setdiscovery: stop limiting the number of local head we initially send

In our testing this limitation provides now real gain and instead triggers
pathological discovery timing for some repository with many heads.

See inline documentation for details.

Some timing below:

Mozilla try repository, (~1M revs, ~35K heads), discovery between 2 clones with
100 head missing on each side

before:
! wall 1.492111 comb 1.490000 user 1.450000 sys 0.040000 (best of 20)
! wall 1.813992 comb 1.820000 user 1.700000 sys 0.120000 (max of 20)
! wall 1.574326 comb 1.573500 user 1.522000 sys 0.051500 (avg of 20)
! wall 1.572583 comb 1.570000 user 1.520000 sys 0.050000 (median of 20)

after:
! wall 1.147834 comb 1.150000 user 1.090000 sys 0.060000 (best of 20)
! wall 1.449144 comb 1.450000 user 1.330000 sys 0.120000 (max of 20)
! wall 1.204618 comb 1.202500 user 1.146500 sys 0.056000 (avg of 20)
! wall 1.194407 comb 1.190000 user 1.140000 sys 0.050000 (median of 20)


pypy (~100 heads, 317 heads) discovery between clones with only 42 common heads

before:
! wall 0.031653 comb 0.030000 user 0.030000 sys 0.000000 (best of 25)
! wall 0.055719 comb 0.050000 user 0.040000 sys 0.010000 (max of 25)
! wall 0.038939 comb 0.039600 user 0.038400 sys 0.001200 (avg of 25)
! wall 0.038660 comb 0.050000 user 0.040000 sys 0.010000 (median of 25)

after:
! wall 0.018754 comb 0.020000 user 0.020000 sys 0.000000 (best of 49)
! wall 0.034505 comb 0.040000 user 0.030000 sys 0.010000 (max of 49)
! wall 0.019631 comb 0.019796 user 0.018367 sys 0.001429 (avg of 49)
! wall 0.019132 comb 0.020000 user 0.020000 sys 0.000000 (median of 49)


Private repository (~1M revs, ~3K heads), discovery from a strip subset, about
100 changesets to be pulled.

before:
! wall 1.837729 comb 1.840000 user 1.790000 sys 0.050000 (best of 20)
! wall 2.203468 comb 2.200000 user 2.100000 sys 0.100000 (max of 20)
! wall 2.049355 comb 2.048500 user 2.002500 sys 0.046000 (avg of 20)
! wall 2.035315 comb 2.040000 user 2.000000 sys 0.040000 (median of 20)

after:
! wall 0.136598 comb 0.130000 user 0.110000 sys 0.020000 (best of 20)
! wall 0.330519 comb 0.330000 user 0.260000 sys 0.070000 (max of 20)
! wall 0.157254 comb 0.155500 user 0.123000 sys 0.032500 (avg of 20)
! wall 0.149870 comb 0.140000 user 0.110000 sys 0.030000 (median of 20)


Same private repo, discovery between two clone with 500 different heads on each
side:

before:
! wall 2.372919 comb 2.370000 user 2.320000 sys 0.050000 (best of 20)
! wall 2.622422 comb 2.610000 user 2.510000 sys 0.100000 (max of 20)
! wall 2.450135 comb 2.450000 user 2.402000 sys 0.048000 (avg of 20)
! wall 2.443896 comb 2.450000 user 2.410000 sys 0.040000 (median of 20)

after:
! wall 0.625497 comb 0.620000 user 0.570000 sys 0.050000 (best of 20)
! wall 0.834723 comb 0.820000 user 0.730000 sys 0.090000 (max of 20)
! wall 0.675725 comb 0.675500 user 0.628000 sys 0.047500 (avg of 20)
! wall 0.671614 comb 0.680000 user 0.640000 sys 0.040000 (median of 20)

mercurial/setdiscovery.py

@@ -275,9 +275,63 b' def findcommonheads(ui, local, remote,'
     # early exit if we know all the specified remote heads already
     ui.debug("query 1; heads\n")
     roundtrips += 1
-    sample = _limitsample(ownheads, initialsamplesize)
-    # indices between sample and externalized version must match
-    sample = list(sample)
+    # 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
+    # large "undecided" set, slower to be clarified than if we asked the
+    # question for all heads right away.
+    #
+    # We are already fetching all server heads using the `heads` commands,
+    # sending a equivalent number of heads the other way should not have a
+    # significant impact.  In addition, it is very likely that we are going to
+    # have to issue "known" request for an equivalent amount of revisions in
+    # order to decide if theses heads are common or missing.
+    #
+    # find a detailled analysis below.
+    #
+    # Case A: local and server both has few heads
+    #
+    #     Ownheads is below initialsamplesize, limit would not have any effect.
+    #
+    # Case B: local has few heads and server has many
+    #
+    #     Ownheads is below initialsamplesize, limit would not have any effect.
+    #
+    # Case C: local and server both has many heads
+    #
+    #     We now transfert some more data, but not significantly more than is
+    #     already transfered to carry the server heads.
+    #
+    # Case D: local has many heads, server has few
+    #
+    #   D.1 local heads are mostly known remotely
+    #
+    #     All the known head will have be part of a `known` request at some
+    #     point for the discovery to finish. Sending them all earlier is
+    #     actually helping.
+    #
+    #     (This case is fairly unlikely, it requires the numerous heads to all
+    #     be merged server side in only a few heads)
+    #
+    #   D.2 local heads are mostly missing remotely
+    #
+    #     To determine that the heads are missing, we'll have to issue `known`
+    #     request for them or one of their ancestors. This amount of `known`
+    #     request will likely be in the same order of magnitude than the amount
+    #     of local heads.
+    #
+    #     The only case where we can be more efficient using `known` request on
+    #     ancestors are case were all the "missing" local heads are based on a
+    #     few changeset, also "missing".  This means we would have a "complex"
+    #     graph (with many heads) attached to, but very independant to a the
+    #     "simple" graph on the server. This is a fairly usual case and have
+    #     not been met in the wild so far.
+    if remote.limitedarguments:
+        sample = _limitsample(ownheads, initialsamplesize)
+        # indices between sample and externalized version must match
+        sample = list(sample)
+    else:
+        sample = ownheads
 
     with remote.commandexecutor() as e:
         fheads = e.callcommand('heads', {})

tests/test-setdiscovery.t

@@ -926,7 +926,7 b' Both many new on top of long history:'
   common heads: 7ead0cba2838
 
 
-One with >200 heads, which used to use up all of the sample:
+One with >200 heads. We now switch to send them all in the initial roundtrip, but still do sampling for the later request.
 
   $ hg init manyheads
   $ cd manyheads
@@ -974,20 +974,17 b' One with >200 heads, which used to use u'
   searching for changes
   taking quick initial sample
   searching: 2 queries
-  query 2; still undecided: 1240, sample size is: 100
+  query 2; still undecided: 1080, sample size is: 100
   sampling from both directions
   searching: 3 queries
-  query 3; still undecided: 1140, sample size is: 200
+  query 3; still undecided: 980, sample size is: 200
   sampling from both directions
   searching: 4 queries
   query 4; still undecided: \d+, sample size is: 200 (re)
   sampling from both directions
   searching: 5 queries
-  query 5; still undecided: \d+, sample size is: 200 (re)
-  sampling from both directions
-  searching: 6 queries
-  query 6; still undecided: \d+, sample size is: \d+ (re)
-  6 total queries in *.????s (glob)
+  query 5; still undecided: 195, sample size is: 195
+  5 total queries in *.????s (glob)
   elapsed time:  * seconds (glob)
   heads summary:
     total common heads:          1
@@ -1116,6 +1113,6 b' fixed in 86c35b7ae300:'
   $ hg -R r1 --config extensions.blackbox= blackbox --config blackbox.track=
   * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> serve --cmdserver chgunix * (glob) (chg !)
   * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> -R r1 outgoing r2 *-T{rev} * --config *extensions.blackbox=* (glob)
-  * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> found 101 common and 1 unknown server heads, 2 roundtrips in *.????s (glob)
+  * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> found 101 common and 1 unknown server heads, 1 roundtrips in *.????s (glob)
   * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> -R r1 outgoing r2 *-T{rev} * --config *extensions.blackbox=* exited 0 after *.?? seconds (glob)
   $ cd ..