Mercurial > hg
view tests/test-util.py @ 40021:c537144fdbef
wireprotov2: support response caching
One of the things I've learned from managing VCS servers over the
years is that they are hard to scale. It is well known that some
companies have very beefy (read: very expensive) servers to power
their VCS needs. It is also known that specialized servers for
various VCS exist in order to facilitate scaling servers. (Mercurial
is in this boat.)
One of the aspects that make a VCS server hard to scale is the
high CPU load incurred by constant client clone/pull operations.
To alleviate the scaling pain associated with data retrieval
operations, I want to integrate caching into the Mercurial wire
protocol server as robustly as possible such that servers can
aggressively cache responses and defer as much server load as
possible.
This commit represents the initial implementation of a general
caching layer in wire protocol version 2.
We define a new interface and behavior for a wire protocol cacher
in repository.py. (This is probably where a reviewer should look
first to understand what is going on.)
The bulk of the added code is in wireprotov2server.py, where we
define how a command can opt in to being cached and integrate
caching into command dispatching.
From a very high-level:
* A command can declare itself as cacheable by providing a callable
that can be used to derive a cache key.
* At dispatch time, if a command is cacheable, we attempt to
construct a cacher and use it for serving the request and/or
caching the request.
* The dispatch layer handles the bulk of the business logic for
caching, making cachers mostly "dumb content stores."
* The mechanism for invalidating cached entries (one of the harder
parts about caching in general) is by varying the cache key when
state changes. As such, cachers don't need to be concerned with
cache invalidation.
Initially, we've hooked up support for caching "manifestdata" and
"filedata" commands. These are the simplest to cache, as they should
be immutable over time. Caching of commands related to changeset
data is a bit harder (because cache validation is impacted by
changes to bookmarks, phases, etc). This will be implemented later.
(Strictly speaking, censoring a file should invalidate caches. I've
added an inline TODO to track this edge case.)
To prove it works, this commit implements a test-only extension
providing in-memory caching backed by an lrucachedict. A new test
showing this extension behaving properly is added. FWIW, the
cacher is ~50 lines of code, demonstrating the relative ease with
which a cache can be added to a server.
While the test cacher is not suitable for production workloads, just
for kicks I performed a clone of just the changeset and manifest data
for the mozilla-unified repository. With a fully warmed cache (of just
the manifest data since changeset data is not cached), server-side
CPU usage dropped from ~73s to ~28s. That's pretty significant and
demonstrates the potential that response caching has on server
scalability!
Differential Revision: https://phab.mercurial-scm.org/D4773
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Wed, 26 Sep 2018 17:16:56 -0700 |
parents | 331ab85e910b |
children | 2372284d9457 |
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# unit tests for mercuril.util utilities from __future__ import absolute_import import contextlib import itertools import unittest from mercurial import pycompat, util, utils @contextlib.contextmanager def mocktimer(incr=0.1, *additional_targets): """Replaces util.timer and additional_targets with a mock The timer starts at 0. On each call the time incremented by the value of incr. If incr is an iterable, then the time is incremented by the next value from that iterable, looping in a cycle when reaching the end. additional_targets must be a sequence of (object, attribute_name) tuples; the mock is set with setattr(object, attribute_name, mock). """ time = [0] try: incr = itertools.cycle(incr) except TypeError: incr = itertools.repeat(incr) def timer(): time[0] += next(incr) return time[0] # record original values orig = util.timer additional_origs = [(o, a, getattr(o, a)) for o, a in additional_targets] # mock out targets util.timer = timer for obj, attr in additional_targets: setattr(obj, attr, timer) try: yield finally: # restore originals util.timer = orig for args in additional_origs: setattr(*args) # attr.s default factory for util.timedstats.start binds the timer we # need to mock out. _start_default = (util.timedcmstats.start.default, 'factory') @contextlib.contextmanager def capturestderr(): """Replace utils.procutil.stderr with a pycompat.bytesio instance The instance is made available as the return value of __enter__. This contextmanager is reentrant. """ orig = utils.procutil.stderr utils.procutil.stderr = pycompat.bytesio() try: yield utils.procutil.stderr finally: utils.procutil.stderr = orig class timedtests(unittest.TestCase): def testtimedcmstatsstr(self): stats = util.timedcmstats() self.assertEqual(str(stats), '<unknown>') self.assertEqual(bytes(stats), b'<unknown>') stats.elapsed = 12.34 self.assertEqual(str(stats), pycompat.sysstr(util.timecount(12.34))) self.assertEqual(bytes(stats), util.timecount(12.34)) def testtimedcmcleanexit(self): # timestamps 1, 4, elapsed time of 4 - 1 = 3 with mocktimer([1, 3], _start_default): with util.timedcm('pass') as stats: # actual context doesn't matter pass self.assertEqual(stats.start, 1) self.assertEqual(stats.elapsed, 3) self.assertEqual(stats.level, 1) def testtimedcmnested(self): # timestamps 1, 3, 6, 10, elapsed times of 6 - 3 = 3 and 10 - 1 = 9 with mocktimer([1, 2, 3, 4], _start_default): with util.timedcm('outer') as outer_stats: with util.timedcm('inner') as inner_stats: # actual context doesn't matter pass self.assertEqual(outer_stats.start, 1) self.assertEqual(outer_stats.elapsed, 9) self.assertEqual(outer_stats.level, 1) self.assertEqual(inner_stats.start, 3) self.assertEqual(inner_stats.elapsed, 3) self.assertEqual(inner_stats.level, 2) def testtimedcmexception(self): # timestamps 1, 4, elapsed time of 4 - 1 = 3 with mocktimer([1, 3], _start_default): try: with util.timedcm('exceptional') as stats: raise ValueError() except ValueError: pass self.assertEqual(stats.start, 1) self.assertEqual(stats.elapsed, 3) self.assertEqual(stats.level, 1) def testtimeddecorator(self): @util.timed def testfunc(callcount=1): callcount -= 1 if callcount: testfunc(callcount) # timestamps 1, 2, 3, 4, elapsed time of 3 - 2 = 1 and 4 - 1 = 3 with mocktimer(1, _start_default): with capturestderr() as out: testfunc(2) self.assertEqual(out.getvalue(), ( b' testfunc: 1.000 s\n' b' testfunc: 3.000 s\n' )) if __name__ == '__main__': import silenttestrunner silenttestrunner.main(__name__)