revlog: add a small cache of unfiltered chunk This can provides a massive boost to the reading of multiple revision and the computation of a valid delta chain. This greatly help operation like `hg log --patch`, delta computation (helping pull/unbundle), linkrev adjustment (helping copy tracing). A first round of benchmark for `hg log --patch --limit 1000` shows improvement in the 10-20% range on "small" repository like pypy or mercurial and large improvements (about 33%) for more complex ones like netbeans and mozilla's. These speeds up are consistent with the improvement to `hg pull` (from a server sending poor deltas) I saw benchmarking this last year. Further benchmark will be run during the freeze. I added some configuration in the experimental space to be able to further test the effect of various tuning for now. This feature should fit well in the "usage/resource profile" configuration that we should land next cycle. When it does not provides a benefit the overhead of the cache seem to be around 2%, a small price for the big improvement. In addition I believe we could shave most of this overhead with a more efficent lru implementation.

--- a/mercurial/localrepo.py	Fri Oct 27 02:57:09 2023 +0200
+++ b/mercurial/localrepo.py	Fri Oct 27 08:54:41 2023 +0200
@@ -1089,6 +1089,16 @@
     if chunkcachesize is not None:
         data_config.chunk_cache_size = chunkcachesize
 
+    if ui.configbool(b'experimental', b'revlog.uncompressed-cache.enabled'):
+        factor = ui.configint(
+            b'experimental', b'revlog.uncompressed-cache.factor'
+        )
+        count = ui.configint(
+            b'experimental', b'revlog.uncompressed-cache.count'
+        )
+        data_config.uncompressed_cache_factor = factor
+        data_config.uncompressed_cache_count = count
+
     delta_config.delta_both_parents = ui.configbool(
         b'storage', b'revlog.optimize-delta-parent-choice'
     )