bisect: avoid copying ancestor list for non-merge commits
During a bisection, hg needs to compute a list of all ancestors for every
candidate commit. This is accomplished via a bottom-up traversal of the set of
candidates, during which each revision's ancestor list is populated using the
ancestor list of its parent(s). Previously, this involved copying the entire
list, which could be very long in if the bisection range was large.
To help improve this, we can observe that each candidate commit is visited
exactly once, at which point its ancestor list is copied into its children's
lists and then dropped. In the case of non-merge commits, a commit's ancestor
list consists exactly of its parent's list plus itself. This means that we can
trivially reuse the parent's existing list for one of its non-merge children,
which avoids copying entirely if that commit is the parent's only child. This
makes bisections over linear ranges of commits much faster.
During some informal testing in the large publicly-available `mozilla-central`
repository, this noticeably sped up bisections over large ranges of history:
Setup:
$ cd mozilla-central
$ hg bisect --reset
$ hg bisect --good 0
$ hg log -r tip -T '{rev}\n'
628417
Test:
$ time hg bisect --bad tip --noupdate
Before:
real 3m35.927s
user 3m35.553s
sys 0m0.319s
After:
real 1m41.142s
user 1m40.810s
sys 0m0.285s
import os
from mercurial.hgweb import hgwebdir_mod
hgwebdir = hgwebdir_mod.hgwebdir
os.mkdir(b'webdir')
os.chdir(b'webdir')
webdir = os.path.realpath(b'.')
def trivial_response(req, res):
return []
def make_hgwebdir(gc_rate=None):
config = os.path.join(webdir, b'hgwebdir.conf')
with open(config, 'wb') as configfile:
configfile.write(b'[experimental]\n')
if gc_rate is not None:
configfile.write(b'web.full-garbage-collection-rate=%d\n' % gc_rate)
hg_wd = hgwebdir(config)
hg_wd._runwsgi = trivial_response
return hg_wd
def process_requests(webdir_instance, number):
# we don't care for now about passing realistic arguments
for _ in range(number):
for chunk in webdir_instance.run_wsgi(None, None):
pass
without_gc = make_hgwebdir(gc_rate=0)
process_requests(without_gc, 5)
assert without_gc.requests_count == 5
assert without_gc.gc_full_collections_done == 0
with_gc = make_hgwebdir(gc_rate=2)
process_requests(with_gc, 5)
assert with_gc.requests_count == 5
assert with_gc.gc_full_collections_done == 2
with_systematic_gc = make_hgwebdir() # default value of the setting
process_requests(with_systematic_gc, 3)
assert with_systematic_gc.requests_count == 3
assert with_systematic_gc.gc_full_collections_done == 3