Developer Document Catalog (3.3.22)

# Integration of performance changes

To deliver a version of bzr with all our planned changes will require significant integration work. Minimally each change needs to integrate with some aspect of the bzr version it’s merged into, but in reality many of these changes while conceptually independent will in fact have to integrate with the other changes we have planned before can have a completed system.

Additionally changes that alter disk formats are inherently more tricky to integrate because we will often need to alter apis throughout the code base to expose the increased or reduced model of the preferred disk format.

You can generate a graph `performance.png` in the source tree from Graphviz “dot” file `performance.dot`. This graphs out the dependencies to let us make accurate assessments of the changes needed in terms of code and API, hopefully minimising the number of different integration steps we have to take, while giving us a broad surface area for development. It’s based on a summary in the next section of this document of the planned changes with their expected collaborators and dependencies. Where a command is listed, the expectation is that all uses of that command - local, remote, dumb transport and smart transport are being addressed together.

The following provides a summary of the planned changes and their expected collaborators within the code base, along with an estimate of whether they are likely to require changes to their collaborators to be considered ‘finished’.

> - Use case target APIs: Each of these is likely to alter the Tree interface. Some few of them focus on Branch and will alter Branch and Repository accordingly. As they are targeted APIs we can deep changes all the way down the stack to the underlying representation to make it all fit well. Presenting a top level API for many things will be possible now as long as the exposed data is audited for things we plan to make optional, or remove: Such things cannot be present in the final API. Writing these APIs now will provide strong feedback to the design process for those things which are considered optional or removable, so these APIs should be implemented before removing or making optional existing data.

> - Deprecating versioned files as a supported API: This collaborates with the Repository API but can probably be done by adding a replacement API for places where the versioned-file api is used. We may well want to keep a concept of ‘a file over time’ or ‘inventories over time’, so the existing repository model of exposing versioned file objects may be ok; what we need to ensure we do is remove the places in the code base where you create or remove or otherwise describe manipulation of the storage by knit rather than talking at the level of file ids and revision ids. The current versioned-file API would be a burden for implementors of a blob based repository format, so the removal of callers, and deprecation of those parts of the API should be done before creating a blob based repository format.

> - Creating a revision validator: Revision validators may depend on storage layer changes to inventories so while we can create a revision validator API, we cannot create the final one until we have the inventory structural changes completed.

> - Annotation caching API: This API is a prerequisite for new repository formats. If written after they are introduced we may find that the repository is lacking in functionality, so the API should be implemented first.

> - _iter_changes based merging: If the current _iter_changes_ API is insufficient, we should know about that before designing the disk format for generating fast _iter_changes_ output.

> - Network-efficient revision graph API: This influences what questions we will want to ask a local repository very quickly; as such it’s a driver for the new repository format and should be in place first if possible. It’s probably not sufficiently different to local operations to make this a hard ordering though.

> - Working tree disk ordering: Knowing the expected order for disk operations may influence the needed use case specific APIs, so having a solid understanding of what is optimal - and why - and whether it is pessimal on non-Linux-kernel platforms is rather important.

> - Be able to version files greater than memory in size: This cannot be achieved until all parts of the library which deal with user files are able to provide access to files larger than memory. Many strategies can be considered for this - such as temporary files on disk, memory mapping etc. We should have enough of a design laid out that developers of repository and tree logic are able to start exposing apis, and considering requirements related to them, to let this happen.

> - Per-file graph access API: This should be implemented on top of or as part of the newer API for accessing data about a file over time. It can be a separate step easily; but as it’s in the same area of the library should not be done in parallel.

> - Repository stacking API: The key dependency/change required for this is that repositories must individually be happy with having partial data - e.g. many ghosts. However the way the API needs to be used should be driven from the command layer in, because it’s unclear at the moment what will work best.

> - Revision stream API: This API will become clear as we streamline commands. On the data insertion side commit will want to generate new data. The commands pull, bundle, merge, push, possibly uncommit will want to copy existing data in a streaming fashion.

> - New container format: It’s hard to tell what the right way to structure the layering is. Probably having smooth layering down to the point that code wants to operate on the containers directly will make this more clear. As bundles will become a read-only branch & repository, the smart server wants streaming-containers, and we are planning a pack based repository, it appears that we will have three different direct container users. However, the bundle user may in fact be fake - because it really is a repository.

> - Separation of annotation cache: Making the disk changes to achieve this depends on the new API being created. Bundles probably want to be annotation-free, so they are a form of implementation of this and will need the on-demand annotation facility.

> - Repository operation disk ordering: Dramatically changing the ordering of disk operations requires a new repository format. We have most of the analysis done to be able to specify the desired ordering, so it should be possible to write such a format now based on the container logic, but without any of the inventory representation or delta representation changes. This would for instance involve pack combining ordering the existing diffs in reverse order.

> - Inventory representation: This has a dependency on what data is dropped from the core and what is kept. Without those changes being known we can implement a new representation, but it won’t be a final one. One of the services the new inventory representation is expected to deliver is one of validators for subtrees – a means of comparing just subtrees of two inventories without comparing all the data within that subtree.

> - Delta storage optimisation: This has a strict dependency on a new repository format. Optimisation takes many forms - we probably cannot complete the desired optimisations under knits though we could use xdelta within a knit-variation.

> - Greatest distance from origin cache: The potential users of this exist today, it is likely able to be implemented immediately, but we are not sure that its needed anymore, so it is being shelved.

> - Removing derivable data: It’s very hard to do this while the derived data is exposed in API’s but not used by commands. Implemented the targeted API’s for our core use cases should allow use to remove accidental use of derived data, making only explicit uses of it visible, and isolating the impact of removing it : allowing us to experiment sensibly. This covers both dropping the per-file merge graph and the hash-based-names proposals.