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 ??
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)