Documentation/technical/racy-git.txt
Signed-off-by: Junio C Hamano <junkio@cox.net>
This commit is contained in:
Junio C Hamano
parent
789a09b487
commit
520cd3eca5
193
Documentation/technical/racy-git.txt
Normal file
193
Documentation/technical/racy-git.txt
Normal file
@ -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.
|
||||||
Loading…
Reference in New Issue
Block a user