ostree repo commit: Speed up composing trees with `--tree=ref`
Running `ostree commit --tree=ref=a --tree=dir=b` involves reading the
whole of a into an `OstreeMutableTree` before composing `b` on top. This
is inefficient if a is a complete rootfs and b is just touching one file.
We process O(size of a + size of b) directories rather than
O(number of touched dirs).
This commit makes `ostree commit` more efficient at composing multiple
directories together. With `ostree_mutable_tree_fill_empty_from_dirtree`
we create a lazy `OstreeMutableTree` which only reads the underlying
information from disk when needed. We don't need to read all the
subdirectories just to get the checksum of a tree already checked into the
repo.
This provides great speedups when composing a rootfs out of multiple other
rootfs as we do in our build system. We compose multiple containers
together with:
ostree commit --tree=ref=base-rootfs --tree=ref=container1 --tree=ref=container2
and it is much faster now.
As a test I ran
time ostree --repo=... commit --orphan --tree=ref=big-rootfs --tree=dir=modified_etc
Where modified_etc contained a modified sudoers file under /etc. I used
`strace` to count syscalls and I seperatly took timing measurements. To
test with a cold cache I ran
sync && echo 3 | sudo tee /proc/sys/vm/drop_caches
Results:
| | Before | After |
| -------------------- | ------ | ----- |
| Time (cold cache) | 8.1s | 0.12s |
| Time (warm cache) | 3.7s | 0.08s |
| `openat` calls | 53589 | 246 |
| `fgetxattr` calls | 78916 | 0 |
I'm not sure if this will have some negative interaction with the
`_ostree_repo_commit_modifier_apply` which is short-circuited here. I
think it was disabled for `--tree=ref=` anyway, but I'm not certain. All
the tests pass anyway.
I originally implemented this in terms of the `OstreeRepoFile` APIs, but
it was *way* less efficient, opening and reading many files unnecessarily.
Closes: #1643
Approved by: cgwalters
projects / ostree.git / commit