diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -69,6 +69,7 @@ from . import (
     pycompat,
     registrar,
     repair,
+    repoview,
     revlog,
     revset,
     revsetlang,
@@ -964,20 +965,73 @@ def debugstate(ui, repo, **opts):
         ),
         (b'', b'rev', [], b'restrict discovery to this set of revs'),
         (b'', b'seed', b'12323', b'specify the random seed use for discovery'),
+        (
+            b'',
+            b'local-as-revs',
+            "",
+            'treat local has having these revisions only',
+        ),
+        (
+            b'',
+            b'remote-as-revs',
+            "",
+            'use local as remote, with only these these revisions',
+        ),
     ]
     + cmdutil.remoteopts,
     _(b'[--rev REV] [OTHER]'),
 )
 def debugdiscovery(ui, repo, remoteurl=b"default", **opts):
-    """runs the changeset discovery protocol in isolation"""
+    """runs the changeset discovery protocol in isolation
+
+    The local peer can be "replaced" by a subset of the local repository by
+    using the `--local-as-revs` flag. Int he same way, usual `remote` peer can
+    be "replaced" by a subset of the local repository using the
+    `--local-as-revs` flag. This is useful to efficiently debug pathological
+    discovery situation.
+    """
     opts = pycompat.byteskwargs(opts)
-    remoteurl, branches = hg.parseurl(ui.expandpath(remoteurl))
-    remote = hg.peer(repo, opts, remoteurl)
-    ui.status(_(b'comparing with %s\n') % util.hidepassword(remoteurl))
+    unfi = repo.unfiltered()
+
+    # setup potential extra filtering
+    local_revs = opts[b"local_as_revs"]
+    remote_revs = opts[b"remote_as_revs"]
 
     # make sure tests are repeatable
     random.seed(int(opts[b'seed']))
 
+    if not remote_revs:
+
+        remoteurl, branches = hg.parseurl(ui.expandpath(remoteurl))
+        remote = hg.peer(repo, opts, remoteurl)
+        ui.status(_(b'comparing with %s\n') % util.hidepassword(remoteurl))
+    else:
+        branches = (None, [])
+        remote_filtered_revs = scmutil.revrange(
+            unfi, [b"not (::(%s))" % remote_revs]
+        )
+        remote_filtered_revs = frozenset(remote_filtered_revs)
+
+        def remote_func(x):
+            return remote_filtered_revs
+
+        repoview.filtertable[b'debug-discovery-remote-filter'] = remote_func
+
+        remote = repo.peer()
+        remote._repo = remote._repo.filtered(b'debug-discovery-remote-filter')
+
+    if local_revs:
+        local_filtered_revs = scmutil.revrange(
+            unfi, [b"not (::(%s))" % local_revs]
+        )
+        local_filtered_revs = frozenset(local_filtered_revs)
+
+        def local_func(x):
+            return local_filtered_revs
+
+        repoview.filtertable[b'debug-discovery-local-filter'] = local_func
+        repo = repo.filtered(b'debug-discovery-local-filter')
+
     data = {}
     if opts.get(b'old'):
 
diff --git a/tests/test-completion.t b/tests/test-completion.t
--- a/tests/test-completion.t
+++ b/tests/test-completion.t
@@ -283,7 +283,7 @@ Show all commands + options
   debugdate: extended
   debugdeltachain: changelog, manifest, dir, template
   debugdirstate: nodates, dates, datesort
-  debugdiscovery: old, nonheads, rev, seed, ssh, remotecmd, insecure
+  debugdiscovery: old, nonheads, rev, seed, local-as-revs, remote-as-revs, ssh, remotecmd, insecure
   debugdownload: output
   debugextensions: template
   debugfileset: rev, all-files, show-matcher, show-stage
diff --git a/tests/test-setdiscovery.t b/tests/test-setdiscovery.t
--- a/tests/test-setdiscovery.t
+++ b/tests/test-setdiscovery.t
@@ -1588,3 +1588,139 @@ returned as common heads.
       common:                    0
       missing:                   1
   common heads: 66f7d451a68b
+
+  $ cd ..
+
+
+Test debuging discovery using different subset of the same repository
+=====================================================================
+
+remote is a local subset
+------------------------
+
+remote will be last 25 heads of the local graph
+
+  $ cd $TESTTMP/manyheads
+  $ hg -R a debugdiscovery \
+  > --debug \
+  > --remote-as-revs 'last(heads(all()), 25)' \
+  > --config devel.discovery.randomize=false
+  query 1; heads
+  searching for changes
+  all remote heads known locally
+  elapsed time:  * seconds (glob)
+  round-trips:                   1
+  heads summary:
+    total common heads:         25
+      also local heads:         25
+      also remote heads:        25
+      both:                     25
+    local heads:               260
+      common:                   25
+      missing:                 235
+    remote heads:               25
+      common:                   25
+      unknown:                   0
+  local changesets:           1340
+    common:                    400
+      heads:                    25
+      roots:                     1
+    missing:                   940
+      heads:                   235
+      roots:                   235
+    first undecided set:       940
+      heads:                   235
+      roots:                   235
+      common:                    0
+      missing:                 940
+  common heads: 0dfd965d91c6 0fe09b60448d 14a17233ce9d 175c0a3072cf 1c51e2c80832 1e51600e0698 24eb5f9bdbab 25ce09526613 36bd00abde57 426989fdefa0 596d87362679 5dd1039ea5c0 5ef24f022278 5f230dc19419 80b39998accb 88f40688ffb5 9e37ddf8c632 abf4d55b075e b2ce801fddfe b368b6ac3ce3 c959bf2e869c c9fba6ba4e2e d783207cf649 d9a51e256f21 e3717a4e3753
+
+local is a local subset
+------------------------
+
+remote will be last 25 heads of the local graph
+
+  $ cd $TESTTMP/manyheads
+  $ hg -R a debugdiscovery b \
+  > --debug \
+  > --local-as-revs 'first(heads(all()), 25)' \
+  > --config devel.discovery.randomize=false
+  comparing with b
+  query 1; heads
+  searching for changes
+  taking quick initial sample
+  query 2; still undecided: 375, sample size is: 81
+  sampling from both directions
+  query 3; still undecided: 3, sample size is: 3
+  3 total queries *s (glob)
+  elapsed time:  * seconds (glob)
+  round-trips:                   3
+  heads summary:
+    total common heads:          1
+      also local heads:          0
+      also remote heads:         0
+      both:                      0
+    local heads:                25
+      common:                    0
+      missing:                  25
+    remote heads:                1
+      common:                    0
+      unknown:                   1
+  local changesets:            400
+    common:                    300
+      heads:                     1
+      roots:                     1
+    missing:                   100
+      heads:                    25
+      roots:                    25
+    first undecided set:       400
+      heads:                    25
+      roots:                     1
+      common:                  300
+      missing:                 100
+  common heads: 3ee37d65064a
+
+both local and remove are subset
+------------------------
+
+remote will be last 25 heads of the local graph
+
+  $ cd $TESTTMP/manyheads
+  $ hg -R a debugdiscovery \
+  > --debug \
+  > --local-as-revs 'first(heads(all()), 25)' \
+  > --remote-as-revs 'last(heads(all()), 25)' \
+  > --config devel.discovery.randomize=false
+  query 1; heads
+  searching for changes
+  taking quick initial sample
+  query 2; still undecided: 375, sample size is: 81
+  sampling from both directions
+  query 3; still undecided: 3, sample size is: 3
+  3 total queries in *s (glob)
+  elapsed time:  * seconds (glob)
+  round-trips:                   3
+  heads summary:
+    total common heads:          1
+      also local heads:          0
+      also remote heads:         0
+      both:                      0
+    local heads:                25
+      common:                    0
+      missing:                  25
+    remote heads:               25
+      common:                    0
+      unknown:                  25
+  local changesets:            400
+    common:                    300
+      heads:                     1
+      roots:                     1
+    missing:                   100
+      heads:                    25
+      roots:                    25
+    first undecided set:       400
+      heads:                    25
+      roots:                     1
+      common:                  300
+      missing:                 100
+  common heads: 3ee37d65064a