diff --git a/mercurial/utils/storageutil.py b/mercurial/utils/storageutil.py
--- a/mercurial/utils/storageutil.py
+++ b/mercurial/utils/storageutil.py
@@ -460,14 +460,29 @@ def emitrevisions(
                 debug_delta_source = "storage"
             baserev = deltaparentrev
         else:
+            if deltaparentrev != nullrev and debug_info is not None:
+                debug_info['denied-base-not-available'] += 1
             # No guarantee the receiver has the delta parent, or Storage has a
             # fulltext revision.
             #
-            # Send delta against last revision (if possible), which in the
-            # common case should be similar enough to this revision that the
-            # delta is reasonable.
-            if deltaparentrev != nullrev and debug_info is not None:
-                debug_info['denied-base-not-available'] += 1
+            # We compute a delta on the fly to send over the wire.
+            #
+            # We start with a try against p1, which in the common case should
+            # be close to this revision content.
+            #
+            # note: we could optimize between p1 and p2 in merges cases.
+            elif is_usable_base(p1rev):
+                if debug_info is not None:
+                    debug_delta_source = "p1"
+                baserev = p1rev
+            # if p1 was not an option, try p2
+            elif is_usable_base(p2rev):
+                if debug_info is not None:
+                    debug_delta_source = "p2"
+                baserev = p2rev
+            # Send delta against prev in despair
+            #
+            # using the closest available ancestors first might be better?
             elif prevrev is not None:
                 if debug_info is not None:
                     debug_delta_source = "prev"