla-dirstate-v2 | default-copies-small.ignored.pbr | 0.0536 -> 0.0417: -0.0119 (0.78)
mozilla-dirstate-v2 | default-copies-small.no-iu.pbr | 0.0482 -> 0.0393: -0.0089 (0.81)
mozilla-dirstate-v2 | default-plain-clean.ignored.pbr | 0.0518 -> 0.0402: -0.0116 (0.78)
mozilla-dirstate-v2 | default-plain-clean.no-iu.pbr | 0.0481 -> 0.0392: -0.0088 (0.82)
mozilla-dirstate-v2 | default-plain-large.all.pbr | 0.1271 -> 0.1218: -0.0052 (0.96)
mozilla-dirstate-v2 | default-plain-small.ignored-unknown.pbr | 0.1225 -> 0.1202: -0.0022 (0.98)
mozilla-dirstate-v2 | default-plain-small.ignored.pbr | 0.0510 -> 0.0418: -0.0092 (0.82)
mozilla-dirstate-v2 | default-plain-small.no-iu.pbr | 0.0480 -> 0.0394: -0.0086 (0.82)
netbeans-dirstate-v1 | default-plain-clean.no-iu.pbr | 0.1442 -> 0.1422: -0.0020 (0.99)
netbeans-dirstate-v2 | default-plain-clean.no-iu.pbr | 0.0325 -> 0.0282: -0.0043 (0.87)
```
Differential Revision: https://phab.mercurial-scm.org/D12175
# pvec.py - probabilistic vector clocks for Mercurial
#
# Copyright 2012 Olivia Mackall <olivia@selenic.com>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
'''
A "pvec" is a changeset property based on the theory of vector clocks
that can be compared to discover relatedness without consulting a
graph. This can be useful for tasks like determining how a
disconnected patch relates to a repository.
Currently a pvec consist of 448 bits, of which 24 are 'depth' and the
remainder are a bit vector. It is represented as a 70-character base85
string.
Construction:
- a root changeset has a depth of 0 and a bit vector based on its hash
- a normal commit has a changeset where depth is increased by one and
one bit vector bit is flipped based on its hash
- a merge changeset pvec is constructed by copying changes from one pvec into
the other to balance its depth
Properties:
- for linear changes, difference in depth is always <= hamming distance
- otherwise, changes are probably divergent
- when hamming distance is < 200, we can reliably detect when pvecs are near
Issues:
- hamming distance ceases to work over distances of ~ 200
- detecting divergence is less accurate when the common ancestor is very close
to either revision or total distance is high
- this could probably be improved by modeling the relation between
delta and hdist
Uses:
- a patch pvec can be used to locate the nearest available common ancestor for
resolving conflicts
- ordering of patches can be established without a DAG
- two head pvecs can be compared to determine whether push/pull/merge is needed
and approximately how many changesets are involved
- can be used to find a heuristic divergence measure between changesets on
different branches
'''
from __future__ import absolute_import
from .node import nullrev
from . import (
pycompat,
util,
)
_size = 448 # 70 chars b85-encoded
_bytes = _size // 8
_depthbits = 24
_depthbytes = _depthbits // 8
_vecbytes = _bytes - _depthbytes
_vecbits = _vecbytes * 8
_radius = (_vecbits - 30) // 2 # high probability vectors are related
def _bin(bs):
'''convert a bytestring to a long'''
v = 0
for b in bs:
v = v * 256 + ord(b)
return v
def _str(v, l):
# type: (int, int) -> bytes
bs = b""
for p in pycompat.xrange(l):
bs = pycompat.bytechr(v & 255) + bs
v >>= 8
return bs
def _split(b):
'''depth and bitvec'''
return _bin(b[:_depthbytes]), _bin(b[_depthbytes:])
def _join(depth, bitvec):
return _str(depth, _depthbytes) + _str(bitvec, _vecbytes)
def _hweight(x):
c = 0
while x:
if x & 1:
c += 1
x >>= 1
return c
_htab = [_hweight(x) for x in pycompat.xrange(256)]
def _hamming(a, b):
'''find the hamming distance between two longs'''
d = a ^ b
c = 0
while d:
c += _htab[d & 0xFF]
d >>= 8
return c
def _mergevec(x, y, c):
# Ideally, this function would be x ^ y ^ ancestor, but finding
# ancestors is a nuisance. So instead we find the minimal number
# of changes to balance the depth and hamming distance
d1, v1 = x
d2, v2 = y
if d1 < d2:
d1, d2, v1, v2 = d2, d1, v2, v1
hdist = _hamming(v1, v2)
ddist = d1 - d2
v = v1
m = v1 ^ v2 # mask of different bits
i = 1
if hdist > ddist:
# if delta = 10 and hdist = 100, then we need to go up 55 steps
# to the ancestor and down 45
changes = (hdist - ddist + 1) // 2
else:
# must make at least one change
changes = 1
depth = d1 + changes
# copy changes from v2
if m:
while changes:
if m & i:
v ^= i
changes -= 1
i <<= 1
else:
v = _flipbit(v, c)
return depth, v
def _flipbit(v, node):
# converting bit strings to longs is slow
bit = (hash(node) & 0xFFFFFFFF) % _vecbits
return v ^ (1 << bit)
def ctxpvec(ctx):
'''construct a pvec for ctx while filling in the cache'''
r = ctx.repo()
if not util.safehasattr(r, "_pveccache"):
r._pveccache = {}
pvc = r._pveccache
if ctx.rev() not in pvc:
cl = r.changelog
for n in pycompat.xrange(ctx.rev() + 1):
if n not in pvc:
node = cl.node(n)
p1, p2 = cl.parentrevs(n)
if p1 == nullrev:
# start with a 'random' vector at root
pvc[n] = (0, _bin((node * 3)[:_vecbytes]))
elif p2 == nullrev:
d, v = pvc[p1]
pvc[n] = (d + 1, _flipbit(v, node))
else:
pvc[n] = _mergevec(pvc[p1], pvc[p2], node)
bs = _join(*pvc[ctx.rev()])
return pvec(util.b85encode(bs))
class pvec(object):
def __init__(self, hashorctx):
if isinstance(hashorctx, bytes):
self._bs = hashorctx
self._depth, self._vec = _split(util.b85decode(hashorctx))
else:
self._vec = ctxpvec(hashorctx)
def __str__(self):
return self._bs
def __eq__(self, b):
return self._vec == b._vec and self._depth == b._depth
def __lt__(self, b):
delta = b._depth - self._depth
if delta < 0:
return False # always correct
if _hamming(self._vec, b._vec) > delta:
return False
return True
def __gt__(self, b):
return b < self
def __or__(self, b):
delta = abs(b._depth - self._depth)
if _hamming(self._vec, b._vec) <= delta:
return False
return True
def __sub__(self, b):
if self | b:
raise ValueError(b"concurrent pvecs")
return self._depth - b._depth
def distance(self, b):
d = abs(b._depth - self._depth)
h = _hamming(self._vec, b._vec)
return max(d, h)
def near(self, b):
dist = abs(b.depth - self._depth)
if dist > _radius or _hamming(self._vec, b._vec) > _radius:
return False