Signed-off-by: Junio C Hamano <junkio@xxxxxxx> --- Junio C Hamano <junkio@xxxxxxx> writes: > We might want to yank out the whole "racy-git avoidance is > costly later so let's delay writing the index out" codepath > later, but that is a separate issue and needs some testing on > large trees to figure it out. After playing with the kernel > tree, I have a feeling that the whole thing may not be worth > it. Addressed to Linus because he originally brought up this issue in <Pine.LNX.4.64.0607310945490.4168@xxxxxxxxxxx>, Johannes CC'ed because he had some comments earlier on the same topic and he is generally a good person to talk to when I have doubts on issues ;-). Documentation/technical/racy-git.txt | 193 ++++++++++++++++++++++++++++++++++ 1 files changed, 193 insertions(+), 0 deletions(-) diff --git a/Documentation/technical/racy-git.txt b/Documentation/technical/racy-git.txt new file mode 100644 index 0000000..7597d04 --- /dev/null +++ b/Documentation/technical/racy-git.txt @@ -0,0 +1,193 @@ +Use of index and Racy git problem +================================= + +Background +---------- + +The index is one of the most important data structure in git. +It represents a virtual working tree state by recording list of +paths and their object names and serves as a staging area to +write out the next tree object to be committed. The state is +"virtual" in the sense that it does not necessarily have to, and +often does not, match the files in the working tree. + +There are cases git needs to examine the differences between the +virtual working tree state in the index and the files in the +working tree. The most obvious case is when the user asks `git +diff` (or its low level implementation, `git diff-files`) or +`git-ls-files --modified`. In addition, git internally checks +if the files in the working tree is different from what are +recorded in the index to avoid stomping on local changes in them +during patch application, switching branches, and merging. + +In order to speed up this comparison between the files in the +working tree and the index entries, the index entries record the +information obtained from the filesystem via `lstat(2)` system +call when they were last updated. When checking if they differ, +git first runs `lstat(2)` on the files and compare the result +with this information (this is what was originally done by the +`ce_match_stat()` function, which the current code does in +`ce_match_stat_basic()` function). If some of these "cached +stat information" fields do not match, git can tell that the +files are modified without even looking at their contents. + +Note: not all members in `struct stat` obtained via `lstat(2)` +are used for this comparison. For example, `st_atime` obviously +is not useful. Currently, git compares the file type (regular +files vs symbolic links) and executable bits (only for regular +files) from `st_mode` member, `st_mtime` and `st_ctime` +timestamps, `st_uid`, `st_gid`, `st_ino`, and `st_size` members. +With a `USE_STDEV` compile-time option, `st_dev` is also +compared, but this is not enabled by default because this member +is not stable on network filesystems. With `USE_NSEC` +compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec` +members are also compared, but this is not enabled by default +because the value of this member becomes meaningless once the +inode is evicted from the inode cache on filesystems that do not +store it on disk. + + +Racy git +-------- + +There is one slight problem with the optimization based on the +cached stat information. Consider this sequence: + + $ git update-index 'foo' + : modify 'foo' in-place without changing its size + +The first `update-index` computes the object name of the +contents of file `foo` and updates the index entry for `foo` +along with the `struct stat` information. If the modification +that follows it happens very fast so that the file's `st_mtime` +timestamp does not change, after this sequence, the cached stat +information the index entry records still exactly match what you +can obtain from the filesystem, but the file `foo` is modified. +This way, git can incorrectly think files in the working tree +are unmodified even though they actually are. This is called +the "racy git" problem (discovered by Pasky), and the entries +that appear clean when they may not be because of this problem +are called "racily clean". + +To avoid this problem, git does two things: + +. When the cached stat information says the file has not been + modified, and the `st_mtime` is the same as (or newer than) + the timestamp of the index file itself (which is the time `git + update-index foo` finished running in the above example), it + also compares the contents with the object registered in the + index entry to make sure they match. + +. When the index file is updated that contains racily clean + entries, cached `st_size` information is truncated to zero + before writing a new version of the index file. + +Because the index file itself is written after collecting all +the stat information from updated paths, `st_mtime` timestamp of +it is usually the same as or newer than any of the paths the +index contains. And no matter how quick the modification that +follows `git update-index foo` finishes, the resulting +`st_mtime` timestamp on `foo` cannot get the timestamp earlier +than the index file. Therefore, index entries that can be +racily clean are limited to the ones that have the same +timestamp as the index file itself. + +The callers that want to check if an index entry matches the +corresponding file in the working tree continue to call +`ce_match_stat()`, but with this change, `ce_match_stat()` uses +`ce_modified_check_fs()` to see if racily clean ones are +actually clean after comparing the cached stat information using +`ce_match_stat_basic()`. + +The problem the latter solves is this sequence: + + $ git update-index 'foo' + : modify 'foo' in-place without changing its size + : wait for enough time + $ git update-index 'bar' + +Without the latter, the timestamp of the index file gets a newer +value, and falsely clean entry `foo` would not be caught by the +timestamp comparison check done with the former logic anymore. +The latter makes sure that the cached stat information for `foo` +would never match with the file in the working tree, so later +checks by `ce_match_stat_basic()` would report the index entry +does not match the file and git does not have to fall back on more +expensive `ce_modified_check_fs()`. + + +Runtime penalty +--------------- + +The runtime penalty of falling back to `ce_modified_check_fs()` +from `ce_match_stat()` can be very expensive when there are many +racily clean entries. An obvious way to artificially create +this situation is to give the same timestamp to all the files in +the working tree in a large project, run `git update-index` on +them, and give the same timestamp to the index file: + + $ date >.datestamp + $ git ls-files | xargs touch -r .datestamp + $ git ls-files | git update-index --stdin + $ touch -r .datestamp .git/index + +This will make all index entries racily clean. The linux-2.6 +project, for example, there are over 20,000 files in the working +tree. On my Athron 64X2 3800+, after the above: + + $ /usr/bin/time git diff-files + 1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k + 0inputs+0outputs (0major+67111minor)pagefaults 0swaps + $ git update-index MAINTAINERS + $ /usr/bin/time git diff-files + 0.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k + 0inputs+0outputs (0major+935minor)pagefaults 0swaps + +Running `git update-index` in the middle checked the racily +clean entries, and left the cached `st_mtime` for all the paths +intact because they were actually clean (so this step took about +the same amount of time as the first `git diff-files`). After +that, they are not racily clean anymore but are truly clean, so +the second invocation of `git diff-files` fully took advantage +of the cached stat information. + + +Avoiding runtime penalty +------------------------ + +In order to avoid the above runtime penalty, the recent "master" +branch (post 1.4.2) has a code that makes sure the index file +gets timestamp newer than the youngest files in the index when +there are many young files with the same timestamp as the +resulting index file would otherwise would have by waiting +before finishing writing the index file out. + +I suspect that in practice the situation where many paths in the +index are all racily clean is quite rare. The only code paths +that can record recent timestamp for large number of paths I +know of are: + +. Initial `git add .` of a large project. + +. `git checkout` of a large project from an empty index into an + unpopulated working tree. + +Note: switching branches with `git checkout` keeps the cached +stat information of existing working tree files that are the +same between the current branch and the new branch, which are +all older than the resulting index file, and they will not +become racily clean. Only the files that are actually checked +out can become racily clean. + +In a large project where raciness avoidance cost really matters, +however, the initial computation of all object names in the +index takes more than one second, and the index file is written +out after all that happens. Therefore the timestamp of the +index file will be more than one seconds later than the the +youngest file in the working tree. This means that in these +cases there actually will not be any racily clean entry in +the resulting index. + +So in summary I think we should not worry about avoiding the +runtime penalty and get rid of the "wait before finishing +writing" code out. -- 1.4.2.g59bb - To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html