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541 /*<![CDATA[*/
542 var asciidoc = { // Namespace.
544 /////////////////////////////////////////////////////////////////////
545 // Table Of Contents generator
546 /////////////////////////////////////////////////////////////////////
548 /* Author: Mihai Bazon, September 2002
549 * http://students.infoiasi.ro/~mishoo
550 *
551 * Table Of Content generator
552 * Version: 0.4
553 *
554 * Feel free to use this script under the terms of the GNU General Public
555 * License, as long as you do not remove or alter this notice.
556 */
558 /* modified by Troy D. Hanson, September 2006. License: GPL */
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637 },
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734 </head>
739 <span id="email"><code><<a href="mailto:chriscool@tuxfamily.org">chriscool@tuxfamily.org</a>></code></span><br />
741 </div>
747 commit that introduced a regression. We show why it is important to
748 have good tools to fight regressions. We describe how "git bisect"
749 works from the outside and the algorithms it uses inside. Then we
750 explain how to take advantage of "git bisect" to improve current
751 practices. And we discuss how "git bisect" could improve in the
752 future.</p></div>
753 </div>
754 </div>
759 Torvalds and maintained by Junio Hamano.</p></div>
760 <div class="paragraph"><p>In Git like in many other Version Control Systems (VCS), the different
761 states of the data that is managed by the system are called
762 commits. And, as VCS are mostly used to manage software source code,
763 sometimes "interesting" changes of behavior in the software are
764 introduced in some commits.</p></div>
766 "bad" behavior, called a bug or a regression. They are interested in
767 these commits because a commit (hopefully) contains a very small set
768 of source code changes. And it’s much easier to understand and
769 properly fix a problem when you only need to check a very small set of
772 "git bisect" set of commands was invented. And it follows of course
775 called "good" commits. And a commit that introduce the behavior we are
776 interested in is called a "first bad commit". Note that there could be
779 be as efficient as possible, it tries to perform a binary search.</p></div>
780 </div>
781 </div>
787 <div class="paragraph"><p>Regressions are a big problem in the software industry. But it’s
788 difficult to put some real numbers behind that claim.</p></div>
794 they cost the U.S. economy an estimated $59.5 billion annually, or
795 about 0.6 percent of the gross domestic product, according to a newly
796 released study commissioned by the Department of Commerce’s National
797 Institute of Standards and Technology (NIST). At the national level,
798 over half of the costs are borne by software users and the remainder
799 by software developers/vendors. The study also found that, although
800 all errors cannot be removed, more than a third of these costs, or an
801 estimated $22.2 billion, could be eliminated by an improved testing
802 infrastructure that enables earlier and more effective identification
803 and removal of software defects. These are the savings associated with
804 finding an increased percentage (but not 100 percent) of errors closer
805 to the development stages in which they are introduced. Currently,
806 over half of all errors are not found until "downstream" in the
807 development process or during post-sale software use.</p></div>
808 </div>
810 </div></div>
815 development costs on identifying and correcting defects, and yet few
816 products of any type other than software are shipped with such high
817 levels of errors.</p></div>
818 </div>
820 </div></div>
824 <div class="paragraph"><p>The path to higher software quality is significantly improved software
825 testing.</p></div>
826 </div>
828 </div></div>
835 bugs. However, studies and surveys over the years have indicated that
836 the majority, over 80%, of the maintenance effort is used for
837 non-corrective actions (Pigosky 1997). This perception is perpetuated
838 by users submitting problem reports that in reality are functionality
839 enhancements to the system.</p></div>
840 </div>
842 </div></div>
844 because you have to watch out for regressions. At least this would
845 make the above studies consistent among themselves.</p></div>
847 time without being improved on much, and then finally thrown away. In
848 this case, of course, regressions may not be a big problem. But on the
849 other hand, there is a lot of big software that is continually
850 developed and maintained during years or even tens of years by a lot
851 of people. And as there are often many people who depend (sometimes
852 critically) on such software, regressions are a really big problem.</p></div>
854 kernel, we can see that a lot of time and effort is spent to fight
855 regressions. The release cycle start with a 2 weeks long merge
856 window. Then the first release candidate (rc) version is tagged. And
858 week between each of them, before the final release.</p></div>
860 supposed to be used to test rc versions and fight bugs and especially
861 regressions. And this time is more than 80% of the release cycle
862 time. But this is not the end of the fight yet, as of course it
863 continues after the release.</p></div>
865 developer) says about his use of git bisect:</p></div>
869 get merged upstream and when the influx of bugs is the highest) - and
870 yes, there have been cases that i used it multiple times a day. My
871 average is roughly once a day.</p></div>
872 </div>
874 </div></div>
875 <div class="paragraph"><p>So regressions are fought all the time by developers, and indeed it is
876 well known that bugs should be fixed as soon as possible, so as soon
877 as they are found. That’s why it is interesting to have good tools for
878 this purpose.</p></div>
879 </div>
883 same as those used to fight regular bugs. The only specific tools are
886 supposed to be used so that all the tests are checked after each
887 commit. This means that they are not very efficient, because many
888 tests are run for no interesting result, and they suffer from
889 combinatorial explosion.</p></div>
891 configuration options and that each test case should pass for each
892 configuration after each commit. So if you have for each release: N
893 configurations, M commits and T test cases, you should perform:</p></div>
897 </div></div>
898 <div class="paragraph"><p>where N, M and T are all growing with the size your software.</p></div>
899 <div class="paragraph"><p>So very soon it will not be possible to completely test everything.</p></div>
900 <div class="paragraph"><p>And if some bugs slip through your test suite, then you can add a test
901 to your test suite. But if you want to use your new improved test
902 suite to find where the bug slipped in, then you will either have to
903 emulate a bisection process or you will perhaps bluntly test each
904 commit backward starting from the "bad" commit you have which may be
905 very wasteful.</p></div>
906 </div>
907 </div>
908 </div>
915 start the search. Then bounds must be set to limit the commit
916 space. This is done usually by giving one "bad" and at least one
922 </div></div>
927 </div></div>
932 </div></div>
934 commit.</p></div>
936 user to test it, like this:</p></div>
941 [2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit</code></pre>
942 </div></div>
944 will be looking for the first commit that has a version like
946 in the top level Makefile. This is a toy example because there are
947 better ways to find this commit with Git than using "git bisect" (for
949 </div>
953 be driven manually by the user or it can be driven automatically by a
954 script or a command.</p></div>
958 that have been described above. For example:</p></div>
961 <pre><code>$ git bisect bad
964 </div></div>
966 find a first bad commit:</p></div>
969 <pre><code>$ git bisect bad
970 2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit
971 commit 2ddcca36c8bcfa251724fe342c8327451988be0d
975 Linux 2.6.26-rc1
978 </div></div>
979 <div class="paragraph"><p>At this point we can see what the commit does, check it out (if it’s
980 not already checked out) or tinker with it, for example:</p></div>
983 <pre><code>$ git show HEAD
984 commit 2ddcca36c8bcfa251724fe342c8327451988be0d
988 Linux 2.6.26-rc1
990 diff --git a/Makefile b/Makefile
992 --- a/Makefile
993 +++ b/Makefile
995 VERSION = 2
996 PATCHLEVEL = 6
997 -SUBLEVEL = 25
998 -EXTRAVERSION =
999 +SUBLEVEL = 26
1000 +EXTRAVERSION = -rc1
1001 NAME = Funky Weasel is Jiggy wit it
1003 # *DOCUMENTATION*</code></pre>
1004 </div></div>
1006 the branch we were in before we started bisecting:</p></div>
1009 <pre><code>$ git bisect reset
1012 Switched to branch 'master'</code></pre>
1013 </div></div>
1014 </div>
1018 to launch a script or command at each bisection step to know if the
1026 $
1027 $ git bisect run grep '^SUBLEVEL = 25' Makefile
1028 running grep ^SUBLEVEL = 25 Makefile
1031 running grep ^SUBLEVEL = 25 Makefile
1032 SUBLEVEL = 25
1035 ...
1036 ...
1037 running grep ^SUBLEVEL = 25 Makefile
1040 running grep ^SUBLEVEL = 25 Makefile
1041 2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit
1042 commit 2ddcca36c8bcfa251724fe342c8327451988be0d
1046 Linux 2.6.26-rc1
1049 bisect run success</code></pre>
1050 </div></div>
1051 <div class="paragraph"><p>In this example, we passed "grep <em>^SUBLEVEL = 25</em> Makefile" as
1052 parameter to "git bisect run". This means that at each step, the grep
1053 command we passed will be launched. And if it exits with code 0 (that
1054 means success) then git bisect will mark the current state as
1056 included, except the special code 125), then the current state will be
1059 make it stop immediately the bisection process. This is useful for
1060 example if the command passed takes too long to complete, because you
1061 can kill it with a signal and it will stop the bisection process.</p></div>
1064 </div>
1068 example if it does not compile because there was a bug preventing it
1069 at that time. This is what the special exit code 125 is for. It tells
1070 "git bisect run" that the current commit should be marked as
1071 untestable and that another one should be chosen and checked out.</p></div>
1078 better bisection point.</p></div>
1080 happen that the regression you are looking for has been introduced by
1081 one of these untestable commits. In this case it’s not possible to
1082 tell for sure which commit introduced the regression.</p></div>
1087 <pre><code>There are only 'skip'ped commits left to test.
1088 The first bad commit could be any of:
1089 15722f2fa328eaba97022898a305ffc8172db6b1
1090 78e86cf3e850bd755bb71831f42e200626fbd1e0
1091 e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace
1092 070eab2303024706f2924822bfec8b9847e4ac1b
1093 We cannot bisect more!</code></pre>
1094 </div></div>
1095 </div>
1098 <div class="paragraph"><p>If you want to show other people your bisection process, you can get a
1099 log using for example:</p></div>
1103 </div></div>
1108 </div></div>
1109 </div>
1110 </div>
1111 </div>
1118 best bisection commit to test at each step is not so simple. Anyway
1119 Linus found and implemented a "truly stupid" algorithm, later improved
1120 by Junio Hamano, that works quite well.</p></div>
1122 commit when there are no skipped commits is the following:</p></div>
1124 <div class="paragraph"><p>a) are ancestor of the "bad" commit (including the "bad" commit itself),
1126 <div class="paragraph"><p>This means that we get rid of the uninteresting commits in the DAG.</p></div>
1130 <pre><code>G-Y-G-W-W-W-X-X-X-X
1131 \ /
1132 W-W-B
1133 /
1134 Y---G-W---W
1135 \ / \
1136 Y-Y X-X-X-X
1139 </div></div>
1141 are other commits, we will get the following graph after this first
1142 step:</p></div>
1145 <pre><code>W-W-W
1146 \
1147 W-W-B
1148 /
1149 W---W</code></pre>
1150 </div></div>
1151 <div class="paragraph"><p>So only the W and B commits will be kept. Because commits X and Y will
1152 have been removed by rules a) and b) respectively, and because commits
1153 G are removed by rule b) too.</p></div>
1155 given by:</p></div>
1159 </div></div>
1160 <div class="paragraph"><p>Also note that we don’t require the commits that are kept to be
1161 descendants of a "good" commit. So in the following example, commits W
1162 and Z will be kept:</p></div>
1165 <pre><code>G-W-W-W-B
1166 /
1167 Z-Z</code></pre>
1168 </div></div>
1170 commit the number of ancestors it has plus one</p></div>
1171 <div class="paragraph"><p>For example with the following graph where H is the "bad" commit and A
1175 <pre><code>A-B-C
1176 \
1177 F-G-H
1178 /
1179 D---E</code></pre>
1180 </div></div>
1185 A-B-C
1186 \6 7 8
1187 F-G-H
1188 1 2/
1189 D---E</code></pre>
1190 </div></div>
1192 <div class="paragraph"><p>where X is the value associated to the commit in step 2) and N is the
1193 total number of commits in the graph.</p></div>
1198 A-B-C
1199 \2 1 0
1200 F-G-H
1201 1 2/
1202 D---E</code></pre>
1203 </div></div>
1204 <div class="paragraph"><p>4) the best bisection point is the commit with the highest associated
1205 number</p></div>
1206 <div class="paragraph"><p>So in the above example the best bisection point is commit C.</p></div>
1207 <div class="paragraph"><p>5) note that some shortcuts are implemented to speed up the algorithm</p></div>
1208 <div class="paragraph"><p>As we know N from the beginning, we know that min(X, N - X) can’t be
1210 to a commit, then we know this is the best bisection point. So in this
1211 case we can just stop processing any other commit and return the
1212 current commit.</p></div>
1213 </div>
1222 </div></div>
1229 a1939d9a142de972094af4dde9a544e577ddef0e (dist=2)
1231 a3864d4f32a3bf5ed177ddef598490a08760b70d (dist=1)
1232 a41baa717dd74f1180abf55e9341bc7a0bb9d556 (dist=1)
1234 </div></div>
1235 </div>
1238 <div class="paragraph"><p>First let’s define "best bisection point". We will say that a commit X
1239 is a best bisection point or a best bisection commit if knowing its
1243 following function is maximum:</p></div>
1247 </div></div>
1249 and information_if_bad(X) is the information we get if X is bad.</p></div>
1251 means that all its descendants are "bad" and all the other commits are
1252 "good". And we will suppose that all commits have an equal probability
1253 of being good or bad, or of being the first bad commit, so knowing the
1254 state of c commits gives always the same amount of information
1255 wherever these c commits are on the graph and whatever c is. (So we
1256 suppose that these commits being for example on a branch or near a
1257 good or a bad commit does not give more or less information).</p></div>
1258 <div class="paragraph"><p>Let’s also suppose that we have a cleaned up graph like one after step
1259 1) in the bisection algorithm above. This means that we can measure
1260 the information we get in terms of number of commit we can remove
1261 from the graph..</p></div>
1268 </div></div>
1269 <div class="paragraph"><p>And this is true because at step 1) b) we remove the ancestors of the
1276 </div></div>
1277 <div class="paragraph"><p>But this is wrong because at step 1) a) we keep only the ancestors of
1278 the bad commit. So we get more information when a commit is marked as
1280 commit that are not ancestors of the new "bad" commit are not the
1281 first bad commit. We don’t know if they are good or bad, but we know
1282 that they are not the first bad commit because they are not ancestor
1285 commits in the graph except those that are ancestors of the new "bad"
1286 commit. This means that:</p></div>
1290 </div></div>
1291 <div class="paragraph"><p>where N is the number of commits in the (cleaned up) graph.</p></div>
1293 should maximize the function:</p></div>
1297 </div></div>
1298 <div class="paragraph"><p>And this is nice because at step 2) we compute number_of_ancestors(X)
1303 <pre><code> G-H-I-J
1304 / \
1305 A-B-C-D-E-F O
1306 \ /
1307 K-L-M-N</code></pre>
1308 </div></div>
1313 </div></div>
1318 G-H-I-J
1320 A-B-C-D-E-F O
1321 \ /
1322 K-L-M-N
1324 </div></div>
1329 G-H-I-J
1331 A-B-C-D-E-F O
1332 \ /
1333 K-L-M-N
1335 </div></div>
1336 <div class="paragraph"><p>So we chose G, H, K or L as the best bisection point, which is better
1337 than F. Because if for example L is bad, then we will know not only
1338 that L, M and N are bad but also that G, H, I and J are not the first
1339 bad commit (since we suppose that there is only one first bad commit
1340 and it must be an ancestor of L).</p></div>
1342 initially supposed.</p></div>
1343 </div>
1348 roughly the following steps:</p></div>
1350 <div class="paragraph"><p>7) if the first commit has not been skipped, we can return it and stop
1351 here</p></div>
1352 <div class="paragraph"><p>8) otherwise filter out all the skipped commits in the sorted list</p></div>
1355 <div class="paragraph"><p>10) multiply this random number with its square root to bias it toward
1357 <div class="paragraph"><p>11) multiply the result by the number of commits in the filtered list
1358 to get an index into this list</p></div>
1360 </div>
1364 commit has been skipped and return it if it is not the case. And in
1365 fact that was the algorithm we used from when "git bisect skip" was
1369 developer) both complained that sometimes the best bisection points
1370 all happened to be in an area where all the commits are
1371 untestable. And in this case the user was asked to test many
1372 untestable commits, which could be very inefficient.</p></div>
1373 <div class="paragraph"><p>Indeed untestable commits are often untestable because a breakage was
1374 introduced at one time, and that breakage was fixed only after many
1375 other commits were introduced.</p></div>
1376 <div class="paragraph"><p>This breakage is of course most of the time unrelated to the breakage
1377 we are trying to locate in the commit graph. But it prevents us to
1380 probability of being untestable themselves. And the best bisection
1381 commits are often found together too (due to the bisection algorithm).</p></div>
1383 bisection commit when the first one has been skipped.</p></div>
1385 information when they are tested. And the commits that will not on
1386 average give a lot of information are the one near the good and bad
1387 commits.</p></div>
1389 bad commits looked like a good choice.</p></div>
1391 commit that has an associated value near the one of the best bisection
1392 commit, and that is on another branch, before using the PRNG. Because
1393 if such a commit exists, then it is not very likely to be untestable
1394 too, so it will probably give more information than a nearly randomly
1395 chosen one.</p></div>
1396 </div>
1405 because the ancestors of the good commits are supposed to be
1407 They cannot be on a branch that has no link with the branch of the
1408 "bad" commit. But it is possible for a good commit to be related to a
1409 bad commit and yet not be neither one of its ancestor nor one of its
1410 descendants.</p></div>
1411 <div class="paragraph"><p>For example, there can be a "main" branch, and a "dev" branch that was
1416 \
1418 </div></div>
1421 <div class="paragraph"><p>Now let’s suppose that commit J is bad and commit G is good and that
1422 we apply the bisection algorithm like it has been previously
1423 described.</p></div>
1425 the ancestors of the good commits because they are supposed to be good
1426 too.</p></div>
1431 </div></div>
1437 are not aware that people working on another branch fixed a bug! It
1438 could also happen that F fixed more than one bug or that it is a
1439 revert of some big development effort that was not ready to be
1440 released.</p></div>
1441 <div class="paragraph"><p>In fact development teams often maintain both a development branch and
1442 a maintenance branch, and it would be quite easy for them if "git
1443 bisect" just worked when they want to bisect a regression on the
1444 development branch that is not on the maintenance branch. They should
1445 be able to start bisecting using:</p></div>
1449 </div></div>
1451 and when some good commits are not ancestors of the bad commit, we
1452 first compute the merge bases between the bad and the good commits and
1453 we chose these merge bases as the first commits that will be checked
1454 out and tested.</p></div>
1456 is stopped with a message like:</p></div>
1459 <pre><code>The merge base BBBBBB is bad.
1460 This means the bug has been fixed between BBBBBB and [GGGGGG,...].</code></pre>
1461 </div></div>
1462 <div class="paragraph"><p>where BBBBBB is the sha1 hash of the bad merge base and [GGGGGG,…]
1463 is a comma separated list of the sha1 of the good commits.</p></div>
1465 continues, but the following message is printed for each skipped merge
1466 base:</p></div>
1469 <pre><code>Warning: the merge base between BBBBBB and [GGGGGG,...] must be skipped.
1470 So we cannot be sure the first bad commit is between MMMMMM and BBBBBB.
1471 We continue anyway.</code></pre>
1472 </div></div>
1474 hash of the merge base that is skipped and [GGGGGG,…] is a comma
1475 separated list of the sha1 of the good commits.</p></div>
1477 usual after this step.</p></div>
1478 </div>
1479 </div>
1480 </div>
1485 <h3 id="_using_test_suites_and_git_bisect_together">Using test suites and git bisect together</h3>
1486 <div class="paragraph"><p>If you both have a test suite and use git bisect, then it becomes less
1487 important to check that all tests pass after each commit. Though of
1488 course it is probably a good idea to have some checks to avoid
1489 breaking too many things because it could make bisecting other bugs
1490 more difficult.</p></div>
1492 and beta releases) that all the T test cases pass for all the N
1498 </div></div>
1499 <div class="paragraph"><p>where c is the number of rounds of test (so a small constant) and b is
1500 the ratio of bug per commit (hopefully a small constant too).</p></div>
1501 <div class="paragraph"><p>So of course it’s much better as it’s O(N * T) vs O(N * T * M) if
1502 you would test everything after each commit.</p></div>
1504 committed and they are also quite good to tell you that you have some
1505 bugs. But they are not so good to tell you where some bugs have been
1506 introduced. To tell you that efficiently, git bisect is needed.</p></div>
1507 <div class="paragraph"><p>The other nice thing with test suites, is that when you have one, you
1508 already know how to test for bad behavior. So you can use this
1509 knowledge to create a new test case for "git bisect" when it appears
1510 that there is a regression. So it will be easier to bisect the bug and
1511 fix it. And then you can add the test case you just created to your
1512 test suite.</p></div>
1513 <div class="paragraph"><p>So if you know how to create test cases and how to bisect, you will be
1514 subject to a virtuous circle:</p></div>
1515 <div class="paragraph"><p>more tests ⇒ easier to create tests ⇒ easier to bisect ⇒ more tests</p></div>
1516 <div class="paragraph"><p>So test suites and "git bisect" are complementary tools that are very
1517 powerful and efficient when used together.</p></div>
1518 </div>
1521 <div class="paragraph"><p>You can very easily automatically bisect broken builds using something
1522 like:</p></div>
1525 <pre><code>$ git bisect start BAD GOOD
1526 $ git bisect run make</code></pre>
1527 </div></div>
1528 </div>
1530 <h3 id="_passing_sh_c_some_commands_to_git_bisect_run">Passing sh -c "some commands" to "git bisect run"</h3>
1534 <pre><code>$ git bisect run sh -c "make || exit 125; ./my_app | grep 'good output'"</code></pre>
1535 </div></div>
1537 scripts to avoid too much typing.</p></div>
1538 </div>
1543 <div class="paragraph"><p>This script can be passed to "git bisect run" to find the commit that
1544 introduced a performance regression:</p></div>
1547 <pre><code>#!/bin/sh
1549 # Build errors are not what I am interested in.
1550 make my_app || exit 255
1552 # We are checking if it stops in a reasonable amount of time, so
1553 # let it run in the background...
1557 # ... and grab its process ID.
1558 pid=$!
1560 # ... and then wait for sufficiently long.
1561 sleep $NORMAL_TIME
1563 # ... and then see if the process is still there.
1564 if kill -0 $pid
1565 then
1566 # It is still running -- that is bad.
1567 kill $pid; sleep 1; kill $pid;
1568 exit 1
1569 else
1570 # It has already finished (the $pid process was no more),
1571 # and we are happy.
1572 exit 0
1573 fi</code></pre>
1574 </div></div>
1575 </div>
1579 knowingly break things, even if some other commits later fix the
1580 breakage.</p></div>
1582 logical change in each commit.</p></div>
1585 first place, as small changes are easier to review even if they are
1586 only reviewed by the committer.</p></div>
1588 helpful to understand why some changes were made.</p></div>
1589 <div class="paragraph"><p>These general best practices are very helpful if you bisect often.</p></div>
1590 </div>
1594 the merge needs no source code conflict resolution. This is because a
1595 semantic change can happen in one branch while the other branch is not
1596 aware of it.</p></div>
1597 <div class="paragraph"><p>For example one branch can change the semantic of a function while the
1598 other branch add more calls to the same function.</p></div>
1601 make regressions very difficult to track down. It can even be
1602 misleading to know the first bad commit if it happens to be such a
1603 merge, because people might think that the bug comes from bad conflict
1604 resolution when it comes from a semantic change in one branch.</p></div>
1605 <div class="paragraph"><p>Anyway "git rebase" can be used to linearize history. This can be used
1606 either to avoid merging in the first place. Or it can be used to
1607 bisect on a linear history instead of the non linear one, as this
1608 should give more information in case of a semantic change in one
1609 branch.</p></div>
1610 <div class="paragraph"><p>Merges can be also made simpler by using smaller branches or by using
1611 many topic branches instead of only long version related branches.</p></div>
1613 like linux-next for the linux kernel.</p></div>
1614 </div>
1617 <div class="paragraph"><p>A special work-flow to process regressions can give great results.</p></div>
1618 <div class="paragraph"><p>Here is an example of a work-flow used by Andreas Ericsson:</p></div>
1620 <li>
1621 <p>
1622 write, in the test suite, a test script that exposes the regression
1623 </p>
1624 </li>
1625 <li>
1626 <p>
1627 use "git bisect run" to find the commit that introduced it
1628 </p>
1629 </li>
1630 <li>
1631 <p>
1632 fix the bug that is often made obvious by the previous step
1633 </p>
1634 </li>
1635 <li>
1636 <p>
1637 commit both the fix and the test script (and if needed more tests)
1638 </p>
1639 </li>
1640 </ul></div>
1641 <div class="paragraph"><p>And here is what Andreas said about this work-flow <a href="#5">[5]</a>:</p></div>
1645 cycle of 142.6 hours (according to our somewhat weird bug-tracker
1646 which just measures wall-clock time). Since we moved to Git, we’ve
1647 lowered that to 16.2 hours. Primarily because we can stay on top of
1648 the bug fixing now, and because everyone’s jockeying to get to fix
1649 bugs (we’re quite proud of how lazy we are to let Git find the bugs
1650 for us). Each new release results in ~40% fewer bugs (almost certainly
1651 due to how we now feel about writing tests).</p></div>
1652 </div>
1654 </div></div>
1656 and "git bisect". In fact it makes it the standard procedure to deal
1657 with regression.</p></div>
1658 <div class="paragraph"><p>In other messages Andreas says that they also use the "best practices"
1659 described above: small logical commits, topic branches, no evil
1660 merge,… These practices all improve the bisectability of the commit
1661 graph, by making it easier and more useful to bisect.</p></div>
1663 is making bisecting easier, more useful and standard.</p></div>
1664 </div>
1666 <h3 id="_involving_qa_people_and_if_possible_end_users">Involving QA people and if possible end users</h3>
1668 tool. It can effectively be used by QA people or even end users (if
1669 they have access to the source code or if they can get access to all
1670 the builds).</p></div>
1672 of whether it was ok to always ask end user to bisect, and very good
1673 points were made to support the point of view that it is ok.</p></div>
1677 <div class="paragraph"><p>What people don’t get is that this is a situation where the "end node
1678 principle" applies. When you have limited resources (here: developers)
1679 you don’t push the bulk of the burden upon them. Instead you push
1680 things out to the resource you have a lot of, the end nodes (here:
1681 users), so that the situation actually scales.</p></div>
1682 </div>
1684 </div></div>
1685 <div class="paragraph"><p>This means that it is often "cheaper" if QA people or end users can do
1686 it.</p></div>
1687 <div class="paragraph"><p>What is interesting too is that end users that are reporting bugs (or
1688 QA people that reproduced a bug) have access to the environment where
1689 the bug happens. So they can often more easily reproduce a
1690 regression. And if they can bisect, then more information will be
1691 extracted from the environment where the bug happens, which means that
1692 it will be easier to understand and then fix the bug.</p></div>
1694 contributions from end users, and to introduce them to QA and
1695 development activities.</p></div>
1696 </div>
1700 complex scripts to be able to fully automate bisecting.</p></div>
1701 <div class="paragraph"><p>Here is what Ingo Molnar says about that <a href="#7">[7]</a>:</p></div>
1704 <div class="paragraph"><p>i have a fully automated bootup-hang bisection script. It is based on
1705 "git-bisect run". I run the script, it builds and boots kernels fully
1706 automatically, and when the bootup fails (the script notices that via
1707 the serial log, which it continuously watches - or via a timeout, if
1709 the script raises my attention via a beep and i power cycle the test
1710 box. (yeah, i should make use of a managed power outlet to 100%
1711 automate it)</p></div>
1712 </div>
1714 </div></div>
1715 </div>
1717 <h3 id="_combining_test_suites_git_bisect_and_other_systems_together">Combining test suites, git bisect and other systems together</h3>
1719 used together. It can be even more powerful if you can combine them
1720 with other systems.</p></div>
1721 <div class="paragraph"><p>For example some test suites could be run automatically at night with
1722 some unusual (or even random) configurations. And if a regression is
1723 found by a test suite, then "git bisect" can be automatically
1724 launched, and its result can be emailed to the author of the first bad
1725 commit found by "git bisect", and perhaps other people too. And a new
1726 entry in the bug tracking system could be automatically created too.</p></div>
1727 </div>
1728 </div>
1729 </div>
1736 avoid areas in the commit graph where commits are untestable. The
1737 problem is that sometimes the first bad commit will be in an
1738 untestable area.</p></div>
1739 <div class="paragraph"><p>To simplify the discussion we will suppose that the untestable area is
1740 a simple string of commits and that it was created by a breakage
1741 introduced by one commit (let’s call it BBC for bisect breaking
1742 commit) and later fixed by another one (let’s call it BFC for bisect
1743 fixing commit).</p></div>
1748 </div></div>
1750 X6 are untestable.</p></div>
1751 <div class="paragraph"><p>In this case if you are bisecting manually, what you can do is create
1752 a special branch that starts just before the BBC. The first commit in
1753 this branch should be the BBC with the BFC squashed into it. And the
1754 other commits in the branch should be the commits between BBC and BFC
1755 rebased on the first commit of the branch and then the commit after
1756 BFC also rebased on.</p></div>
1760 <pre><code> (BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z'
1761 /
1762 ...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z-...</code></pre>
1763 </div></div>
1765 <div class="paragraph"><p>You can easily create such a branch with Git using interactive rebase.</p></div>
1770 </div></div>
1772 <div class="paragraph"><p>After that you can start bisecting as usual in the new branch and you
1773 should eventually find the first bad commit.</p></div>
1778 </div></div>
1779 <div class="paragraph"><p>If you are using "git bisect run", you can use the same manual fix up
1780 as above, and then start another "git bisect run" in the special
1781 branch. Or as the "git bisect" man page says, the script passed to
1782 "git bisect run" can apply a patch before it compiles and test the
1785 "git bisect" will be able to find the first bad commit. And the script
1786 should not forget to remove the patch once the testing is done before
1787 exiting from the script.</p></div>
1789 apply the fix, and in this case you should use "git reset --hard
1790 HEAD^" to revert the cherry-pick after testing and before returning
1791 from the script.)</p></div>
1793 clunky. Using special branches is nice because these branches can be
1794 shared by developers like usual branches, but the risk is that people
1795 will get many such branches. And it disrupts the normal "git bisect"
1796 work-flow. So, if you want to use "git bisect run" completely
1797 automatically, you have to add special code in your script to restart
1798 bisection in the special branches.</p></div>
1799 <div class="paragraph"><p>Anyway one can notice in the above special branch example that the Z'
1800 and Z commits should point to the same source code state (the same
1802 same changes as Z just in a slightly different order.</p></div>
1804 not need to add anything to a script. It would just work for anyone in
1805 the project sharing the special branches and the replacements.</p></div>
1809 <pre><code> (BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z'-...
1810 /
1811 ...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z</code></pre>
1812 </div></div>
1813 <div class="paragraph"><p>That’s why the "git replace" command was created. Technically it
1816 (that are stored in "refs/tags"), and that means that they can
1817 automatically be shared like branches or tags among developers.</p></div>
1819 commits in already released history, for example to change the commit
1820 message or the author. And it can also be used instead of git "grafts"
1821 to link a repository with another old repository.</p></div>
1822 <div class="paragraph"><p>In fact it’s this last feature that "sold" it to the Git community, so
1826 replacements refs in "refs/replace/", but it would be perhaps better
1827 if the replacement refs that are useful only for bisecting would be in
1828 "refs/replace/bisect/". This way the replacement refs could be used
1829 only for bisecting, while other refs directly in "refs/replace/" would
1830 be used nearly all the time.</p></div>
1831 </div>
1835 add some redundancy to the tests performed so that it would be more
1836 reliable when tracking sporadic bugs.</p></div>
1838 called sporadic bugs do not appear in all the kernel builds because
1839 they are very dependent on the compiler output.</p></div>
1840 <div class="paragraph"><p>The idea is that every 3 test for example, "git bisect" could ask the
1841 user to test a commit that has already been found to be "good" or
1842 "bad" (because one of its descendants or one of its ancestors has been
1844 has been previously incorrectly classified then the bisection can be
1845 aborted early, hopefully before too many mistakes have been made. Then
1846 the user will have to look at what happened and then restart the
1847 bisection using a fixed bisect log.</p></div>
1848 <div class="paragraph"><p>There is already a project called BBChop created by Ealdwulf Wuffinga
1849 on Github that does something like that using Bayesian Search Theory
1853 <div class="paragraph"><p>BBChop is like <em>git bisect</em> (or equivalent), but works when your bug
1854 is intermittent. That is, it works in the presence of false negatives
1855 (when a version happens to work this time even though it contains the
1856 bug). It assumes that there are no false positives (in principle, the
1857 same approach would work, but adding it may be non-trivial).</p></div>
1858 </div>
1860 </div></div>
1862 users to have something integrated in Git.</p></div>
1863 </div>
1864 </div>
1865 </div>
1869 <div class="paragraph"><p>We have seen that regressions are an important problem, and that "git
1870 bisect" has nice features that complement very well practices and
1871 other tools, especially test suites, that are generally used to fight
1872 regressions. But it might be needed to change some work-flows and
1873 (bad) habits to get the most out of it.</p></div>
1875 and some new features could help in some cases, but overall "git
1876 bisect" works already very well, is used a lot, and is already very
1877 useful. To back up that last claim, let’s give the final word to Ingo
1878 Molnar when he was asked by the author how much time does he think
1883 <div class="paragraph"><p>About ten years ago did i do my first <em>bisection</em> of a Linux patch
1884 queue. That was prior the Git (and even prior the BitKeeper) days. I
1885 literally days spent sorting out patches, creating what in essence
1886 were standalone commits that i guessed to be related to that bug.</p></div>
1887 <div class="paragraph"><p>It was a tool of absolute last resort. I’d rather spend days looking
1889 <div class="paragraph"><p>With Git bisect it’s a breeze: in the best case i can get a ~15 step
1891 manual help or when bisecting multiple, overlapping bugs, it’s rarely
1892 more than an hour.</p></div>
1893 <div class="paragraph"><p>In fact it’s invaluable because there are bugs i would never even
1895 patterns that were immediately hopeless for me to debug - at best i
1896 could send the crash/bug signature to lkml and hope that someone else
1897 can think of something.</p></div>
1899 about the bug: that it’s non-deterministic - timing or kernel image
1900 layout dependent.</p></div>
1901 <div class="paragraph"><p>So git bisect is unconditional goodness - and feel free to quote that
1902 ;-)</p></div>
1903 </div>
1905 </div></div>
1906 </div>
1907 </div>
1911 <div class="paragraph"><p>Many thanks to Junio Hamano for his help in reviewing this paper, for
1912 reviewing the patches I sent to the Git mailing list, for discussing
1913 some ideas and helping me improve them, for improving "git bisect" a
1914 lot and for his awesome work in maintaining and developing Git.</p></div>
1915 <div class="paragraph"><p>Many thanks to Ingo Molnar for giving me very useful information that
1916 appears in this paper, for commenting on this paper, for his
1918 on the linux kernel mailing lists.</p></div>
1922 another when I worked on Git, especially to Andreas Ericsson, Johannes
1923 Schindelin, H. Peter Anvin, Daniel Barkalow, Bill Lear, John Hawley,
1924 Shawn O. Pierce, Jeff King, Sam Vilain, Jon Seymour.</p></div>
1926 author to given a talk and for publishing this paper.</p></div>
1927 </div>
1928 </div>
1933 <li>
1934 <p>
1935 <a id="1"></a>[1] <a href="https://web.archive.org/web/20091206032101/http://www.nist.gov/public_affairs/releases/n02-10.htm"><em>Software Errors Cost U.S. Economy $59.5 Billion Annually</em>. Nist News Release.</a> See also <a href="https://www.nist.gov/system/files/documents/director/planning/report02-3.pdf"><em>The Economic Impacts of Inadequate Infratructure for Software Testing</em>. Nist Planning Report 02-3</a>, Executive Summary and Chapter 8.
1936 </p>
1937 </li>
1938 <li>
1939 <p>
1940 <a id="2"></a>[2] <a href="https://www.oracle.com/java/technologies/javase/codeconventions-introduction.html"><em>Code Conventions for the Java Programming Language: 1. Introduction</em>. Sun Microsystems.</a>
1941 </p>
1942 </li>
1943 <li>
1944 <p>
1945 <a id="3"></a>[3] <a href="https://en.wikipedia.org/wiki/Software_maintenance"><em>Software maintenance</em>. Wikipedia.</a>
1946 </p>
1947 </li>
1948 <li>
1949 <p>
1950 <a id="4"></a>[4] <a href="https://lore.kernel.org/git/7vps5xsbwp.fsf_-_@assigned-by-dhcp.cox.net/">Junio C Hamano. <em>Automated bisect success story</em>.</a>
1951 </p>
1952 </li>
1953 <li>
1954 <p>
1955 <a id="5"></a>[5] <a href="https://lwn.net/Articles/317154/">Christian Couder. <em>Fully automated bisecting with "git bisect run"</em>. LWN.net.</a>
1956 </p>
1957 </li>
1958 <li>
1959 <p>
1960 <a id="6"></a>[6] <a href="https://lwn.net/Articles/277872/">Jonathan Corbet. <em>Bisection divides users and developers</em>. LWN.net.</a>
1961 </p>
1962 </li>
1963 <li>
1964 <p>
1965 <a id="7"></a>[7] <a href="https://lore.kernel.org/lkml/20071207113734.GA14598@elte.hu/">Ingo Molnar. <em>Re: BUG 2.6.23-rc3 can’t see sd partitions on Alpha</em>. Linux-kernel mailing list.</a>
1966 </p>
1967 </li>
1968 <li>
1969 <p>
1970 <a id="8"></a>[8] <a href="https://www.kernel.org/pub/software/scm/git/docs/git-bisect.html">Junio C Hamano and the git-list. <em>git-bisect(1) Manual Page</em>. Linux Kernel Archives.</a>
1971 </p>
1972 </li>
1973 <li>
1974 <p>
1975 <a id="9"></a>[9] <a href="https://github.com/Ealdwulf/bbchop">Ealdwulf. <em>bbchop</em>. GitHub.</a>
1976 </p>
1977 </li>
1978 </ul></div>
1979 </div>
1980 </div>
1981 </div>
1985 Last updated
1987 </div>
1988 </div>
1989 </body>
1990 </html>