D4505: util: lower water mark when removing nodes after cost limit reached

[indygreg (Gregory Szorc)]

This revision was automatically updated to reflect the committed changes.
Closed by commit rHGf296c0b366c8: util: lower water mark when removing nodes after cost limit reached (authored by indygreg, committed by ).

REPOSITORY
  rHG Mercurial

CHANGES SINCE LAST UPDATE
  https://phab.mercurial-scm.org/D4505?vs=10824&id=10944

REVISION DETAIL
  https://phab.mercurial-scm.org/D4505

AFFECTED FILES
  mercurial/util.py
  tests/test-lrucachedict.py

CHANGE DETAILS

diff --git a/tests/test-lrucachedict.py b/tests/test-lrucachedict.py
--- a/tests/test-lrucachedict.py
+++ b/tests/test-lrucachedict.py
@@ -314,10 +314,10 @@
         # Inserting new element should free multiple elements so we hit
         # low water mark.
         d.insert('e', 'vd', cost=25)
-        self.assertEqual(len(d), 3)
+        self.assertEqual(len(d), 2)
         self.assertNotIn('a', d)
         self.assertNotIn('b', d)
-        self.assertIn('c', d)
+        self.assertNotIn('c', d)
         self.assertIn('d', d)
         self.assertIn('e', d)
 
diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -1472,11 +1472,21 @@
         # to walk the linked list and doing this in a loop would be
         # quadratic. So we find the first non-empty node and then
         # walk nodes until we free up enough capacity.
+        #
+        # If we only removed the minimum number of nodes to free enough
+        # cost at insert time, chances are high that the next insert would
+        # also require pruning. This would effectively constitute quadratic
+        # behavior for insert-heavy workloads. To mitigate this, we set a
+        # target cost that is a percentage of the max cost. This will tend
+        # to free more nodes when the high water mark is reached, which
+        # lowers the chances of needing to prune on the subsequent insert.
+        targetcost = int(self.maxcost * 0.75)
+
         n = self._head.prev
         while n.key is _notset:
             n = n.prev
 
-        while len(self) > 1 and self.totalcost > self.maxcost:
+        while len(self) > 1 and self.totalcost > targetcost:
             del self._cache[n.key]
             self.totalcost -= n.cost
             n.markempty()



To: indygreg, #hg-reviewers
Cc: mercurial-devel