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

See the inline comment for the reasoning here. This is a pretty
common strategy for garbage collectors, other cache-like primtives.

The performance impact is substantial:

$ hg perflrucachedict --size 4 --gets 1000000 --sets 1000000 --mixed 1000000 --costlimit 100
! inserts w/ cost limit
! wall 1.659181 comb 1.650000 user 1.650000 sys 0.000000 (best of 7)
! wall 1.722122 comb 1.720000 user 1.720000 sys 0.000000 (best of 6)
! mixed w/ cost limit
! wall 1.139955 comb 1.140000 user 1.140000 sys 0.000000 (best of 9)
! wall 1.182513 comb 1.180000 user 1.180000 sys 0.000000 (best of 9)

$ hg perflrucachedict --size 1000 --gets 1000000 --sets 1000000 --mixed 1000000 --costlimit 10000
! inserts
! wall 0.679546 comb 0.680000 user 0.680000 sys 0.000000 (best of 15)
! sets
! wall 0.825147 comb 0.830000 user 0.830000 sys 0.000000 (best of 13)
! inserts w/ cost limit
! wall 25.105273 comb 25.080000 user 25.080000 sys 0.000000 (best of 3)
! wall  1.724397 comb  1.720000 user  1.720000 sys 0.000000 (best of 6)
! mixed
! wall 0.807096 comb 0.810000 user 0.810000 sys 0.000000 (best of 13)
! mixed w/ cost limit
! wall 12.104470 comb 12.070000 user 12.070000 sys 0.000000 (best of 3)
! wall  1.190563 comb  1.190000 user  1.190000 sys 0.000000 (best of 9)

$ hg perflrucachedict --size 1000 --gets 1000000 --sets 1000000 --mixed 1000000 --costlimit 10000 --mixedgetfreq 90
! inserts
! wall 0.711177 comb 0.710000 user 0.710000 sys 0.000000 (best of 14)
! sets
! wall 0.846992 comb 0.850000 user 0.850000 sys 0.000000 (best of 12)
! inserts w/ cost limit
! wall 25.963028 comb 25.960000 user 25.960000 sys 0.000000 (best of 3)
! wall  2.184311 comb  2.180000 user  2.180000 sys 0.000000 (best of 5)
! mixed
! wall 0.728256 comb 0.730000 user 0.730000 sys 0.000000 (best of 14)
! mixed w/ cost limit
! wall 3.174256 comb 3.170000 user 3.170000 sys 0.000000 (best of 4)
! wall 0.773186 comb 0.770000 user 0.770000 sys 0.000000 (best of 13)

$ hg perflrucachedict --size 100000 --gets 1000000 --sets 1000000 --mixed 1000000 --mixedgetfreq 90 --costlimit 5000000
! gets
! wall 1.191368 comb 1.190000 user 1.190000 sys 0.000000 (best of 9)
! wall 1.195304 comb 1.190000 user 1.190000 sys 0.000000 (best of 9)
! inserts
! wall 0.950995 comb 0.950000 user 0.950000 sys 0.000000 (best of 11)
! inserts w/ cost limit
! wall 1.589732 comb 1.590000 user 1.590000 sys 0.000000 (best of 7)
! sets
! wall 1.094941 comb 1.100000 user 1.090000 sys 0.010000 (best of 9)
! mixed
! wall 0.936420 comb 0.940000 user 0.930000 sys 0.010000 (best of 10)
! mixed w/ cost limit
! wall 0.882780 comb 0.870000 user 0.870000 sys 0.000000 (best of 11)

This puts us ~2x slower than caches without cost accounting. And for
read-heavy workloads (the prime use cases for caches), performance is
nearly identical.

In the worst case (pure write workloads with cost accounting enabled),
we're looking at ~1.5us per insert on large caches. That seems "fast
enough."

Differential Revision: https://phab.mercurial-scm.org/D4505

mercurial/util.py

@@ -1472,11 +1472,21 b' class lrucachedict(object):'
         # 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()

tests/test-lrucachedict.py

@@ -314,10 +314,10 b' class testlrucachedict(unittest.TestCase'
         # 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)