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diff --git a/Documentation/git-bisect-lk2009.txt b/Documentation/git-bisect-lk2009.txt new file mode 100644 index 0000000..e015f5b --- /dev/null +++ b/Documentation/git-bisect-lk2009.txt @@ -0,0 +1,1358 @@ +Fighting regressions with git bisect +==================================== +:Author: Christian Couder +:Email: chriscool@tuxfamily.org +:Date: 2009/11/08 + +Abstract +-------- + +"git bisect" enables software users and developers to easily find the +commit that introduced a regression. We show why it is important to +have good tools to fight regressions. We describe how "git bisect" +works from the outside and the algorithms it uses inside. Then we +explain how to take advantage of "git bisect" to improve current +practices. And we discuss how "git bisect" could improve in the +future. + + +Introduction to "git bisect" +---------------------------- + +Git is a Distributed Version Control system (DVCS) created by Linus +Torvalds and maintained by Junio Hamano. + +In Git like in many other Version Control Systems (VCS), the different +states of the data that is managed by the system are called +commits. And, as VCS are mostly used to manage software source code, +sometimes "interesting" changes of behavior in the software are +introduced in some commits. + +In fact people are specially interested in commits that introduce a +"bad" behavior, called a bug or a regression. They are interested in +these commits because a commit (hopefully) contains a very small set +of source code changes. And it's much easier to understand and +properly fix a problem when you only need to check a very small set of +changes, than when you don't know where look in the first place. + +So to help people find commits that introduce a "bad" behavior, the +"git bisect" set of commands was invented. And it follows of course +that in "git bisect" parlance, commits where the "interesting +behavior" is present are called "bad" commits, while other commits are +called "good" commits. And a commit that introduce the behavior we are +interested in is called a "first bad commit". Note that there could be +more than one "first bad commit" in the commit space we are searching. + +So "git bisect" is designed to help find a "first bad commit". And to +be as efficient as possible, it tries to perform a binary search. + + +Fighting regressions overview +----------------------------- + +Regressions: a big problem +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Regressions are a big problem in the software industry. But it's +difficult to put some real numbers behind that claim. + +There are some numbers about bugs in general, like a NIST study in +2002 <<1>> that said: + +_____________ +Software bugs, or errors, are so prevalent and so detrimental that +they cost the U.S. economy an estimated $59.5 billion annually, or +about 0.6 percent of the gross domestic product, according to a newly +released study commissioned by the Department of Commerce's National +Institute of Standards and Technology (NIST). At the national level, +over half of the costs are borne by software users and the remainder +by software developers/vendors. The study also found that, although +all errors cannot be removed, more than a third of these costs, or an +estimated $22.2 billion, could be eliminated by an improved testing +infrastructure that enables earlier and more effective identification +and removal of software defects. These are the savings associated with +finding an increased percentage (but not 100 percent) of errors closer +to the development stages in which they are introduced. Currently, +over half of all errors are not found until "downstream" in the +development process or during post-sale software use. +_____________ + +And then: + +_____________ +Software developers already spend approximately 80 percent of +development costs on identifying and correcting defects, and yet few +products of any type other than software are shipped with such high +levels of errors. +_____________ + +Eventually the conclusion started with: + +_____________ +The path to higher software quality is significantly improved software +testing. +_____________ + +There are other estimates saying that 80% of the cost related to +software is about maintenance <<2>>. + +Though, according to Wikipedia <<3>>: + +_____________ +A common perception of maintenance is that it is merely fixing +bugs. However, studies and surveys over the years have indicated that +the majority, over 80%, of the maintenance effort is used for +non-corrective actions (Pigosky 1997). This perception is perpetuated +by users submitting problem reports that in reality are functionality +enhancements to the system. +_____________ + +But we can guess that improving on existing software is very costly +because you have to watch out for regressions. At least this would +make the above studies consistent among themselves. + +Of course some kind of software is developed, then used during some +time without being improved on much, and then finally thrown away. In +this case, of course, regressions may not be a big problem. But on the +other hand, there is a lot of big software that is continually +developed and maintained during years or even tens of years by a lot +of people. And as there are often many people who depend (sometimes +critically) on such software, regressions are a really big problem. + +One such software is the Linux kernel. And if we look at the Linux +kernel, we can see that a lot of time and effort is spent to fight +regressions. The release cycle start with a 2 weeks long merge +window. Then the first release candidate (rc) version is tagged. And +after that about 7 or 8 more rc versions will appear with around one +week between each of them, before the final release. + +The time between the first rc release and the final release is +supposed to be used to test rc versions and fight bugs and especially +regressions. And this time is more than 80% of the release cycle +time. But this is not the end of the fight yet, as of course it +continues after the release. + +And then this is what Ingo Molnar (a well known Linux kernel +developer) says about his use of git bisect: + +_____________ +I most actively use it during the merge window (when a lot of trees +get merged upstream and when the influx of bugs is the highest) - and +yes, there have been cases that i used it multiple times a day. My +average is roughly once a day. +_____________ + +So regressions are fought all the time by developers, and indeed it is +well known that bugs should be fixed as soon as possible, so as soon +as they are found. That's why it is interesting to have good tools for +this purpose. + +Other tools to fight regressions +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +So what are the tools used to fight regressions? They are nearly the +same as those used to fight regular bugs. The only specific tools are +test suites and tools similar as "git bisect". + +Test suites are very nice. But when they are used alone, they are +supposed to be used so that all the tests are checked after each +commit. This means that they are not very efficient, because many +tests are run for no interesting result, and they suffer from +combinational explosion. + +In fact the problem is that big software often has many different +configuration options and that each test case should pass for each +configuration after each commit. So if you have for each release: N +configurations, M commits and T test cases, you should perform: + +------------- +N * M * T tests +------------- + +where N, M and T are all growing with the size your software. + +So very soon it will not be possible to completely test everything. + +And if some bugs slip through your test suite, then you can add a test +to your test suite. But if you want to use your new improved test +suite to find where the bug slipped in, then you will either have to +emulate a bisection process or you will perhaps bluntly test each +commit backward starting from the "bad" commit you have which may be +very wasteful. + +"git bisect" overview +--------------------- + +Starting a bisection +~~~~~~~~~~~~~~~~~~~~ + +The first "git bisect" subcommand to use is "git bisect start" to +start the search. Then bounds must be set to limit the commit +space. This is done usually by giving one "bad" and at least one +"good" commit. They can be passed in the initial call to "git bisect +start" like this: + +------------- +$ git bisect start [BAD [GOOD...]] +------------- + +or they can be set using: + +------------- +$ git bisect bad [COMMIT] +------------- + +and: + +------------- +$ git bisect good [COMMIT...] +------------- + +where BAD, GOOD and COMMIT are all names that can be resolved to a +commit. + +Then "git bisect" will checkout a commit of its choosing and ask the +user to test it, like this: + +------------- +$ git bisect start v2.6.27 v2.6.25 +Bisecting: 10928 revisions left to test after this (roughly 14 steps) +[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit +------------- + +Note that the example that we will use is really a toy example, we +will be looking for the first commit that has a version like +"2.6.26-something", that is the commit that has a "SUBLEVEL = 26" line +in the top level Makefile. This is a toy example because there are +better ways to find this commit with Git than using "git bisect" (for +example "git blame" or "git log -S<string>"). + +Driving a bisection manually +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +At this point there are basically 2 ways to drive the search. It can +be driven manually by the user or it can be driven automatically by a +script or a command. + +If the user is driving it, then at each step of the search, the user +will have to test the current commit and say if it is "good" or "bad" +using the "git bisect good" or "git bisect bad" commands respectively +that have been described above. For example: + +------------- +$ git bisect bad +Bisecting: 5480 revisions left to test after this (roughly 13 steps) +[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm +------------- + +And after a few more steps like that, "git bisect" will eventually +find a first bad commit: + +------------- +$ git bisect bad +2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit +commit 2ddcca36c8bcfa251724fe342c8327451988be0d +Author: Linus Torvalds <torvalds@linux-foundation.org> +Date: Sat May 3 11:59:44 2008 -0700 + + Linux 2.6.26-rc1 + +:100644 100644 5cf82581... 4492984e... M Makefile +------------- + +At this point we can see what the commit does, check it out (if it's +not already checked out) or tinker with it, for example: + +------------- +$ git show HEAD +commit 2ddcca36c8bcfa251724fe342c8327451988be0d +Author: Linus Torvalds <torvalds@linux-foundation.org> +Date: Sat May 3 11:59:44 2008 -0700 + + Linux 2.6.26-rc1 + +diff --git a/Makefile b/Makefile +index 5cf8258..4492984 100644 +--- a/Makefile ++++ b/Makefile +@@ -1,7 +1,7 @@ + VERSION = 2 + PATCHLEVEL = 6 +-SUBLEVEL = 25 +-EXTRAVERSION = ++SUBLEVEL = 26 ++EXTRAVERSION = -rc1 + NAME = Funky Weasel is Jiggy wit it + + # *DOCUMENTATION* +------------- + +And when we are finished we can use "git bisect reset" to go back to +the branch we were in before we started bisecting: + +------------- +$ git bisect reset +Checking out files: 100% (21549/21549), done. +Previous HEAD position was 2ddcca3... Linux 2.6.26-rc1 +Switched to branch 'master' +------------- + +Driving a bisection automatically +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The other way to drive the bisection process is to tell "git bisect" +to launch a script or command at each bisection step to know if the +current commit is "good" or "bad". To do that, we use the "git bisect +run" command. For example: + +------------- +$ git bisect start v2.6.27 v2.6.25 +Bisecting: 10928 revisions left to test after this (roughly 14 steps) +[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit +$ +$ git bisect run grep '^SUBLEVEL = 25' Makefile +running grep ^SUBLEVEL = 25 Makefile +Bisecting: 5480 revisions left to test after this (roughly 13 steps) +[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm +running grep ^SUBLEVEL = 25 Makefile +SUBLEVEL = 25 +Bisecting: 2740 revisions left to test after this (roughly 12 steps) +[671294719628f1671faefd4882764886f8ad08cb] V4L/DVB(7879): Adding cx18 Support for mxl5005s +... +... +running grep ^SUBLEVEL = 25 Makefile +Bisecting: 0 revisions left to test after this (roughly 0 steps) +[2ddcca36c8bcfa251724fe342c8327451988be0d] Linux 2.6.26-rc1 +running grep ^SUBLEVEL = 25 Makefile +2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit +commit 2ddcca36c8bcfa251724fe342c8327451988be0d +Author: Linus Torvalds <torvalds@linux-foundation.org> +Date: Sat May 3 11:59:44 2008 -0700 + + Linux 2.6.26-rc1 + +:100644 100644 5cf82581... 4492984e... M Makefile +bisect run success +------------- + +In this example, we passed "grep '^SUBLEVEL = 25' Makefile" as +parameter to "git bisect run". This means that at each step, the grep +command we passed will be launched. And if it exits with code 0 (that +means success) then git bisect will mark the current state as +"good". If it exits with code 1 (or any code between 1 and 127 +included, except the special code 125), then the current state will be +marked as "bad". + +Exit code between 128 and 255 are special to "git bisect run". They +make it stop immediately the bisection process. This is useful for +example if the command passed takes too long to complete, because you +can kill it with a signal and it will stop the bisection process. + +It can also be useful in scripts passed to "git bisect run" to "exit +255" if some very abnormal situation is detected. + +Avoiding untestable commits +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Sometimes it happens that the current state cannot be tested, for +example if it does not compile because there was a bug preventing it +at that time. This is what the special exit code 125 is for. It tells +"git bisect run" that the current commit should be marked as +untestable and that another one should be chosen and checked out. + +If the bisection process is driven manually, you can use "git bisect +skip" to do the same thing. (In fact the special exit code 125 makes +"git bisect run" use "git bisect skip" in the background.) + +Or if you want more control, you can inspect the current state using +for example "git bisect visualize". It will launch gitk (or "git log" +if the `DISPLAY` environment variable is not set) to help you find a +better bisection point. + +Either way, if you have a string of untestable commits, it might +happen that the regression you are looking for has been introduced by +one of these untestable commits. In this case it's not possible to +tell for sure which commit introduced the regression. + +So if you used "git bisect skip" (or the run script exited with +special code 125) you could get a result like this: + +------------- +There are only 'skip'ped commits left to test. +The first bad commit could be any of: +15722f2fa328eaba97022898a305ffc8172db6b1 +78e86cf3e850bd755bb71831f42e200626fbd1e0 +e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace +070eab2303024706f2924822bfec8b9847e4ac1b +We cannot bisect more! +------------- + +Saving a log and replaying it +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you want to show other people your bisection process, you can get a +log using for example: + +------------- +$ git bisect log > bisect_log.txt +------------- + +And it is possible to replay it using: + +------------- +$ git bisect replay bisect_log.txt +------------- + + +"git bisect" details +-------------------- + +Bisection algorithm +~~~~~~~~~~~~~~~~~~~ + +As the Git commits form a directed acyclic graph (DAG), finding the +best bisection commit to test at each step is not so simple. Anyway +Linus found and implemented a "truly stupid" algorithm, later improved +by Junio Hamano, that works quite well. + +So the algorithm used by "git bisect" to find the best bisection +commit when there are no skipped commits is the following: + +1) keep only the commits that: + +a) are ancestor of the "bad" commit (including the "bad" commit itself), +b) are not ancestor of a "good" commit (excluding the "good" commits). + +This means that we get rid of the uninteresting commits in the DAG. + +For example if we start with a graph like this: + +------------- +G-Y-G-W-W-W-X-X-X-X + \ / + W-W-B + / +Y---G-W---W + \ / \ +Y-Y X-X-X-X + +-> time goes this way -> +------------- + +where B is the "bad" commit, "G" are "good" commits and W, X, and Y +are other commits, we will get the following graph after this first +step: + +------------- +W-W-W + \ + W-W-B + / +W---W +------------- + +So only the W and B commits will be kept. Because commits X and Y will +have been removed by rules a) and b) respectively, and because commits +G are removed by rule b) too. + +Note for Git users, that it is equivalent as keeping only the commit +given by: + +------------- +git rev-list BAD --not GOOD1 GOOD2... +------------- + +Also note that we don't require the commits that are kept to be +descendants of a "good" commit. So in the following example, commits W +and Z will be kept: + +------------- +G-W-W-W-B + / +Z-Z +------------- + +2) starting from the "good" ends of the graph, associate to each +commit the number of ancestors it has plus one + +For example with the following graph where H is the "bad" commit and A +and D are some parents of some "good" commits: + +------------- +A-B-C + \ + F-G-H + / +D---E +------------- + +this will give: + +------------- +1 2 3 +A-B-C + \6 7 8 + F-G-H +1 2/ +D---E +------------- + +3) associate to each commit: min(X, N - X) + +where X is the value associated to the commit in step 2) and N is the +total number of commits in the graph. + +In the above example we have N = 8, so this will give: + +------------- +1 2 3 +A-B-C + \2 1 0 + F-G-H +1 2/ +D---E +------------- + +4) the best bisection point is the commit with the highest associated +number + +So in the above example the best bisection point is commit C. + +5) note that some shortcuts are implemented to speed up the algorithm + +As we know N from the beginning, we know that min(X, N - X) can't be +greater than N/2. So during steps 2) and 3), if we would associate N/2 +to a commit, then we know this is the best bisection point. So in this +case we can just stop processing any other commit and return the +current commit. + +Bisection algorithm debugging +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For any commit graph, you can see the number associated with each +commit using "git rev-list --bisect-all". + +For example, for the above graph, a command like: + +------------- +$ git rev-list --bisect-all BAD --not GOOD1 GOOD2 +------------- + +would output something like: + +------------- +e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace (dist=3) +15722f2fa328eaba97022898a305ffc8172db6b1 (dist=2) +78e86cf3e850bd755bb71831f42e200626fbd1e0 (dist=2) +a1939d9a142de972094af4dde9a544e577ddef0e (dist=2) +070eab2303024706f2924822bfec8b9847e4ac1b (dist=1) +a3864d4f32a3bf5ed177ddef598490a08760b70d (dist=1) +a41baa717dd74f1180abf55e9341bc7a0bb9d556 (dist=1) +9e622a6dad403b71c40979743bb9d5be17b16bd6 (dist=0) +------------- + +Bisection algorithm discussed +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +First let's define "best bisection point". We will say that a commit X +is a best bisection point or a best bisection commit if knowing its +state ("good" or "bad") gives as much information as possible whether +the state of the commit happens to be "good" or "bad". + +This means that the best bisection commits are the commits where the +following function is maximum: + +------------- +f(X) = min(information_if_good(X), information_if_bad(X)) +------------- + +where information_if_good(X) is the information we get if X is good +and information_if_bad(X) is the information we get if X is bad. + +Now we will suppose that there is only one "first bad commit". This +means that all its descendants are "bad" and all the other commits are +"good". And we will suppose that all commits have an equal probability +of being good or bad, or of being the first bad commit, so knowing the +state of c commits gives always the same amount of information +wherever these c commits are on the graph and whatever c is. (So we +suppose that these commits being for example on a branch or near a +good or a bad commit does not give more or less information). + +Let's also suppose that we have a cleaned up graph like one after step +1) in the bisection algorithm above. This means that we can measure +the information we get in terms of number of commit we can remove from +the graph.. + +And let's take a commit X in the graph. + +If X is found to be "good", then we know that its ancestors are all +"good", so we want to say that: + +------------- +information_if_good(X) = number_of_ancestors(X) (TRUE) +------------- + +And this is true because at step 1) b) we remove the ancestors of the +"good" commits. + +If X is found to be "bad", then we know that its descendants are all +"bad", so we want to say that: + +------------- +information_if_bad(X) = number_of_descendants(X) (WRONG) +------------- + +But this is wrong because at step 1) a) we keep only the ancestors of +the bad commit. So we get more information when a commit is marked as +"bad", because we also know that the ancestors of the previous "bad" +commit that are not ancestors of the new "bad" commit are not the +first bad commit. We don't know if they are good or bad, but we know +that they are not the first bad commit because they are not ancestor +of the new "bad" commit. + +So when a commit is marked as "bad" we know we can remove all the +commits in the graph except those that are ancestors of the new "bad" +commit. This means that: + +------------- +information_if_bad(X) = N - number_of_ancestors(X) (TRUE) +------------- + +where N is the number of commits in the (cleaned up) graph. + +So in the end this means that to find the best bisection commits we +should maximize the function: + +------------- +f(X) = min(number_of_ancestors(X), N - number_of_ancestors(X)) +------------- + +And this is nice because at step 2) we compute number_of_ancestors(X) +and so at step 3) we compute f(X). + +Let's take the following graph as an example: + +------------- + G-H-I-J + / \ +A-B-C-D-E-F O + \ / + K-L-M-N +------------- + +If we compute the following non optimal function on it: + +------------- +g(X) = min(number_of_ancestors(X), number_of_descendants(X)) +------------- + +we get: + +------------- + 4 3 2 1 + G-H-I-J +1 2 3 4 5 6/ \0 +A-B-C-D-E-F O + \ / + K-L-M-N + 4 3 2 1 +------------- + +but with the algorithm used by git bisect we get: + +------------- + 7 7 6 5 + G-H-I-J +1 2 3 4 5 6/ \0 +A-B-C-D-E-F O + \ / + K-L-M-N + 7 7 6 5 +------------- + +So we chose G, H, K or L as the best bisection point, which is better +than F. Because if for example L is bad, then we will know not only +that L, M and N are bad but also that G, H, I and J are not the first +bad commit (since we suppose that there is only one first bad commit +and it must be an ancestor of L). + +So the current algorithm seems to be the best possible given what we +initially supposed. + +Skip algorithm +~~~~~~~~~~~~~~ + +When some commits have been skipped (using "git bisect skip"), then +the bisection algorithm is the same for step 1) to 3). But then we use +roughly the following steps: + +6) sort the commit by decreasing associated value + +7) if the first commit has not been skipped, we can return it and stop +here + +8) otherwise filter out all the skipped commits in the sorted list + +9) use a pseudo random number generator (PRNG) to generate a random +number between 0 and 1 + +10) multiply this random number with its square root to bias it toward +0 + +11) multiply the result by the number of commits in the filtered list +to get an index into this list + +12) return the commit at the computed index + +Skip algorithm discussed +~~~~~~~~~~~~~~~~~~~~~~~~ + +After step 7) (in the skip algorithm), we could check if the second +commit has been skipped and return it if it is not the case. And in +fact that was the algorithm we used from when "git bisect skip" was +developed in Git version 1.5.4 (released on February 1st 2008) until +Git version 1.6.4 (released July 29th 2009). + +But Ingo Molnar and H. Peter Anvin (another well known linux kernel +developer) both complained that sometimes the best bisection points +all happened to be in an area where all the commits are +untestable. And in this case the user was asked to test many +untestable commits, which could be very inefficient. + +Indeed untestable commits are often untestable because a breakage was +introduced at one time, and that breakage was fixed only after many +other commits were introduced. + +This breakage is of course most of the time unrelated to the breakage +we are trying to locate in the commit graph. But it prevents us to +know if the interesting "bad behavior" is present or not. + +So it is a fact that commits near an untestable commit have a high +probability of being untestable themselves. And the best bisection +commits are often found together too (due to the bisection algorithm). + +This is why it is a bad idea to just chose the next best unskipped +bisection commit when the first one has been skipped. + +We found that most commits on the graph may give quite a lot of +information when they are tested. And the commits that will not on +average give a lot of information are the one near the good and bad +commits. + +So using a PRNG with a bias to favor commits away from the good and +bad commits looked like a good choice. + +One obvious improvement to this algorithm would be to look for a +commit that has an associated value near the one of the best bisection +commit, and that is on another branch, before using the PRNG. Because +if such a commit exists, then it is not very likely to be untestable +too, so it will probably give more information than a nearly randomly +chosen one. + +Checking merge bases +~~~~~~~~~~~~~~~~~~~~ + +There is another tweak in the bisection algorithm that has not been +described in the "bisection algorithm" above. + +We supposed in the previous examples that the "good" commits were +ancestors of the "bad" commit. But this is not a requirement of "git +bisect". + +Of course the "bad" commit cannot be an ancestor of a "good" commit, +because the ancestors of the good commits are supposed to be +"good". And all the "good" commits must be related to the bad commit. +They cannot be on a branch that has no link with the branch of the +"bad" commit. But it is possible for a good commit to be related to a +bad commit and yet not be neither one of its ancestor nor one of its +descendants. + +For example, there can be a "main" branch, and a "dev" branch that was +forked of the main branch at a commit named "D" like this: + +------------- +A-B-C-D-E-F-G <--main + \ + H-I-J <--dev +------------- + +The commit "D" is called a "merge base" for branch "main" and "dev" +because it's the best common ancestor for these branches for a merge. + +Now let's suppose that commit J is bad and commit G is good and that +we apply the bisection algorithm like it has been previously +described. + +As described in step 1) b) of the bisection algorithm, we remove all +the ancestors of the good commits because they are supposed to be good +too. + +So we would be left with only: + +------------- +H-I-J +------------- + +But what happens if the first bad commit is "B" and if it has been +fixed in the "main" branch by commit "F"? + +The result of such a bisection would be that we would find that H is +the first bad commit, when in fact it's B. So that would be wrong! + +And yes it can happen in practice that people working on one branch +are not aware that people working on another branch fixed a bug! It +could also happen that F fixed more than one bug or that it is a +revert of some big development effort that was not ready to be +released. + +In fact development teams often maintain both a development branch and +a maintenance branch, and it would be quite easy for them if "git +bisect" just worked when they want to bisect a regression on the +development branch that is not on the maintenance branch. They should +be able to start bisecting using: + +------------- +$ git bisect start dev main +------------- + +To enable that additional nice feature, when a bisection is started +and when some good commits are not ancestors of the bad commit, we +first compute the merge bases between the bad and the good commits and +we chose these merge bases as the first commits that will be checked +out and tested. + +If it happens that one merge base is bad, then the bisection process +is stopped with a message like: + +------------- +The merge base BBBBBB is bad. +This means the bug has been fixed between BBBBBB and [GGGGGG,...]. +------------- + +where BBBBBB is the sha1 hash of the bad merge base and [GGGGGG,...] +is a comma separated list of the sha1 of the good commits. + +If some of the merge bases are skipped, then the bisection process +continues, but the following message is printed for each skipped merge +base: + +------------- +Warning: the merge base between BBBBBB and [GGGGGG,...] must be skipped. +So we cannot be sure the first bad commit is between MMMMMM and BBBBBB. +We continue anyway. +------------- + +where BBBBBB is the sha1 hash of the bad commit, MMMMMM is the sha1 +hash of the merge base that is skipped and [GGGGGG,...] is a comma +separated list of the sha1 of the good commits. + +So if there is no bad merge base, the bisection process continues as +usual after this step. + +Best bisecting practices +------------------------ + +Using test suites and git bisect together +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you both have a test suite and use git bisect, then it becomes less +important to check that all tests pass after each commit. Though of +course it is probably a good idea to have some checks to avoid +breaking too many things because it could make bisecting other bugs +more difficult. + +You can focus your efforts to check at a few points (for example rc +and beta releases) that all the T test cases pass for all the N +configurations. And when some tests don't pass you can use "git +bisect" (or better "git bisect run"). So you should perform roughly: + +------------- +c * N * T + b * M * log2(M) tests +------------- + +where c is the number of rounds of test (so a small constant) and b is +the ratio of bug per commit (hopefully a small constant too). + +So of course it's much better as it's O(N * T) vs O(N * T * M) if +you would test everything after each commit. + +This means that test suites are good to prevent some bugs from being +committed and they are also quite good to tell you that you have some +bugs. But they are not so good to tell you where some bugs have been +introduced. To tell you that efficiently, git bisect is needed. + +The other nice thing with test suites, is that when you have one, you +already know how to test for bad behavior. So you can use this +knowledge to create a new test case for "git bisect" when it appears +that there is a regression. So it will be easier to bisect the bug and +fix it. And then you can add the test case you just created to your +test suite. + +So if you know how to create test cases and how to bisect, you will be +subject to a virtuous circle: + +more tests => easier to create tests => easier to bisect => more tests + +So test suites and "git bisect" are complementary tools that are very +powerful and efficient when used together. + +Bisecting build failures +~~~~~~~~~~~~~~~~~~~~~~~~ + +You can very easily automatically bisect broken builds using something +like: + +------------- +$ git bisect start BAD GOOD +$ git bisect run make +------------- + +Passing sh -c "some commands" to "git bisect run" +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For example: + +------------- +$ git bisect run sh -c "make || exit 125; ./my_app | grep 'good output'" +------------- + +On the other hand if you do this often, then it can be worth having +scripts to avoid too much typing. + +Finding performance regressions +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Here is an example script that comes slightly modified from a real +world script used by Junio Hamano <<4>>. + +This script can be passed to "git bisect run" to find the commit that +introduced a performance regression: + +------------- +#!/bin/sh + +# Build errors are not what I am interested in. +make my_app || exit 255 + +# We are checking if it stops in a reasonable amount of time, so +# let it run in the background... + +./my_app >log 2>&1 & + +# ... and grab its process ID. +pid=$! + +# ... and then wait for sufficiently long. +sleep $NORMAL_TIME + +# ... and then see if the process is still there. +if kill -0 $pid +then + # It is still running -- that is bad. + kill $pid; sleep 1; kill $pid; + exit 1 +else + # It has already finished (the $pid process was no more), + # and we are happy. + exit 0 +fi +------------- + +Following general best practices +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +It is obviously a good idea not to have commits with changes that +knowingly break things, even if some other commits later fix the +breakage. + +It is also a good idea when using any VCS to have only one small +logical change in each commit. + +The smaller the changes in your commit, the most effective "git +bisect" will be. And you will probably need "git bisect" less in the +first place, as small changes are easier to review even if they are +only reviewed by the committer. + +Another good idea is to have good commit messages. They can be very +helpful to understand why some changes were made. + +These general best practices are very helpful if you bisect often. + +Avoiding bug prone merges +~~~~~~~~~~~~~~~~~~~~~~~~~ + +First merges by themselves can introduce some regressions even when +the merge needs no source code conflict resolution. This is because a +semantic change can happen in one branch while the other branch is not +aware of it. + +For example one branch can change the semantic of a function while the +other branch add more calls to the same function. + +This is made much worse if many files have to be fixed to resolve +conflicts. That's why such merges are called "evil merges". They can +make regressions very difficult to track down. It can even be +misleading to know the first bad commit if it happens to be such a +merge, because people might think that the bug comes from bad conflict +resolution when it comes from a semantic change in one branch. + +Anyway "git rebase" can be used to linearize history. This can be used +either to avoid merging in the first place. Or it can be used to +bisect on a linear history instead of the non linear one, as this +should give more information in case of a semantic change in one +branch. + +Merges can be also made simpler by using smaller branches or by using +many topic branches instead of only long version related branches. + +And testing can be done more often in special integration branches +like linux-next for the linux kernel. + +Adapting your work-flow +~~~~~~~~~~~~~~~~~~~~~~~ + +A special work-flow to process regressions can give great results. + +Here is an example of a work-flow used by Andreas Ericsson: + +* write, in the test suite, a test script that exposes the regression +* use "git bisect run" to find the commit that introduced it +* fix the bug that is often made obvious by the previous step +* commit both the fix and the test script (and if needed more tests) + +And here is what Andreas said about this work-flow <<5>>: + +_____________ +To give some hard figures, we used to have an average report-to-fix +cycle of 142.6 hours (according to our somewhat weird bug-tracker +which just measures wall-clock time). Since we moved to Git, we've +lowered that to 16.2 hours. Primarily because we can stay on top of +the bug fixing now, and because everyone's jockeying to get to fix +bugs (we're quite proud of how lazy we are to let Git find the bugs +for us). Each new release results in ~40% fewer bugs (almost certainly +due to how we now feel about writing tests). +_____________ + +Clearly this work-flow uses the virtuous circle between test suites +and "git bisect". In fact it makes it the standard procedure to deal +with regression. + +In other messages Andreas says that they also use the "best practices" +described above: small logical commits, topic branches, no evil +merge,... These practices all improve the bisectability of the commit +graph, by making it easier and more useful to bisect. + +So a good work-flow should be designed around the above points. That +is making bisecting easier, more useful and standard. + +Involving QA people and if possible end users +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +One nice about "git bisect" is that it is not only a developer +tool. It can effectively be used by QA people or even end users (if +they have access to the source code or if they can get access to all +the builds). + +There was a discussion at one point on the linux kernel mailing list +of whether it was ok to always ask end user to bisect, and very good +points were made to support the point of view that it is ok. + +For example David Miller wrote <<6>>: + +_____________ +What people don't get is that this is a situation where the "end node +principle" applies. When you have limited resources (here: developers) +you don't push the bulk of the burden upon them. Instead you push +things out to the resource you have a lot of, the end nodes (here: +users), so that the situation actually scales. +_____________ + +This means that it is often "cheaper" if QA people or end users can do +it. + +What is interesting too is that end users that are reporting bugs (or +QA people that reproduced a bug) have access to the environment where +the bug happens. So they can often more easily reproduce a +regression. And if they can bisect, then more information will be +extracted from the environment where the bug happens, which means that +it will be easier to understand and then fix the bug. + +For open source projects it can be a good way to get more useful +contributions from end users, and to introduce them to QA and +development activities. + +Using complex scripts +~~~~~~~~~~~~~~~~~~~~~ + +In some cases like for kernel development it can be worth developing +complex scripts to be able to fully automate bisecting. + +Here is what Ingo Molnar says about that <<7>>: + +_____________ +i have a fully automated bootup-hang bisection script. It is based on +"git-bisect run". I run the script, it builds and boots kernels fully +automatically, and when the bootup fails (the script notices that via +the serial log, which it continuously watches - or via a timeout, if +the system does not come up within 10 minutes it's a "bad" kernel), +the script raises my attention via a beep and i power cycle the test +box. (yeah, i should make use of a managed power outlet to 100% +automate it) +_____________ + +Combining test suites, git bisect and other systems together +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +We have seen that test suites an git bisect are very powerful when +used together. It can be even more powerful if you can combine them +with other systems. + +For example some test suites could be run automatically at night with +some unusual (or even random) configurations. And if a regression is +found by a test suite, then "git bisect" can be automatically +launched, and its result can be emailed to the author of the first bad +commit found by "git bisect", and perhaps other people too. And a new +entry in the bug tracking system could be automatically created too. + + +The future of bisecting +----------------------- + +"git replace" +~~~~~~~~~~~~~ + +We saw earlier that "git bisect skip" is now using a PRNG to try to +avoid areas in the commit graph where commits are untestable. The +problem is that sometimes the first bad commit will be in an +untestable area. + +To simplify the discussion we will suppose that the untestable area is +a simple string of commits and that it was created by a breakage +introduced by one commit (let's call it BBC for bisect breaking +commit) and later fixed by another one (let's call it BFC for bisect +fixing commit). + +For example: + +------------- +...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z-... +------------- + +where we know that Y is good and BFC is bad, and where BBC and X1 to +X6 are untestable. + +In this case if you are bisecting manually, what you can do is create +a special branch that starts just before the BBC. The first commit in +this branch should be the BBC with the BFC squashed into it. And the +other commits in the branch should be the commits between BBC and BFC +rebased on the first commit of the branch and then the commit after +BFC also rebased on. + +For example: + +------------- + (BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z' + / +...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z-... +------------- + +where commits quoted with ' have been rebased. + +You can easily create such a branch with Git using interactive rebase. + +For example using: + +------------- +$ git rebase -i Y Z +------------- + +and then moving BFC after BBC and squashing it. + +After that you can start bisecting as usual in the new branch and you +should eventually find the first bad commit. + +For example: + +------------- +$ git bisect start Z' Y +------------- + +If you are using "git bisect run", you can use the same manual fix up +as above, and then start another "git bisect run" in the special +branch. Or as the "git bisect" man page says, the script passed to +"git bisect run" can apply a patch before it compiles and test the +software <<8>>. The patch should turn a current untestable commits +into a testable one. So the testing will result in "good" or "bad" and +"git bisect" will be able to find the first bad commit. And the script +should not forget to remove the patch once the testing is done before +exiting from the script. + +(Note that instead of a patch you can use "git cherry-pick BFC" to +apply the fix, and in this case you should use "git reset --hard +HEAD^" to revert the cherry-pick after testing and before returning +from the script.) + +But the above ways to work around untestable areas are a little bit +clunky. Using special branches is nice because these branches can be +shared by developers like usual branches, but the risk is that people +will get many such branches. And it disrupts the normal "git bisect" +work-flow. So, if you want to use "git bisect run" completely +automatically, you have to add special code in your script to restart +bisection in the special branches. + +Anyway one can notice in the above special branch example that the Z' +and Z commits should point to the same source code state (the same +"tree" in git parlance). That's because Z' result from applying the +same changes as Z just in a slightly different order. + +So if we could just "replace" Z by Z' when we bisect, then we would +not need to add anything to a script. It would just work for anyone in +the project sharing the special branches and the replacements. + +With the example above that would give: + +------------- + (BBC+BFC)-X1'-X2'-X3'-X4'-X5'-X6'-Z'-... + / +...-Y-BBC-X1-X2-X3-X4-X5-X6-BFC-Z +------------- + +That's why the "git replace" command was created. Technically it +stores replacements "refs" in the "refs/replace/" hierarchy. These +"refs" are like branches (that are stored in "refs/heads/") or tags +(that are stored in "refs/tags"), and that means that they can +automatically be shared like branches or tags among developers. + +"git replace" is a very powerful mechanism. It can be used to fix +commits in already released history, for example to change the commit +message or the author. And it can also be used instead of git "grafts" +to link a repository with another old repository. + +In fact it's this last feature that "sold" it to the Git community, so +it is now in the "master" branch of Git's Git repository and it should +be released in Git 1.6.5 in October or November 2009. + +One problem with "git replace" is that currently it stores all the +replacements refs in "refs/replace/", but it would be perhaps better +if the replacement refs that are useful only for bisecting would be in +"refs/replace/bisect/". This way the replacement refs could be used +only for bisecting, while other refs directly in "refs/replace/" would +be used nearly all the time. + +Bisecting sporadic bugs +~~~~~~~~~~~~~~~~~~~~~~~ + +Another possible improvement to "git bisect" would be to optionally +add some redundancy to the tests performed so that it would be more +reliable when tracking sporadic bugs. + +This has been requested by some kernel developers because some bugs +called sporadic bugs do not appear in all the kernel builds because +they are very dependent on the compiler output. + +The idea is that every 3 test for example, "git bisect" could ask the +user to test a commit that has already been found to be "good" or +"bad" (because one of its descendants or one of its ancestors has been +found to be "good" or "bad" respectively). If it happens that a commit +has been previously incorrectly classified then the bisection can be +aborted early, hopefully before too many mistakes have been made. Then +the user will have to look at what happened and then restart the +bisection using a fixed bisect log. + +There is already a project called BBChop created by Ealdwulf Wuffinga +on Github that does something like that using Bayesian Search Theory +<<9>>: + +_____________ +BBChop is like 'git bisect' (or equivalent), but works when your bug +is intermittent. That is, it works in the presence of false negatives +(when a version happens to work this time even though it contains the +bug). It assumes that there are no false positives (in principle, the +same approach would work, but adding it may be non-trivial). +_____________ + +But BBChop is independent of any VCS and it would be easier for Git +users to have something integrated in Git. + +Conclusion +---------- + +We have seen that regressions are an important problem, and that "git +bisect" has nice features that complement very well practices and +other tools, especially test suites, that are generally used to fight +regressions. But it might be needed to change some work-flows and +(bad) habits to get the most out of it. + +Some improvements to the algorithms inside "git bisect" are possible +and some new features could help in some cases, but overall "git +bisect" works already very well, is used a lot, and is already very +useful. To back up that last claim, let's give the final word to Ingo +Molnar when he was asked by the author how much time does he think +"git bisect" saves him when he uses it: + +_____________ +a _lot_. + +About ten years ago did i do my first 'bisection' of a Linux patch +queue. That was prior the Git (and even prior the BitKeeper) days. I +literally days spent sorting out patches, creating what in essence +were standalone commits that i guessed to be related to that bug. + +It was a tool of absolute last resort. I'd rather spend days looking +at printk output than do a manual 'patch bisection'. + +With Git bisect it's a breeze: in the best case i can get a ~15 step +kernel bisection done in 20-30 minutes, in an automated way. Even with +manual help or when bisecting multiple, overlapping bugs, it's rarely +more than an hour. + +In fact it's invaluable because there are bugs i would never even +_try_ to debug if it wasn't for git bisect. In the past there were bug +patterns that were immediately hopeless for me to debug - at best i +could send the crash/bug signature to lkml and hope that someone else +can think of something. + +And even if a bisection fails today it tells us something valuable +about the bug: that it's non-deterministic - timing or kernel image +layout dependent. + +So git bisect is unconditional goodness - and feel free to quote that +;-) +_____________ + +Acknowledgments +--------------- + +Many thanks to Junio Hamano for his help in reviewing this paper, for +reviewing the patches I sent to the Git mailing list, for discussing +some ideas and helping me improve them, for improving "git bisect" a +lot and for his awesome work in maintaining and developing Git. + +Many thanks to Ingo Molnar for giving me very useful information that +appears in this paper, for commenting on this paper, for his +suggestions to improve "git bisect" and for evangelizing "git bisect" +on the linux kernel mailing lists. + +Many thanks to Linus Torvalds for inventing, developing and +evangelizing "git bisect", Git and Linux. + +Many thanks to the many other great people who helped one way or +another when I worked on Git, especially to Andreas Ericsson, Johannes +Schindelin, H. Peter Anvin, Daniel Barkalow, Bill Lear, John Hawley, +Shawn O. Pierce, Jeff King, Sam Vilain, Jon Seymour. + +Many thanks to the Linux-Kongress program committee for choosing the +author to given a talk and for publishing this paper. + +References +---------- + +- [[[1]]] http://www.nist.gov/public_affairs/releases/n02-10.htm['Software Errors Cost U.S. Economy $59.5 Billion Annually'. Nist News Release.] +- [[[2]]] http://java.sun.com/docs/codeconv/html/CodeConventions.doc.html#16712['Code Conventions for the Java Programming Language'. Sun Microsystems.] +- [[[3]]] http://en.wikipedia.org/wiki/Software_maintenance['Software maintenance'. Wikipedia.] +- [[[4]]] http://article.gmane.org/gmane.comp.version-control.git/45195/[Junio C Hamano. 'Automated bisect success story'. Gmane.] +- [[[5]]] http://lwn.net/Articles/317154/[Christian Couder. 'Fully automated bisecting with "git bisect run"'. LWN.net.] +- [[[6]]] http://lwn.net/Articles/277872/[Jonathan Corbet. 'Bisection divides users and developers'. LWN.net.] +- [[[7]]] http://article.gmane.org/gmane.linux.scsi/36652/[Ingo Molnar. 'Re: BUG 2.6.23-rc3 can't see sd partitions on Alpha'. Gmane.] +- [[[8]]] http://www.kernel.org/pub/software/scm/git/docs/git-bisect.html[Junio C Hamano and the git-list. 'git-bisect(1) Manual Page'. Linux Kernel Archives.] +- [[[9]]] http://github.com/Ealdwulf/bbchop[Ealdwulf. 'bbchop'. GitHub.] |