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197 lines
8.6 KiB
197 lines
8.6 KiB
Use of index and Racy git problem |
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================================= |
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Background |
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---------- |
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The index is one of the most important data structures in git. |
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It represents a virtual working tree state by recording list of |
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paths and their object names and serves as a staging area to |
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write out the next tree object to be committed. The state is |
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"virtual" in the sense that it does not necessarily have to, and |
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often does not, match the files in the working tree. |
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There are cases git needs to examine the differences between the |
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virtual working tree state in the index and the files in the |
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working tree. The most obvious case is when the user asks `git |
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diff` (or its low level implementation, `git diff-files`) or |
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`git-ls-files --modified`. In addition, git internally checks |
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if the files in the working tree are different from what are |
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recorded in the index to avoid stomping on local changes in them |
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during patch application, switching branches, and merging. |
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In order to speed up this comparison between the files in the |
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working tree and the index entries, the index entries record the |
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information obtained from the filesystem via `lstat(2)` system |
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call when they were last updated. When checking if they differ, |
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git first runs `lstat(2)` on the files and compares the result |
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with this information (this is what was originally done by the |
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`ce_match_stat()` function, but the current code does it in |
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`ce_match_stat_basic()` function). If some of these "cached |
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stat information" fields do not match, git can tell that the |
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files are modified without even looking at their contents. |
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Note: not all members in `struct stat` obtained via `lstat(2)` |
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are used for this comparison. For example, `st_atime` obviously |
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is not useful. Currently, git compares the file type (regular |
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files vs symbolic links) and executable bits (only for regular |
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files) from `st_mode` member, `st_mtime` and `st_ctime` |
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timestamps, `st_uid`, `st_gid`, `st_ino`, and `st_size` members. |
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With a `USE_STDEV` compile-time option, `st_dev` is also |
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compared, but this is not enabled by default because this member |
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is not stable on network filesystems. With `USE_NSEC` |
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compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec` |
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members are also compared, but this is not enabled by default |
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because in-core timestamps can have finer granularity than |
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on-disk timestamps, resulting in meaningless changes when an |
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inode is evicted from the inode cache. See commit 8ce13b0 |
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of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git |
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([PATCH] Sync in core time granuality with filesystems, |
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2005-01-04). |
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Racy git |
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-------- |
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There is one slight problem with the optimization based on the |
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cached stat information. Consider this sequence: |
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: modify 'foo' |
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$ git update-index 'foo' |
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: modify 'foo' again, in-place, without changing its size |
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The first `update-index` computes the object name of the |
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contents of file `foo` and updates the index entry for `foo` |
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along with the `struct stat` information. If the modification |
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that follows it happens very fast so that the file's `st_mtime` |
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timestamp does not change, after this sequence, the cached stat |
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information the index entry records still exactly match what you |
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would see in the filesystem, even though the file `foo` is now |
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different. |
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This way, git can incorrectly think files in the working tree |
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are unmodified even though they actually are. This is called |
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the "racy git" problem (discovered by Pasky), and the entries |
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that appear clean when they may not be because of this problem |
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are called "racily clean". |
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To avoid this problem, git does two things: |
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. When the cached stat information says the file has not been |
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modified, and the `st_mtime` is the same as (or newer than) |
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the timestamp of the index file itself (which is the time `git |
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update-index foo` finished running in the above example), it |
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also compares the contents with the object registered in the |
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index entry to make sure they match. |
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. When the index file is updated that contains racily clean |
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entries, cached `st_size` information is truncated to zero |
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before writing a new version of the index file. |
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Because the index file itself is written after collecting all |
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the stat information from updated paths, `st_mtime` timestamp of |
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it is usually the same as or newer than any of the paths the |
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index contains. And no matter how quick the modification that |
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follows `git update-index foo` finishes, the resulting |
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`st_mtime` timestamp on `foo` cannot get a value earlier |
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than the index file. Therefore, index entries that can be |
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racily clean are limited to the ones that have the same |
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timestamp as the index file itself. |
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The callers that want to check if an index entry matches the |
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corresponding file in the working tree continue to call |
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`ce_match_stat()`, but with this change, `ce_match_stat()` uses |
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`ce_modified_check_fs()` to see if racily clean ones are |
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actually clean after comparing the cached stat information using |
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`ce_match_stat_basic()`. |
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The problem the latter solves is this sequence: |
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$ git update-index 'foo' |
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: modify 'foo' in-place without changing its size |
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: wait for enough time |
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$ git update-index 'bar' |
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Without the latter, the timestamp of the index file gets a newer |
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value, and falsely clean entry `foo` would not be caught by the |
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timestamp comparison check done with the former logic anymore. |
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The latter makes sure that the cached stat information for `foo` |
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would never match with the file in the working tree, so later |
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checks by `ce_match_stat_basic()` would report that the index entry |
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does not match the file and git does not have to fall back on more |
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expensive `ce_modified_check_fs()`. |
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Runtime penalty |
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--------------- |
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The runtime penalty of falling back to `ce_modified_check_fs()` |
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from `ce_match_stat()` can be very expensive when there are many |
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racily clean entries. An obvious way to artificially create |
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this situation is to give the same timestamp to all the files in |
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the working tree in a large project, run `git update-index` on |
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them, and give the same timestamp to the index file: |
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$ date >.datestamp |
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$ git ls-files | xargs touch -r .datestamp |
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$ git ls-files | git update-index --stdin |
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$ touch -r .datestamp .git/index |
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This will make all index entries racily clean. The linux-2.6 |
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project, for example, there are over 20,000 files in the working |
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tree. On my Athlon 64 X2 3800+, after the above: |
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$ /usr/bin/time git diff-files |
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1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k |
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0inputs+0outputs (0major+67111minor)pagefaults 0swaps |
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$ git update-index MAINTAINERS |
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$ /usr/bin/time git diff-files |
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0.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k |
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0inputs+0outputs (0major+935minor)pagefaults 0swaps |
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Running `git update-index` in the middle checked the racily |
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clean entries, and left the cached `st_mtime` for all the paths |
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intact because they were actually clean (so this step took about |
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the same amount of time as the first `git diff-files`). After |
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that, they are not racily clean anymore but are truly clean, so |
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the second invocation of `git diff-files` fully took advantage |
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of the cached stat information. |
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Avoiding runtime penalty |
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------------------------ |
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In order to avoid the above runtime penalty, post 1.4.2 git used |
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to have a code that made sure the index file |
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got timestamp newer than the youngest files in the index when |
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there are many young files with the same timestamp as the |
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resulting index file would otherwise would have by waiting |
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before finishing writing the index file out. |
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I suspected that in practice the situation where many paths in the |
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index are all racily clean was quite rare. The only code paths |
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that can record recent timestamp for large number of paths are: |
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. Initial `git add .` of a large project. |
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. `git checkout` of a large project from an empty index into an |
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unpopulated working tree. |
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Note: switching branches with `git checkout` keeps the cached |
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stat information of existing working tree files that are the |
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same between the current branch and the new branch, which are |
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all older than the resulting index file, and they will not |
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become racily clean. Only the files that are actually checked |
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out can become racily clean. |
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In a large project where raciness avoidance cost really matters, |
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however, the initial computation of all object names in the |
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index takes more than one second, and the index file is written |
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out after all that happens. Therefore the timestamp of the |
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index file will be more than one seconds later than the |
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youngest file in the working tree. This means that in these |
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cases there actually will not be any racily clean entry in |
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the resulting index. |
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Based on this discussion, the current code does not use the |
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"workaround" to avoid the runtime penalty that does not exist in |
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practice anymore. This was done with commit 0fc82cff on Aug 15, |
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2006.
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