[PATCH] patchcopies: backout and optimisation that backfired

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at octobus.net>
# Date 1570072771 14400
#      Wed Oct 02 23:19:31 2019 -0400
# Node ID 07010e0cae5cab2ff0c375f24c1d8185ead8e957
# Parent  03e769278ef31f648ba5c49be719da5b73587607
# EXP-Topic patchcopies-regression
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 07010e0cae5c
patchcopies: backout and optimisation that backfired

In 8a0136f69027 we introduced a new "introrev" function. And in d98fb3f42f33, we
made it smart enough to speedup various pathological case.

However, it turns out that is can also introduce massive slow down in some other
cases. To clarify the situation, I ran `perfpathcopies` on 6989 pair of revision
in the pypy repository (`hg perfhelper-pathcopies`. The result is massively in favor of dropping this
condition:

Here are various numbers (before this changeset/after this changesets)

            source       destination  before      after    abs-time    ratio
worth cases 0c90acafea5d 7a260b2c4511    1.251143 1.775874   -0.524731 1.419401
            795743bd02b0 daa08b2d951c    1.276092 1.722316   -0.446224 1.349680
            d7b4ace71d7e a4fedb1664ee    1.046802 1.491993   -0.445191 1.425287
            464d641a9035 4d87b3d82dc2    1.072234 1.509769   -0.437535 1.408059
            a305590465d6 a84f8ceb8740    1.322278 1.738900   -0.416622 1.315079
worse  1%   dbfbfcf077e9 89183b3a84f3    1.471880 1.735137   -0.263257 1.178858
worth  5%   688a132b06b6 573e529840d7    0.090268 0.118713   -0.028445 1.315117
worth 10%   0bfaff4207c3 dca96cba7aee    1.790686 1.795441   -0.004755 1.002655
worth 25%   e99ce6af254c a7dedcc55d55    0.001954 0.002108   -0.000154 1.078813
median      9e7c5b33e755 c6c97bf46846    0.001925 0.001901    0.000024 0.987532
best 25%    3f055112fae9 df1447de24d7    0.002923 0.002163    0.000760 0.739993
best 10%    da67e5b7be9e f2d371471588    0.473663 0.441357    0.032306 0.931795
best  5%    b02f6ce0d05b 9b4bd629bb44    1.664841 1.427323    0.237518 0.857333
best  1%    73ea83bf410b ed910c5221d9    3.005100 0.138998    2.866102 0.046254
best cases  80b492d79663 1456861b1ea6 1464.845562 3.942297 1460.903265 0.002691
            ace7255d9a26 682589af6612 1594.849940 2.467435 1592.382505 0.001547
            c3b14617fbd7 743a0fcaa4eb 1618.130257 2.636641 1615.493616 0.001629
            c3b14617fbd7 9ba6ab77fd29 1621.882135 2.670315 1619.211820 0.001646
            ef865da04745 421ca854cd86 1631.803875 2.646586 1629.157289 0.001622


As one can see, the average case is not really impacted. However, the worth case
we get after this changeset are much better than the one we had before it. We
have 30 pairs where improvement is > 10 minutes.


This reflect in the combined time for all pairs
before: 37700s
after:   1898s (-95%)

If we remove these last 30 case, we still see a significant improvements:

before: 2347s
after:  1898s (-20%)

diff --git a/mercurial/copies.py b/mercurial/copies.py
--- a/mercurial/copies.py
+++ b/mercurial/copies.py
@@ -160,8 +160,6 @@ def _tracefile(fctx, am, basemf, limit):
             return path
         if basemf and basemf.get(path, None) == f.filenode():
             return path
-        if not f.isintroducedafter(limit):
-            return None
 
 def _dirstatecopies(repo, match=None):
     ds = repo.dirstate