1Use of index and Racy git problem 2================================= 3 4Background 5---------- 6 7The index is one of the most important data structures in git. 8It represents a virtual working tree state by recording list of 9paths and their object names and serves as a staging area to 10write out the next tree object to be committed. The state is 11"virtual" in the sense that it does not necessarily have to, and 12often does not, match the files in the working tree. 13 14There are cases git needs to examine the differences between the 15virtual working tree state in the index and the files in the 16working tree. The most obvious case is when the user asks `git 17diff` (or its low level implementation, `git diff-files`) or 18`git-ls-files --modified`. In addition, git internally checks 19if the files in the working tree are different from what are 20recorded in the index to avoid stomping on local changes in them 21during patch application, switching branches, and merging. 22 23In order to speed up this comparison between the files in the 24working tree and the index entries, the index entries record the 25information obtained from the filesystem via `lstat(2)` system 26call when they were last updated. When checking if they differ, 27git first runs `lstat(2)` on the files and compares the result 28with this information (this is what was originally done by the 29`ce_match_stat()` function, but the current code does it in 30`ce_match_stat_basic()` function). If some of these "cached 31stat information" fields do not match, git can tell that the 32files are modified without even looking at their contents. 33 34Note: not all members in `struct stat` obtained via `lstat(2)` 35are used for this comparison. For example, `st_atime` obviously 36is not useful. Currently, git compares the file type (regular 37files vs symbolic links) and executable bits (only for regular 38files) from `st_mode` member, `st_mtime` and `st_ctime` 39timestamps, `st_uid`, `st_gid`, `st_ino`, and `st_size` members. 40With a `USE_STDEV` compile-time option, `st_dev` is also 41compared, but this is not enabled by default because this member 42is not stable on network filesystems. With `USE_NSEC` 43compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec` 44members are also compared, but this is not enabled by default 45because the value of this member becomes meaningless once the 46inode is evicted from the inode cache on filesystems that do not 47store it on disk. 48 49 50Racy git 51-------- 52 53There is one slight problem with the optimization based on the 54cached stat information. Consider this sequence: 55 56 : modify 'foo' 57 $ git update-index 'foo' 58 : modify 'foo' again, in-place, without changing its size 59 60The first `update-index` computes the object name of the 61contents of file `foo` and updates the index entry for `foo` 62along with the `struct stat` information. If the modification 63that follows it happens very fast so that the file's `st_mtime` 64timestamp does not change, after this sequence, the cached stat 65information the index entry records still exactly match what you 66would see in the filesystem, even though the file `foo` is now 67different. 68This way, git can incorrectly think files in the working tree 69are unmodified even though they actually are. This is called 70the "racy git" problem (discovered by Pasky), and the entries 71that appear clean when they may not be because of this problem 72are called "racily clean". 73 74To avoid this problem, git does two things: 75 76. When the cached stat information says the file has not been 77 modified, and the `st_mtime` is the same as (or newer than) 78 the timestamp of the index file itself (which is the time `git 79 update-index foo` finished running in the above example), it 80 also compares the contents with the object registered in the 81 index entry to make sure they match. 82 83. When the index file is updated that contains racily clean 84 entries, cached `st_size` information is truncated to zero 85 before writing a new version of the index file. 86 87Because the index file itself is written after collecting all 88the stat information from updated paths, `st_mtime` timestamp of 89it is usually the same as or newer than any of the paths the 90index contains. And no matter how quick the modification that 91follows `git update-index foo` finishes, the resulting 92`st_mtime` timestamp on `foo` cannot get a value earlier 93than the index file. Therefore, index entries that can be 94racily clean are limited to the ones that have the same 95timestamp as the index file itself. 96 97The callers that want to check if an index entry matches the 98corresponding file in the working tree continue to call 99`ce_match_stat()`, but with this change, `ce_match_stat()` uses 100`ce_modified_check_fs()` to see if racily clean ones are 101actually clean after comparing the cached stat information using 102`ce_match_stat_basic()`. 103 104The problem the latter solves is this sequence: 105 106 $ git update-index 'foo' 107 : modify 'foo' in-place without changing its size 108 : wait for enough time 109 $ git update-index 'bar' 110 111Without the latter, the timestamp of the index file gets a newer 112value, and falsely clean entry `foo` would not be caught by the 113timestamp comparison check done with the former logic anymore. 114The latter makes sure that the cached stat information for `foo` 115would never match with the file in the working tree, so later 116checks by `ce_match_stat_basic()` would report that the index entry 117does not match the file and git does not have to fall back on more 118expensive `ce_modified_check_fs()`. 119 120 121Runtime penalty 122--------------- 123 124The runtime penalty of falling back to `ce_modified_check_fs()` 125from `ce_match_stat()` can be very expensive when there are many 126racily clean entries. An obvious way to artificially create 127this situation is to give the same timestamp to all the files in 128the working tree in a large project, run `git update-index` on 129them, and give the same timestamp to the index file: 130 131 $ date >.datestamp 132 $ git ls-files | xargs touch -r .datestamp 133 $ git ls-files | git update-index --stdin 134 $ touch -r .datestamp .git/index 135 136This will make all index entries racily clean. The linux-2.6 137project, for example, there are over 20,000 files in the working 138tree. On my Athlon 64 X2 3800+, after the above: 139 140 $ /usr/bin/time git diff-files 141 1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k 142 0inputs+0outputs (0major+67111minor)pagefaults 0swaps 143 $ git update-index MAINTAINERS 144 $ /usr/bin/time git diff-files 145 0.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k 146 0inputs+0outputs (0major+935minor)pagefaults 0swaps 147 148Running `git update-index` in the middle checked the racily 149clean entries, and left the cached `st_mtime` for all the paths 150intact because they were actually clean (so this step took about 151the same amount of time as the first `git diff-files`). After 152that, they are not racily clean anymore but are truly clean, so 153the second invocation of `git diff-files` fully took advantage 154of the cached stat information. 155 156 157Avoiding runtime penalty 158------------------------ 159 160In order to avoid the above runtime penalty, post 1.4.2 git used 161to have a code that made sure the index file 162got timestamp newer than the youngest files in the index when 163there are many young files with the same timestamp as the 164resulting index file would otherwise would have by waiting 165before finishing writing the index file out. 166 167I suspected that in practice the situation where many paths in the 168index are all racily clean was quite rare. The only code paths 169that can record recent timestamp for large number of paths are: 170 171. Initial `git add .` of a large project. 172 173. `git checkout` of a large project from an empty index into an 174 unpopulated working tree. 175 176Note: switching branches with `git checkout` keeps the cached 177stat information of existing working tree files that are the 178same between the current branch and the new branch, which are 179all older than the resulting index file, and they will not 180become racily clean. Only the files that are actually checked 181out can become racily clean. 182 183In a large project where raciness avoidance cost really matters, 184however, the initial computation of all object names in the 185index takes more than one second, and the index file is written 186out after all that happens. Therefore the timestamp of the 187index file will be more than one seconds later than the 188youngest file in the working tree. This means that in these 189cases there actually will not be any racily clean entry in 190the resulting index. 191 192Based on this discussion, the current code does not use the 193"workaround" to avoid the runtime penalty that does not exist in 194practice anymore. This was done with commit 0fc82cff on Aug 15, 1952006.