deltas: skip if projected compressed size is bigger than previous snapshot
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(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
further performance boost (timing from `hg perfrevlogwrite`):
before: 3.010646 seconds
after: 2.609307 seconds
--- a/mercurial/revlogutils/deltas.py Mon Jan 21 22:46:18 2019 +0100
+++ b/mercurial/revlogutils/deltas.py Mon Jan 21 22:46:31 2019 +0100
@@ -966,6 +966,8 @@
snapshotlimit = revinfo.textlen >> snapshotdepth
if snapshotlimit < lowestrealisticdeltalen:
return None
+ if revlog.length(base) < lowestrealisticdeltalen:
+ return None
header, data = revlog.compress(delta)
deltalen = len(header) + len(data)
offset = revlog.end(len(revlog) - 1)