deltas: skip if projected compressed size does not match text size constraint
If we have a delta, we check constraints against a lower bound estimate of the
resulting compressed delta. We then checks this projected size against the ½ⁿ
size constraints. 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
further performance boost (timing from `hg perfrevlogwrite`):
before: 3.145906 seconds
after: 3.010646 seconds
--- a/mercurial/revlogutils/deltas.py Mon Jan 21 22:37:30 2019 +0100
+++ b/mercurial/revlogutils/deltas.py Mon Jan 21 22:46:18 2019 +0100
@@ -960,6 +960,12 @@
delta = revinfo.cachedelta[1]
if delta is None:
delta = self._builddeltadiff(base, revinfo, fh)
+ # snapshotdept need to be neither None nor 0 level snapshot
+ if revlog.upperboundcomp is not None and snapshotdepth:
+ lowestrealisticdeltalen = len(delta) // revlog.upperboundcomp
+ snapshotlimit = revinfo.textlen >> snapshotdepth
+ if snapshotlimit < lowestrealisticdeltalen:
+ return None
header, data = revlog.compress(delta)
deltalen = len(header) + len(data)
offset = revlog.end(len(revlog) - 1)