deltas: skip if projected delta size is bigger than previous snapshot
Before computing any delta, we get a basic estimation of the delta size we can
expect and the resulted compressed value. We then checks this projected size
against the `size(snapshotⁿ) > size(snapshotⁿ⁺¹)` constraint. This allows to
exclude potential base candidates before doing any expensive computation.
This only apply to the intermediate-snapshot case since this constraint only
apply to them.
For some pathological cases of a private repository this step provide a
significant performance boost (timing from `hg perfrevlogwrite`):
before: 14.115908 seconds
after: 3.145906 seconds
--- a/mercurial/revlogutils/deltas.py Thu Apr 25 22:30:14 2019 +0200
+++ b/mercurial/revlogutils/deltas.py Thu Apr 25 22:50:33 2019 +0200
@@ -698,6 +698,12 @@
# delta lower bound is larger than accepted upper bound
continue
+ # check the relative constraint on the delta size
+ revlength = revlog.length(rev)
+ if revlength < lowestrealisticdeltalen:
+ # delta probable lower bound is larger than target base
+ continue
+
group.append(rev)
if group:
# XXX: in the sparse revlog case, group can become large,