--- a/mercurial/configitems.py	Tue Jan 19 00:20:42 2021 +0100
+++ b/mercurial/configitems.py	Sun Jan 17 23:21:33 2021 +0100
@@ -729,6 +729,18 @@
     b'discovery.randomize',
     default=True,
 )
+# Control the initial size of the discovery sample
+coreconfigitem(
+    b'devel',
+    b'discovery.sample-size',
+    default=200,
+)
+# Control the initial size of the discovery for initial change
+coreconfigitem(
+    b'devel',
+    b'discovery.sample-size.initial',
+    default=100,
+)
 _registerdiffopts(section=b'diff')
 coreconfigitem(
     b'email',

--- a/mercurial/setdiscovery.py	Tue Jan 19 00:20:42 2021 +0100
+++ b/mercurial/setdiscovery.py	Sun Jan 17 23:21:33 2021 +0100
@@ -286,8 +286,6 @@
     ui,
     local,
     remote,
-    initialsamplesize=100,
-    fullsamplesize=200,
     abortwhenunrelated=True,
     ancestorsof=None,
     audit=None,
@@ -315,7 +313,8 @@
         ownheads = [rev for rev in cl.headrevs() if rev != nullrev]
 
     initial_head_exchange = ui.configbool(b'devel', b'discovery.exchange-heads')
-
+    initialsamplesize = ui.configint(b'devel', b'discovery.sample-size.initial')
+    fullsamplesize = ui.configint(b'devel', b'discovery.sample-size')
     # We also ask remote about all the local heads. That set can be arbitrarily
     # large, so we used to limit it size to `initialsamplesize`. We no longer
     # do as it proved counter productive. The skipped heads could lead to a

--- a/tests/test-setdiscovery.t	Tue Jan 19 00:20:42 2021 +0100
+++ b/tests/test-setdiscovery.t	Sun Jan 17 23:21:33 2021 +0100
@@ -1328,25 +1328,25 @@
   updating to branch b
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
 
-  $ hg -R a debugdiscovery b --debug --verbose --config progress.debug=true --config devel.discovery.randomize=false
+  $ hg -R a debugdiscovery b --debug --verbose --config progress.debug=true --config devel.discovery.randomize=false  --config devel.discovery.sample-size.initial=50
   comparing with b
   query 1; heads
   searching for changes
   taking quick initial sample
   searching: 2 queries
-  query 2; still undecided: 1080, sample size is: 100
+  query 2; still undecided: 1080, sample size is: 50
   sampling from both directions
   searching: 3 queries
-  query 3; still undecided: 980, sample size is: 200
+  query 3; still undecided: 1030, sample size is: 200
   sampling from both directions
   searching: 4 queries
-  query 4; still undecided: 497, sample size is: 210
+  query 4; still undecided: 547, sample size is: 210
   sampling from both directions
   searching: 5 queries
-  query 5; still undecided: 285, sample size is: 220
+  query 5; still undecided: 336, sample size is: 220
   sampling from both directions
   searching: 6 queries
-  query 6; still undecided: 63, sample size is: 63
+  query 6; still undecided: 114, sample size is: 114
   6 total queries in *.????s (glob)
   elapsed time:  * seconds (glob)
   round-trips:                   6
@@ -1412,22 +1412,30 @@
       missing:                1040
   common heads: 3ee37d65064a
 
-  $ hg -R a debugdiscovery b --debug --config devel.discovery.exchange-heads=false --config devel.discovery.randomize=false --config devel.discovery.grow-sample.rate=1.01
+  $ hg -R a debugdiscovery b --debug --config devel.discovery.exchange-heads=false --config devel.discovery.randomize=false --config devel.discovery.grow-sample.rate=1.20 --config devel.discovery.sample-size=50
   comparing with b
   searching for changes
   sampling from both directions
-  query 1; still undecided: 1340, sample size is: 200
+  query 1; still undecided: 1340, sample size is: 50
+  sampling from both directions
+  query 2; still undecided: 995, sample size is: 60
   sampling from both directions
-  query 2; still undecided: 795, sample size is: 202
+  query 3; still undecided: 913, sample size is: 72
   sampling from both directions
-  query 3; still undecided: 525, sample size is: 204
+  query 4; still undecided: 816, sample size is: 204
+  sampling from both directions
+  query 5; still undecided: 612, sample size is: 153
   sampling from both directions
-  query 4; still undecided: 252, sample size is: 206
+  query 6; still undecided: 456, sample size is: 123
+  sampling from both directions
+  query 7; still undecided: 332, sample size is: 147
   sampling from both directions
-  query 5; still undecided: 44, sample size is: 44
-  5 total queries in *s (glob)
-  elapsed time: * seconds (glob)
-  round-trips:                   5
+  query 8; still undecided: 184, sample size is: 176
+  sampling from both directions
+  query 9; still undecided: 8, sample size is: 8
+  9 total queries in *s (glob)
+  elapsed time:  * seconds (glob)
+  round-trips:                   9
   heads summary:
     total common heads:          1
       also local heads:          0