comparison mercurial/hgweb/hgwebdir_mod.py @ 47802:de2e04fe4897

hgwebdir: avoid systematic full garbage collection

Forcing a systematic full garbage collection upon each request
can serioulsy harm performance. This is reported as
https://bz.mercurial-scm.org/show_bug.cgi?id=6075

With this change we're performing the full collection according
to a new setting, `experimental.web.full-garbage-collection-rate`.
The default value is 1, which doesn't change the behavior and will
allow us to test on real use cases. If the value is 0, no full garbage
collection occurs.

Regardless of the value of the setting, a partial garbage collection
still occurs upon each request (not attempting to collect objects from
the oldest generation). This should be enough to take care of
reference cycles that have been created by the last request
(assessment of this requires changing the setting, not to be 1).

In my experience chasing memory leaks in Mercurial servers,
the full collection never reclaimed any memory, but this is with
Python 3 and biased towards small repositories.

On the other hand, as explained in the Python developer docs [1],
frequent full collections are very harmful in terms of performance if
lots of objects survive the collection, and hence stay in the
oldest generation. Note that `gc.collect()` is indeed trying to
collect the oldest generation [2]. This happens usually in two cases:

- unwanted lingering objects (i.e., an actual memory leak that
  the GC cannot do anything about). Sadly, we have lots of those
  these days.
- desireable long-term objects, typically in caches (not inner caches
  carried by repositories, which should be collected with them). This
  is a subject of interest for the Heptapod project.

In short, the flat rate that this change still permits is
probably a bad idea in most cases, and the default value can
be tweaked later on (or even be set to 0) according to experiments
in the wild.

The test is inspired from test-hgwebdir-paths.py

[1] https://devguide.python.org/garbage_collector/#collecting-the-oldest-generation
[2] https://docs.python.org/3/library/gc.html#gc.collect

Differential Revision: https://phab.mercurial-scm.org/D11204

47795:b1e1559f5a45

        self.baseui = baseui
        self.ui = None
        self.lastrefresh = 0
        self.motd = None
        self.refresh()
        if not baseui:
            # set up environment for new ui
            extensions.loadall(self.ui)
            extensions.populateui(self.ui)

                    name = name[len(prefix) :]
                repos.append((name.lstrip(b'/'), repo))

        self.repos = repos
        self.ui = u
        encoding.encoding = self.ui.config(b'web', b'encoding')
        self.style = self.ui.config(b'web', b'style')
        self.templatepath = self.ui.config(
            b'web', b'templates', untrusted=False
        )

                for r in self._runwsgi(req, res):
                    yield r
            finally:
                # There are known cycles in localrepository that prevent
                # those objects (and tons of held references) from being
                # collected through normal refcounting. We mitigate those
                # leaks by performing an explicit GC on every request.
                # TODO remove this once leaks are fixed.
                # TODO only run this on requests that create localrepository
                # instances instead of every request.
                gc.collect()

    def _runwsgi(self, req, res):
        try:
            self.refresh()

47802:de2e04fe4897

        self.baseui = baseui
        self.ui = None
        self.lastrefresh = 0
        self.motd = None
        self.refresh()
        self.requests_count = 0
        if not baseui:
            # set up environment for new ui
            extensions.loadall(self.ui)
            extensions.populateui(self.ui)

                    name = name[len(prefix) :]
                repos.append((name.lstrip(b'/'), repo))

        self.repos = repos
        self.ui = u
        self.gc_full_collect_rate = self.ui.configint(
            b'experimental', b'web.full-garbage-collection-rate'
        )
        self.gc_full_collections_done = 0
        encoding.encoding = self.ui.config(b'web', b'encoding')
        self.style = self.ui.config(b'web', b'style')
        self.templatepath = self.ui.config(
            b'web', b'templates', untrusted=False
        )

                for r in self._runwsgi(req, res):
                    yield r
            finally:
                # There are known cycles in localrepository that prevent
                # those objects (and tons of held references) from being
                # collected through normal refcounting.
                # In some cases, the resulting memory consumption can
                # be tamed by performing explicit garbage collections.
                # In presence of actual leaks or big long-lived caches, the
                # impact on performance of such collections can become a
                # problem, hence the rate shouldn't be set too low.
                # See "Collecting the oldest generation" in
                # https://devguide.python.org/garbage_collector
                # for more about such trade-offs.
                rate = self.gc_full_collect_rate

                # this is not thread safe, but the consequence (skipping
                # a garbage collection) is arguably better than risking
                # to have several threads perform a collection in parallel
                # (long useless wait on all threads).
                self.requests_count += 1
                if rate > 0 and self.requests_count % rate == 0:
                    gc.collect()
                    self.gc_full_collections_done += 1
                else:
                    gc.collect(generation=1)

    def _runwsgi(self, req, res):
        try:
            self.refresh()