changeset 46149:294d5aca4ff5

copies: iterate over children directly (instead of parents) Before this change we would gather all parent → child edges and iterate on all parent, gathering copy information for children and aggregating them from there. They are not strict requirement for edges to be processed in that specific order. We could also simply iterate over all "children" revision and aggregate data from both parents at the same time. This patch does that. It make various things simpler: * since both parents are processed at the same time, we no longer need to cache data for merge (see next changeset for details), * we no longer need nested loop to process data, * we no longer need to store partial merge data for a rev from distinct loop interaction to another when processing merges, * we no longer need to build a full parent -> children mapping (we only rely on a simpler "parent -> number of children" map (for memory efficiency), * the data access pattern is now simpler (from lower revisions to higher revisions) and entirely predicable. That predictability open the way to prefetching and parallel processing. So that new iterations order requires simpler code and open the way to interesting optimisation. The effect on performance is quite good. In the worse case, we don't see any significant negative impact. And in the best case, the reduction of roundtrip to Python provide us with a significant speed. Some example below: Repo Case Source-Rev Dest-Rev # of revisions old time new time Difference Factor time per rev --------------------------------------------------------------------------------------------------------------------------------------------------------------- mozilla-try x00000_revs_x00000_added_0_copies dc8a3ca7010e d16fde900c9c : 34414 revs, 0.962867 s, 0.502584 s, -0.460283 s, × 0.5220, 14 µs/rev mozilla-try x0000_revs_xx000_added_x_copies 156f6e2674f2 4d0f2c178e66 : 8598 revs, 0.110717 s, 0.076323 s, -0.034394 s, × 0.6894, 8 µs/rev # full comparison between the previous changeset and this one Repo Case Source-Rev Dest-Rev # of revisions old time new time Difference Factor time per rev --------------------------------------------------------------------------------------------------------------------------------------------------------------- mercurial x_revs_x_added_0_copies ad6b123de1c7 39cfcef4f463 : 1 revs, 0.000048 s, 0.000041 s, -0.000007 s, × 0.8542, 41 µs/rev mercurial x_revs_x_added_x_copies 2b1c78674230 0c1d10351869 : 6 revs, 0.000153 s, 0.000102 s, -0.000051 s, × 0.6667, 17 µs/rev mercurial x000_revs_x000_added_x_copies 81f8ff2a9bf2 dd3267698d84 : 1032 revs, 0.004209 s, 0.004254 s, +0.000045 s, × 1.0107, 4 µs/rev pypy x_revs_x_added_0_copies aed021ee8ae8 099ed31b181b : 9 revs, 0.000203 s, 0.000282 s, +0.000079 s, × 1.3892, 31 µs/rev pypy x_revs_x000_added_0_copies 4aa4e1f8e19a 359343b9ac0e : 1 revs, 0.000059 s, 0.000048 s, -0.000011 s, × 0.8136, 48 µs/rev pypy x_revs_x_added_x_copies ac52eb7bbbb0 72e022663155 : 7 revs, 0.000194 s, 0.000211 s, +0.000017 s, × 1.0876, 30 µs/rev pypy x_revs_x00_added_x_copies c3b14617fbd7 ace7255d9a26 : 1 revs, 0.000380 s, 0.000375 s, -0.000005 s, × 0.9868, 375 µs/rev pypy x_revs_x000_added_x000_copies df6f7a526b60 a83dc6a2d56f : 6 revs, 0.010588 s, 0.010574 s, -0.000014 s, × 0.9987, 1762 µs/rev pypy x000_revs_xx00_added_0_copies 89a76aede314 2f22446ff07e : 4785 revs, 0.048961 s, 0.049974 s, +0.001013 s, × 1.0207, 10 µs/rev pypy x000_revs_x000_added_x_copies 8a3b5bfd266e 2c68e87c3efe : 6780 revs, 0.083612 s, 0.084300 s, +0.000688 s, × 1.0082, 12 µs/rev pypy x000_revs_x000_added_x000_copies 89a76aede314 7b3dda341c84 : 5441 revs, 0.058579 s, 0.060128 s, +0.001549 s, × 1.0264, 11 µs/rev pypy x0000_revs_x_added_0_copies d1defd0dc478 c9cb1334cc78 : 43645 revs, 0.736783 s, 0.686542 s, -0.050241 s, × 0.9318, 15 µs/rev pypy x0000_revs_xx000_added_0_copies bf2c629d0071 4ffed77c095c : 2 revs, 0.022050 s, 0.009277 s, -0.012773 s, × 0.4207, 4638 µs/rev pypy x0000_revs_xx000_added_x000_copies 08ea3258278e d9fa043f30c0 : 11316 revs, 0.120800 s, 0.114733 s, -0.006067 s, × 0.9498, 10 µs/rev netbeans x_revs_x_added_0_copies fb0955ffcbcd a01e9239f9e7 : 2 revs, 0.000140 s, 0.000081 s, -0.000059 s, × 0.5786, 40 µs/rev netbeans x_revs_x000_added_0_copies 6f360122949f 20eb231cc7d0 : 2 revs, 0.000114 s, 0.000107 s, -0.000007 s, × 0.9386, 53 µs/rev netbeans x_revs_x_added_x_copies 1ada3faf6fb6 5a39d12eecf4 : 3 revs, 0.000224 s, 0.000173 s, -0.000051 s, × 0.7723, 57 µs/rev netbeans x_revs_x00_added_x_copies 35be93ba1e2c 9eec5e90c05f : 9 revs, 0.000723 s, 0.000698 s, -0.000025 s, × 0.9654, 77 µs/rev netbeans x000_revs_xx00_added_0_copies eac3045b4fdd 51d4ae7f1290 : 1421 revs, 0.009665 s, 0.009248 s, -0.000417 s, × 0.9569, 6 µs/rev netbeans x000_revs_x000_added_x_copies e2063d266acd 6081d72689dc : 1533 revs, 0.014820 s, 0.015446 s, +0.000626 s, × 1.0422, 10 µs/rev netbeans x000_revs_x000_added_x000_copies ff453e9fee32 411350406ec2 : 5750 revs, 0.076049 s, 0.074373 s, -0.001676 s, × 0.9780, 12 µs/rev netbeans x0000_revs_xx000_added_x000_copies 588c2d1ced70 1aad62e59ddd : 66949 revs, 0.683603 s, 0.639870 s, -0.043733 s, × 0.9360, 9 µs/rev mozilla-central x_revs_x_added_0_copies 3697f962bb7b 7015fcdd43a2 : 2 revs, 0.000161 s, 0.000088 s, -0.000073 s, × 0.5466, 44 µs/rev mozilla-central x_revs_x000_added_0_copies dd390860c6c9 40d0c5bed75d : 8 revs, 0.000234 s, 0.000199 s, -0.000035 s, × 0.8504, 24 µs/rev mozilla-central x_revs_x_added_x_copies 8d198483ae3b 14207ffc2b2f : 9 revs, 0.000247 s, 0.000171 s, -0.000076 s, × 0.6923, 19 µs/rev mozilla-central x_revs_x00_added_x_copies 98cbc58cc6bc 446a150332c3 : 7 revs, 0.000630 s, 0.000592 s, -0.000038 s, × 0.9397, 84 µs/rev mozilla-central x_revs_x000_added_x000_copies 3c684b4b8f68 0a5e72d1b479 : 3 revs, 0.003286 s, 0.003151 s, -0.000135 s, × 0.9589, 1050 µs/rev mozilla-central x_revs_x0000_added_x0000_copies effb563bb7e5 c07a39dc4e80 : 6 revs, 0.062441 s, 0.061612 s, -0.000829 s, × 0.9867, 10268 µs/rev mozilla-central x000_revs_xx00_added_0_copies 6100d773079a 04a55431795e : 1593 revs, 0.005423 s, 0.005381 s, -0.000042 s, × 0.9923, 3 µs/rev mozilla-central x000_revs_x000_added_x_copies 9f17a6fc04f9 2d37b966abed : 41 revs, 0.005919 s, 0.003742 s, -0.002177 s, × 0.6322, 91 µs/rev mozilla-central x000_revs_x000_added_x000_copies 7c97034feb78 4407bd0c6330 : 7839 revs, 0.062597 s, 0.061983 s, -0.000614 s, × 0.9902, 7 µs/rev mozilla-central x0000_revs_xx000_added_0_copies 9eec5917337d 67118cc6dcad : 615 revs, 0.043551 s, 0.019861 s, -0.023690 s, × 0.4560, 32 µs/rev mozilla-central x0000_revs_xx000_added_x000_copies f78c615a656c 96a38b690156 : 30263 revs, 0.192475 s, 0.188101 s, -0.004374 s, × 0.9773, 6 µs/rev mozilla-central x00000_revs_x0000_added_x0000_copies 6832ae71433c 4c222a1d9a00 : 153721 revs, 1.955575 s, 1.806696 s, -0.148879 s, × 0.9239, 11 µs/rev mozilla-central x00000_revs_x00000_added_x000_copies 76caed42cf7c 1daa622bbe42 : 204976 revs, 2.886501 s, 2.682987 s, -0.203514 s, × 0.9295, 13 µs/rev mozilla-try x_revs_x_added_0_copies aaf6dde0deb8 9790f499805a : 2 revs, 0.001181 s, 0.000852 s, -0.000329 s, × 0.7214, 426 µs/rev mozilla-try x_revs_x000_added_0_copies d8d0222927b4 5bb8ce8c7450 : 2 revs, 0.001189 s, 0.000859 s, -0.000330 s, × 0.7225, 429 µs/rev mozilla-try x_revs_x_added_x_copies 092fcca11bdb 936255a0384a : 4 revs, 0.000563 s, 0.000150 s, -0.000413 s, × 0.2664, 37 µs/rev mozilla-try x_revs_x00_added_x_copies b53d2fadbdb5 017afae788ec : 2 revs, 0.001548 s, 0.001158 s, -0.000390 s, × 0.7481, 579 µs/rev mozilla-try x_revs_x000_added_x000_copies 20408ad61ce5 6f0ee96e21ad : 1 revs, 0.027782 s, 0.027240 s, -0.000542 s, × 0.9805, 27240 µs/rev mozilla-try x_revs_x0000_added_x0000_copies effb563bb7e5 c07a39dc4e80 : 6 revs, 0.062781 s, 0.062824 s, +0.000043 s, × 1.0007, 10470 µs/rev mozilla-try x000_revs_xx00_added_0_copies 6100d773079a 04a55431795e : 1593 revs, 0.005778 s, 0.005463 s, -0.000315 s, × 0.9455, 3 µs/rev mozilla-try x000_revs_x000_added_x_copies 9f17a6fc04f9 2d37b966abed : 41 revs, 0.006192 s, 0.004238 s, -0.001954 s, × 0.6844, 103 µs/rev mozilla-try x000_revs_x000_added_x000_copies 1346fd0130e4 4c65cbdabc1f : 6657 revs, 0.065391 s, 0.064113 s, -0.001278 s, × 0.9805, 9 µs/rev mozilla-try x0000_revs_x_added_0_copies 63519bfd42ee a36a2a865d92 : 40314 revs, 0.317216 s, 0.294063 s, -0.023153 s, × 0.9270, 7 µs/rev mozilla-try x0000_revs_x_added_x_copies 9fe69ff0762d bcabf2a78927 : 38690 revs, 0.303119 s, 0.281493 s, -0.021626 s, × 0.9287, 7 µs/rev mozilla-try x0000_revs_xx000_added_x_copies 156f6e2674f2 4d0f2c178e66 : 8598 revs, 0.110717 s, 0.076323 s, -0.034394 s, × 0.6894, 8 µs/rev mozilla-try x0000_revs_xx000_added_0_copies 9eec5917337d 67118cc6dcad : 615 revs, 0.045739 s, 0.020390 s, -0.025349 s, × 0.4458, 33 µs/rev mozilla-try x0000_revs_xx000_added_x000_copies 89294cd501d9 7ccb2fc7ccb5 : 97052 revs, 3.098021 s, 3.023879 s, -0.074142 s, × 0.9761, 31 µs/rev mozilla-try x0000_revs_x0000_added_x0000_copies e928c65095ed e951f4ad123a : 52031 revs, 0.771480 s, 0.735549 s, -0.035931 s, × 0.9534, 14 µs/rev mozilla-try x00000_revs_x_added_0_copies 6a320851d377 1ebb79acd503 : 363753 revs, 18.813422 s, 18.568900 s, -0.244522 s, × 0.9870, 51 µs/rev mozilla-try x00000_revs_x00000_added_0_copies dc8a3ca7010e d16fde900c9c : 34414 revs, 0.962867 s, 0.502584 s, -0.460283 s, × 0.5220, 14 µs/rev mozilla-try x00000_revs_x_added_x_copies 5173c4b6f97c 95d83ee7242d : 362229 revs, 18.684923 s, 18.356645 s, -0.328278 s, × 0.9824, 50 µs/rev mozilla-try x00000_revs_x000_added_x_copies 9126823d0e9c ca82787bb23c : 359344 revs, 18.296305 s, 18.250393 s, -0.045912 s, × 0.9975, 50 µs/rev mozilla-try x00000_revs_x0000_added_x0000_copies 8d3fafa80d4b eb884023b810 : 192665 revs, 3.061887 s, 2.792459 s, -0.269428 s, × 0.9120, 14 µs/rev mozilla-try x00000_revs_x00000_added_x0000_copies 1b661134e2ca 1ae03d022d6d : 228985 revs, 103.869641 s, 107.697264 s, +3.827623 s, × 1.0369, 470 µs/rev mozilla-try x00000_revs_x00000_added_x000_copies 9b2a99adc05e 8e29777b48e6 : 382065 revs, 64.262957 s, 63.961040 s, -0.301917 s, × 0.9953, 167 µs/rev Differential Revision: https://phab.mercurial-scm.org/D9422
author Pierre-Yves David <pierre-yves.david@octobus.net>
date Mon, 14 Dec 2020 11:32:24 +0100
parents 70a9eb899637
children a132aa5979ec
files mercurial/copies.py rust/hg-core/src/copy_tracing.rs rust/hg-cpython/src/copy_tracing.rs
diffstat 3 files changed, 231 insertions(+), 144 deletions(-) [+]
line wrap: on
line diff
--- a/mercurial/copies.py	Mon Dec 14 11:32:20 2020 +0100
+++ b/mercurial/copies.py	Mon Dec 14 11:32:24 2020 +0100
@@ -288,40 +288,92 @@
 
     cl = repo.changelog
     isancestor = cl.isancestorrev
-    missingrevs = cl.findmissingrevs(common=[a.rev()], heads=[b.rev()])
-    mrset = set(missingrevs)
+
+    # To track rename from "A" to B, we need to gather all parent → children
+    # edges that are contains in `::B` but not in `::A`.
+    #
+    #
+    # To do so, we need to gather all revisions exclusive¹ to "B" (ie¹: `::b -
+    # ::a`) and also all the "roots point", ie the parents of the exclusive set
+    # that belong to ::a. These are exactly all the revisions needed to express
+    # the parent → children we need to combine.
+    #
+    # [1] actually, we need to gather all the edges within `(::a)::b`, ie:
+    # excluding paths that leads to roots that are not ancestors of `a`. We
+    # keep this out of the explanation because it is hard enough without this special case..
+
+    parents = cl._uncheckedparentrevs
+    graph_roots = (nullrev, nullrev)
+
+    ancestors = cl.ancestors([a.rev()], inclusive=True)
+    revs = cl.findmissingrevs(common=[a.rev()], heads=[b.rev()])
     roots = set()
-    for r in missingrevs:
-        for p in cl.parentrevs(r):
-            if p == nullrev:
-                continue
-            if p not in children:
-                children[p] = [r]
-            else:
-                children[p].append(r)
-            if p not in mrset:
-                roots.add(p)
+    has_graph_roots = False
+
+    # iterate over `only(B, A)`
+    for r in revs:
+        ps = parents(r)
+        if ps == graph_roots:
+            has_graph_roots = True
+        else:
+            p1, p2 = ps
+
+            # find all the "root points" (see larger comment above)
+            if p1 != nullrev and p1 in ancestors:
+                roots.add(p1)
+            if p2 != nullrev and p2 in ancestors:
+                roots.add(p2)
     if not roots:
         # no common revision to track copies from
         return {}
-    min_root = min(roots)
-
-    from_head = set(
-        cl.reachableroots(min_root, [b.rev()], list(roots), includepath=True)
-    )
-
-    iterrevs = set(from_head)
-    iterrevs &= mrset
-    iterrevs.update(roots)
-    iterrevs.remove(b.rev())
-    revs = sorted(iterrevs)
+    if has_graph_roots:
+        # this deal with the special case mentionned in the [1] footnotes. We
+        # must filter out revisions that leads to non-common graphroots.
+        roots = list(roots)
+        m = min(roots)
+        h = [b.rev()]
+        roots_to_head = cl.reachableroots(m, h, roots, includepath=True)
+        roots_to_head = set(roots_to_head)
+        revs = [r for r in revs if r in roots_to_head]
 
     if repo.filecopiesmode == b'changeset-sidedata':
+        # When using side-data, we will process the edges "from" the children.
+        # We iterate over the childre, gathering previous collected data for
+        # the parents. Do know when the parents data is no longer necessary, we
+        # keep a counter of how many children each revision has.
+        #
+        # An interresting property of `children_count` is that it only contains
+        # revision that will be relevant for a edge of the graph. So if a
+        # children has parent not in `children_count`, that edges should not be
+        # processed.
+        children_count = dict((r, 0) for r in roots)
+        for r in revs:
+            for p in cl.parentrevs(r):
+                if p == nullrev:
+                    continue
+                children_count[r] = 0
+                if p in children_count:
+                    children_count[p] += 1
         revinfo = _revinfo_getter(repo, match)
         return _combine_changeset_copies(
-            revs, children, b.rev(), revinfo, match, isancestor
+            revs, children_count, b.rev(), revinfo, match, isancestor
         )
     else:
+        # When not using side-data, we will process the edges "from" the parent.
+        # so we need a full mapping of the parent -> children relation.
+        children = dict((r, []) for r in roots)
+        for r in revs:
+            for p in cl.parentrevs(r):
+                if p == nullrev:
+                    continue
+                children[r] = []
+                if p in children:
+                    children[p].append(r)
+        x = revs.pop()
+        assert x == b.rev()
+        revs.extend(roots)
+        revs.sort()
+
         revinfo = _revinfo_getter_extra(repo)
         return _combine_changeset_copies_extra(
             revs, children, b.rev(), revinfo, match, isancestor
@@ -329,12 +381,12 @@
 
 
 def _combine_changeset_copies(
-    revs, children, targetrev, revinfo, match, isancestor
+    revs, children_count, targetrev, revinfo, match, isancestor
 ):
     """combine the copies information for each item of iterrevs
 
     revs: sorted iterable of revision to visit
-    children: a {parent: [children]} mapping.
+    children_count: a {parent: <number-of-relevant-children>} mapping.
     targetrev: the final copies destination revision (not in iterrevs)
     revinfo(rev): a function that return (p1, p2, p1copies, p2copies, removed)
     match: a matcher
@@ -346,69 +398,76 @@
 
     if rustmod is not None and alwaysmatch:
         return rustmod.combine_changeset_copies(
-            list(revs), children, targetrev, revinfo, isancestor
+            list(revs), children_count, targetrev, revinfo, isancestor
         )
 
     isancestor = cached_is_ancestor(isancestor)
 
     all_copies = {}
-    # iterate over all the "parent" side of copy tracing "edge"
-    for r in revs:
-        # fetch potential previously computed data for that parent
-        copies = all_copies.pop(r, None)
-        if copies is None:
-            # this is a root
-            copies = {}
+    # iterate over all the "children" side of copy tracing "edge"
+    for current_rev in revs:
+        p1, p2, changes = revinfo(current_rev)
+        current_copies = None
 
-        # iterate over all known children to chain the existing data with the
+        # iterate over all parents to chain the existing data with the
         # data from the parent → child edge.
-        for i, c in enumerate(children[r]):
-            p1, p2, changes = revinfo(c)
-            childcopies = {}
+        for parent, parent_rev in ((1, p1), (2, p2)):
+            if parent_rev == nullrev:
+                continue
+            remaining_children = children_count.get(parent_rev)
+            if remaining_children is None:
+                continue
+            remaining_children -= 1
+            children_count[parent_rev] = remaining_children
+            if remaining_children:
+                copies = all_copies.get(parent_rev, None)
+            else:
+                copies = all_copies.pop(parent_rev, None)
 
-            # select the right parent → child edge
-            if r == p1:
-                parent = 1
-                if changes is not None:
+            if copies is None:
+                # this is a root
+                copies = {}
+
+            newcopies = copies
+            # chain the data in the edge with the existing data
+            if changes is not None:
+                childcopies = {}
+                if parent == 1:
                     childcopies = changes.copied_from_p1
-            else:
-                assert r == p2
-                parent = 2
-                if changes is not None:
+                elif parent == 2:
                     childcopies = changes.copied_from_p2
-            if not alwaysmatch:
-                childcopies = {
-                    dst: src for dst, src in childcopies.items() if match(dst)
-                }
 
-            # chain the data in the edge with the existing data
-            newcopies = copies
-            if childcopies:
-                newcopies = copies.copy()
-                for dest, source in pycompat.iteritems(childcopies):
-                    prev = copies.get(source)
-                    if prev is not None and prev[1] is not None:
-                        source = prev[1]
-                    newcopies[dest] = (c, source)
-                assert newcopies is not copies
-            if changes is not None and changes.removed:
-                if newcopies is copies:
+                if not alwaysmatch:
+                    childcopies = {
+                        dst: src
+                        for dst, src in childcopies.items()
+                        if match(dst)
+                    }
+                if childcopies:
                     newcopies = copies.copy()
-                for f in changes.removed:
-                    if f in newcopies:
-                        if newcopies is copies:
-                            # copy on write to avoid affecting potential other
-                            # branches.  when there are no other branches, this
-                            # could be avoided.
-                            newcopies = copies.copy()
-                        newcopies[f] = (c, None)
+                    for dest, source in pycompat.iteritems(childcopies):
+                        prev = copies.get(source)
+                        if prev is not None and prev[1] is not None:
+                            source = prev[1]
+                        newcopies[dest] = (current_rev, source)
+                    assert newcopies is not copies
+                if changes.removed:
+                    if newcopies is copies:
+                        newcopies = copies.copy()
+                    for f in changes.removed:
+                        if f in newcopies:
+                            if newcopies is copies:
+                                # copy on write to avoid affecting potential other
+                                # branches.  when there are no other branches, this
+                                # could be avoided.
+                                newcopies = copies.copy()
+                            newcopies[f] = (current_rev, None)
 
             # check potential need to combine the data from another parent (for
             # that child). See comment below for details.
-            othercopies = all_copies.get(c)
-            if othercopies is None:
-                all_copies[c] = newcopies
-            elif newcopies is othercopies:
+            if current_copies is None:
+                current_copies = newcopies
+            elif current_copies is newcopies:
                 # nothing to merge:
                 pass
             else:
@@ -419,17 +478,11 @@
                 # This is an arbitrary choice made anew when implementing
                 # changeset based copies. It was made without regards with
                 # potential filelog related behavior.
-                if parent == 1:
-                    if newcopies is copies:
-                        newcopies = copies.copy()
-                    minor, major = othercopies, newcopies
-                else:
-                    # we do not know if the other dict is a copy or not, so we
-                    # need to blindly copy it. Future change should make this
-                    # unnecessary.
-                    minor, major = newcopies, othercopies.copy()
-                copies = _merge_copies_dict(minor, major, isancestor, changes)
-                all_copies[c] = copies
+                assert parent == 2
+                current_copies = _merge_copies_dict(
+                    newcopies, current_copies, isancestor, changes
+                )
+        all_copies[current_rev] = current_copies
 
     # filter out internal details and return a {dest: source mapping}
     final_copies = {}
--- a/rust/hg-core/src/copy_tracing.rs	Mon Dec 14 11:32:20 2020 +0100
+++ b/rust/hg-core/src/copy_tracing.rs	Mon Dec 14 11:32:24 2020 +0100
@@ -1,6 +1,7 @@
 use crate::utils::hg_path::HgPath;
 use crate::utils::hg_path::HgPathBuf;
 use crate::Revision;
+use crate::NULL_REVISION;
 
 use im_rc::ordmap::DiffItem;
 use im_rc::ordmap::OrdMap;
@@ -328,7 +329,7 @@
 ///                                 ancestor of another
 pub fn combine_changeset_copies<A: Fn(Revision, Revision) -> bool, D>(
     revs: Vec<Revision>,
-    children: HashMap<Revision, Vec<Revision>>,
+    mut children_count: HashMap<Revision, usize>,
     target_rev: Revision,
     rev_info: RevInfoMaker<D>,
     is_ancestor: &A,
@@ -337,57 +338,69 @@
     let mut oracle = AncestorOracle::new(is_ancestor);
 
     for rev in revs {
-        // Retrieve data computed in a previous iteration
-        let copies = all_copies.remove(&rev);
-        let copies = match copies {
-            Some(c) => c,
-            None => TimeStampedPathCopies::default(), // root of the walked set
-        };
-
-        let current_children = match children.get(&rev) {
-            Some(c) => c,
-            None => panic!("inconsistent `revs` and `children`"),
-        };
-
-        for child in current_children {
-            // We will chain the copies information accumulated for `rev` with
-            // the individual copies information for each of its children.
-            // Creating a new PathCopies for each `rev` → `children` vertex.
-            let mut d: DataHolder<D> = DataHolder { data: None };
-            let (p1, p2, changes) = rev_info(*child, &mut d);
+        let mut d: DataHolder<D> = DataHolder { data: None };
+        let (p1, p2, changes) = rev_info(rev, &mut d);
 
-            let parent = if rev == p1 {
-                Parent::FirstParent
-            } else {
-                assert_eq!(rev, p2);
-                Parent::SecondParent
-            };
-            let new_copies =
-                add_from_changes(&copies, &changes, parent, *child);
+        // We will chain the copies information accumulated for the parent with
+        // the individual copies information the curent revision.  Creating a
+        // new TimeStampedPath for each `rev` → `children` vertex.
+        let mut copies: Option<TimeStampedPathCopies> = None;
+        if p1 != NULL_REVISION {
+            // Retrieve data computed in a previous iteration
+            let parent_copies = get_and_clean_parent_copies(
+                &mut all_copies,
+                &mut children_count,
+                p1,
+            );
+            if let Some(parent_copies) = parent_copies {
+                // combine it with data for that revision
+                let vertex_copies = add_from_changes(
+                    &parent_copies,
+                    &changes,
+                    Parent::FirstParent,
+                    rev,
+                );
+                // keep that data around for potential later combination
+                copies = Some(vertex_copies);
+            }
+        }
+        if p2 != NULL_REVISION {
+            // Retrieve data computed in a previous iteration
+            let parent_copies = get_and_clean_parent_copies(
+                &mut all_copies,
+                &mut children_count,
+                p2,
+            );
+            if let Some(parent_copies) = parent_copies {
+                // combine it with data for that revision
+                let vertex_copies = add_from_changes(
+                    &parent_copies,
+                    &changes,
+                    Parent::SecondParent,
+                    rev,
+                );
 
-            // Merge has two parents needs to combines their copy information.
-            //
-            // If the vertex from the other parent was already processed, we
-            // will have a value for the child ready to be used. We need to
-            // grab it and combine it with the one we already
-            // computed. If not we can simply store the newly
-            // computed data. The processing happening at
-            // the time of the second parent will take care of combining the
-            // two TimeStampedPathCopies instance.
-            match all_copies.remove(child) {
-                None => {
-                    all_copies.insert(child, new_copies);
-                }
-                Some(other_copies) => {
-                    let (minor, major) = match parent {
-                        Parent::FirstParent => (other_copies, new_copies),
-                        Parent::SecondParent => (new_copies, other_copies),
-                    };
-                    let merged_copies =
-                        merge_copies_dict(minor, major, &changes, &mut oracle);
-                    all_copies.insert(child, merged_copies);
-                }
-            };
+                copies = match copies {
+                    None => Some(vertex_copies),
+                    // Merge has two parents needs to combines their copy
+                    // information.
+                    //
+                    // If we got data from both parents, We need to combine
+                    // them.
+                    Some(copies) => Some(merge_copies_dict(
+                        vertex_copies,
+                        copies,
+                        &changes,
+                        &mut oracle,
+                    )),
+                };
+            }
+        }
+        match copies {
+            Some(copies) => {
+                all_copies.insert(rev, copies);
+            }
+            _ => {}
         }
     }
 
@@ -405,6 +418,32 @@
     result
 }
 
+/// fetch previous computed information
+///
+/// If no other children are expected to need this information, we drop it from
+/// the cache.
+///
+/// If parent is not part of the set we are expected to walk, return None.
+fn get_and_clean_parent_copies(
+    all_copies: &mut HashMap<Revision, TimeStampedPathCopies>,
+    children_count: &mut HashMap<Revision, usize>,
+    parent_rev: Revision,
+) -> Option<TimeStampedPathCopies> {
+    let count = children_count.get_mut(&parent_rev)?;
+    *count -= 1;
+    if *count == 0 {
+        match all_copies.remove(&parent_rev) {
+            Some(c) => Some(c),
+            None => Some(TimeStampedPathCopies::default()),
+        }
+    } else {
+        match all_copies.get(&parent_rev) {
+            Some(c) => Some(c.clone()),
+            None => Some(TimeStampedPathCopies::default()),
+        }
+    }
+}
+
 /// Combine ChangedFiles with some existing PathCopies information and return
 /// the result
 fn add_from_changes(
--- a/rust/hg-cpython/src/copy_tracing.rs	Mon Dec 14 11:32:20 2020 +0100
+++ b/rust/hg-cpython/src/copy_tracing.rs	Mon Dec 14 11:32:24 2020 +0100
@@ -23,7 +23,7 @@
 pub fn combine_changeset_copies_wrapper(
     py: Python,
     revs: PyList,
-    children: PyDict,
+    children_count: PyDict,
     target_rev: Revision,
     rev_info: PyObject,
     is_ancestor: PyObject,
@@ -93,20 +93,15 @@
 
             (p1, p2, files)
         });
-    let children: PyResult<_> = children
+    let children_count: PyResult<_> = children_count
         .items(py)
         .iter()
-        .map(|(k, v)| {
-            let v: &PyList = v.cast_as(py)?;
-            let v: PyResult<_> =
-                v.iter(py).map(|child| Ok(child.extract(py)?)).collect();
-            Ok((k.extract(py)?, v?))
-        })
+        .map(|(k, v)| Ok((k.extract(py)?, v.extract(py)?)))
         .collect();
 
     let res = combine_changeset_copies(
         revs?,
-        children?,
+        children_count?,
         target_rev,
         rev_info_maker,
         &is_ancestor_wrap,