diff -r b1e1559f5a45 -r de2e04fe4897 mercurial/hgweb/hgwebdir_mod.py
--- a/mercurial/hgweb/hgwebdir_mod.py	Wed Jul 28 13:45:07 2021 +0300
+++ b/mercurial/hgweb/hgwebdir_mod.py	Tue Jul 20 17:20:19 2021 +0200
@@ -285,6 +285,7 @@
         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)
@@ -341,6 +342,10 @@
 
         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(
@@ -383,12 +388,27 @@
             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()
+                # 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: