~bzr-pqm/bzr/bzr.dev

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.