The commit backend at Google is cloud-based (and so are the other
backends); it reads and writes commits from/to a server, which stores
them in a database. That makes latency much higher than for disk-based
backends. To reduce the latency, we have a local daemon process that
caches and prefetches objects. There are still many cases where
latency is high, such as when diffing two uncached commits. We can
improve that by changing some of our (jj's) algorithms to read many
objects concurrently from the backend. In the case of tree-diffing, we
can fetch one level (depth) of the tree at a time. There are several
ways of doing that:
* Make the backend methods `async`
* Use many threads for reading from the backend
* Add backend methods for batch reading
I don't think we typically need CPU parallelism, so it's wasteful to
have hundreds of threads running in order to fetch hundreds of objects
in parallel (especially when using a synchronous backend like the Git
backend). Batching would work well for the tree-diffing case, but it's
not as composable as `async`. For example, if we wanted to fetch some
commits at the same time as we were doing a diff, it's hard to see how
to do that with batching. Using async seems like our best bet.
I didn't make the backend interface's write functions async because
writes are already async with the daemon we have at Google. That
daemon will hash the object and immediately return, and then send the
object to the server in the background. I think any cloud-based
solution will need a similar daemon process. However, we may need to
reconsider this if/when jj gets used on a server with a custom backend
that writes directly to a database (i.e. no async daemon in between).
I've tried to measure the performance impact. That's the largest
difference I've been able to measure was on `jj diff
--ignore-working-copy -s --from v5.0 --to v6.0` in the Linux repo,
which increases from 749 ms to 773 ms (3.3%). In most cases I've
tested, there's no measurable difference. I've tried diffing from the
root commit, as well as `jj --ignore-working-copy log --no-graph -r
'::v3.0 & author(torvalds)' -T 'commit_id ++ "\n"'` (to test a
commit-heavy load).
https://github.com/cargo-bins/cargo-binstall
Note that `jj` will only become installable once the next release
is published to crates.io. For this reason, I am planning to
wait until then before documenting the fact that `jj` can be
installed this way.
At that point, `cargo binstall jj-cli` should be sufficient.
Before then, it's possible to test that this will work by doing
```
cargo binstall jj-cli --force --strategies crate-meta-data --log-level debug --dry-run --manifest-path cli/Cargo.toml
```
Without --dry-run, this should install the 0.9 release if run
on `cli/Cargo.toml` form this commit.
Among other things, this prevented `jj` from working with
`cargo binstall`.
The trick is taken from
https://github.com/rust-lang/cargo/issues/2911#issuecomment-1483256987.
We could now also remove `--bin jj` from the installation commands
in `install-and-setup.md`, but I'm not sure we should. That argument
makes it clear that the binary is `jj`, not `jj-cli`.
Fixes#216.
I think the feature is requested by enough users that we should
include it by default, also for people who install from source (we
include it in the `packaging` feature already).
It increases the size of the binary from 16.5 MiB to 17.8 MiB. I
suspect we'd see some of that increase in size soon anyway, as I'm
probably going to use Tokio for making async backend requests.
There should be no reason to include `criterion` without the `bench`
feature. Adding the `dep:` prefix to any dependency makes cargo not
expose the implicit `criterion` feature.
The VS Code "Better TOML" plugin (which I think most of our VS Code developers use?) doesn't support the `x.y = z` syntax at the top level, even though it's valid TOML.
This is also useful if we ever want to add additional properties in different sub-crates (although unlikely for the near future).
Summary: There's no need to go around specifying `rust-version` or `edition` or
`version` several times, now that we have a global workspace. Instead, inherit
workspace metadata from the top-level Cargo.toml file.
Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: Iaf905445978ed2b3377239dcdb8a6c32
Summary: This moves all dependencies across the jj-lib and jj-cli crates into
the top-level Cargo file; with that, we can change each crate instead to just
inherit the workspace version, with the toggled features enabled, by setting
a dependency such as:
dep.workspace = true
in the relevant Cargo.toml file.
This doesn't actually change any of the build semantics (from what I can tell)
nor the lockfile, and seems to respond normally. There are more cleanups that
can follow.
Two notes:
- Dependabot seems to work fine, based on what I've seen in other repos.
- `cargo add` doesn't seem to know how to add packages to a top-level
`workspace.dependencies` field; instead you can `cargo add -p jj-cli`
and move the entries, at least.
Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: I307827e5f15c0d8ea8e2a80ec793d3c7
Summary: In preparation for unifying all workspace dependencies across all
crates, let's go ahead and move the jj-cli crate into its own new directory.
This will also be a nicer and more uniform layout as we add new `jj-*` crates.
Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: Icf94e7ae5f290dc8e181215727b38ada
