~bzr-pqm/bzr/bzr.dev

Commit Performance Notes

========================

.. contents:: :local:

Changes to commit

-----------------

We want to improve the commit code in two phases.

Phase one is to have a better separation from the format-specific logic,

the user interface, and the general process of committing.

Phase two is to have better interfaces by which a good workingtree format

can efficiently pass data to a good storage format.  If we get phase one

right, it will be relatively easy and non-disruptive to bring this in.

Commit: The Minimum Work Required

---------------------------------

Here is a description of the minimum work that commit must do.  We

want to make sure that our design doesn't cost too much more than this

minimum.  I am trying to do this without making too many assumptions

about the underlying storage, but am assuming that the ui and basic

architecture (wt, branch, repo) stays about the same.

The basic purpose of commit is to:

1. create and store a new revision based on the contents of the working tree

2. make this the new basis revision for the working tree

We can do a selected commit of only some files or subtrees.

The best performance we could hope for is:

- stat each versioned selected working file once

- read from the workingtree and write into the repository any new file texts

- in general, do work proportional to the size of the shape (eg

inventory) of the old and new selected trees, and to the total size of

the modified files

In more detail:

1.0 - Store new file texts: if a versioned file contains a new text

there is no avoiding storing it.  To determine which ones have changed

we must go over the workingtree and at least stat each file.  If the

file is modified since it was last hashed, it must be read in.

Ideally we would read it only once, and either notice that it has not

changed, or store it at that point.

On the other hand we want new code to be able to handle files that are

larger than will fit in memory.  We may then need to read each file up

to two times: once to determine if there is a new text and calculate

its hash, and again to store it.

1.1 - Store a tree-shape description (ie inventory or similar.)  This

describes the non-file objects, and provides a reference from the

Revision to the texts within it.

1.2 - Generate and store a new revision object.

1.3 - Do delta-compression on the stored objects.  (git notably does

not do this at commit time, deferring this entirely until later.)

This requires finding the appropriate basis for each modified file: in

the current scheme we get the file id, last-revision from the

dirstate, look into the knit for that text, extract that text in

total, generate a delta, then store that into the knit.  Most delta

operations are O(n**2) to O(n**3) in the size of the modified files.

1.4 - Cache annotation information for the changes: at the moment this

is done as part of the delta storage.  There are some flaws in that

approach, such as that it is not updated when ghosts are filled, and

the annotation can't be re-run with new diff parameters.

2.1 - Make the new revision the basis for the tree, and clear the list

of parents.  Strictly this is all that's logically necessary, unless

the working tree format requires more work.

The dirstate format does require more work, because it caches the

parent tree data for each file within the working tree data.  In

practice this means that every commit rewrites the entire dirstate

file - we could try to avoid rewriting the whole file but this may be

difficult because variable-length data (the last-changed revision id)

is inserted into many rows.

The current dirstate design then seems to mean that any commit of a

single file imposes a cost proportional to the size of the current

workingtree.  Maybe there are other benefits that outweigh this.

Alternatively if it was fast enough for operations to always look at

the original storage of the parent trees we could do without the

cache.

2.2 - Record the observed file hashes into the workingtree control

files.  For the files that we just committed, we have the information

to store a valid hash cache entry: we know their stat information and

the sha1 of the file contents.  This is not strictly necessary to the

speed of commit, but it will be useful later in avoiding reading those

files, and the only cost of doing it now is writing it out.

In fact there are some user interface niceties that complicate this:

3 - Before starting the commit proper, we prompt for a commit message

and in that commit message editor we show a list of the files that

will be committed: basically the output of bzr status.  This is

basically the same as the list of changes we detect while storing the

commit, but because the user will sometimes change the tree after

opening the commit editor and expect the final state to be committed I

think we do have to look for changes twice.  Since it takes the user a

while to enter a message this is not a big problem as long as both the

status summary and the commit are individually fast.

4 - As the commit proceeds (or after?) we show another status-like

summary.  Just printing the names of modified files as they're stored

would be easy.  Recording deleted and renamed files or directories is

more work: this can only be done by reference to the primary parent

tree and requires it be read in.  Worse, reporting renames requires

searching by id across the entire parent tree.   Possibly full

reporting should be a default-off verbose option because it does

require more work beyond the commit itself.

5 - Bazaar currently allows for missing files to be automatically

marked as removed at the time of commit.  Leaving aside the ui

consequences, this means that we have to update the working inventory

to mark these files as removed.  Since as discussed above we always

have to rewrite the dirstate on commit this is not substantial, though

we should make sure we do this in one pass, not two.  I have

previously proposed to make this behaviour a non-default option.

We may need to run hooks or generate signatures during commit, but

they don't seem to have substantial performance consequences.

If one wanted to optimize solely for the speed of commit I think

hash-addressed  file-per-text storage like in git (or bzr 0.1) is very

good.  Remarkably, it does not need to read the inventory for the

previous revision.  For each versioned file, we just need to get its

hash, either by reading the file or validating its stat data.  If that

hash is not already in the repository, the file is just copied in and

compressed.  As directories are traversed, they're turned into texts

and stored as well, and then finally the revision is too.  This does

depend on later doing some delta compression of these texts.

Variations on this are possible.  Rather than writing a single file

into the repository for each text, we could fold them into a single

collation or pack file.  That would create a smaller number of files

in the repository, but looking up a single text would require looking

into their indexes rather than just asking the filesystem.

Rather than using hashes we can use file-id/rev-id pairs as at

present, which has several consequences pro and con.

Commit vs Status

----------------

At first glance, commit simply stores the changes status reports. In fact,

this isn't technically correct: commit considers some files modified that

status does not. The notes below were put together by John Arbash Meinel

and Aaron Bentley in May 2007 to explain the finer details of commit to

Ian Clatworthy. They are recorded here as they are likely to be useful to

others new to Bazaar ...

1) **Unknown files have a different effect.** With --no-strict (the default)

   they have no effect and can be completely ignored. With --strict they

   should cause the commit to abort (so you don't forget to add the two new

   test files that you just created).

2) **Multiple parents.** 'status' always compares 2 trees, typically the

   last-committed tree and the current working tree. 'commit' will compare

   more trees if there has been a merge.

  a) The "last modified" property for files.

     A file may be marked as changed since the last commit, but that

     change may have come in from the merge, and the change could have

     happened several commits back. There are several edge cases to be

     handled here, like if both branches modified the same file, or if

     just one branch modified it.

  b) The trickier case is when a file appears unmodified since last

     commit, but it was modified versus one of the merged branches. I

     believe there are a few ways this can happen, like if a merged

     branch changes a file and then reverts it back (you still update

     the 'last modified' field).

     In general, if both sides disagree on the 'last-modified' flag,

     then you need to generate a new entry pointing 'last-modified' at

     this revision (because you are resolving the differences between

     the 2 parents).

3) **Automatic deletion of 'missing' files.** This is a point that we go

   back and forth on. I think the basic idea is that 'bzr commit' by

   default should abort if it finds a 'missing' file (in case that file was

   renamed rather than deleted), but 'bzr commit --auto' can add unknown

   files and remove missing files automatically.

4) **sha1 for newly added files.** status doesn't really need this: it should

   only care that the file is not present in base, but is present now. In

   some ways commit doesn't care either, since it needs to read and sha the

   file itself anyway.

5) **Nested trees.** status doesn't recurse into nested trees, but commit does.

   This is just because not all of the nested-trees work has been merged yet.

   A tree-reference is considered modified if the subtree has been

   committed since the last containing-tree commit.  But commit needs to

   recurse into every subtree, to ensure that a commit is done if the

   subtree has changed since its last commit.  _iter_changes only reports

   on tree-references that are modified, so it can't be used for doing

   subtree commits.

Avoiding Work: Smarter Change Detection

---------------------------------------

Commit currently walks through every file building an inventory. Here is

Aaron's brain dump on a better way ...

_iter_changes won't tell us about tree references that haven't changed,

even if those subtrees have changed.  (Unless we ask for unchanged

files, which we don't want to do, of course.)

There is an iter_references method, but using it looks just as expensive

as calling kind().

I did some work on updating commit to use iter_changes, but found for

multi-parent trees, I had to fall back to the slow inventory comparison

approach.

Really, I think we need a call akin to iter_changes that handles

multiple parents, and knows to emit entries when InventoryEntry.revision

is all that's changed.

Avoiding Work: Better Layering

------------------------------

For each file, commit is currently doing more work than it should. Here is

John's take on a better way ...

Note that "_iter_changes" *does* have to touch every path on disk, but

it just can do it in a more efficient manner. (It doesn't have to create

an InventoryEntry for all the ones that haven't changed).

I agree with Aaron that we need something a little different than

_iter_changes. Both because of handling multiple parents, as well as we

don't want it to actually read the files if we have a stat-cache miss.

Specifically, the commit code *has* to read the files because it is

going to add the text to the repository, and we want it to compute the

sha1 at *that* time, so we are guaranteed to have the valid sha (rather

than just whatever the last cached one was). So we want the code to

return 'None' if it doesn't have an up-to-date sha1, rather than reading

the file and computing it, just before it returns it to the parent.

The commit code (0.16) should really be restructured. It's layering is

pretty wrong.

Specifically, calling "kind()" requires a stat of the file. But we have

to do a stat to get the size/whether the record is up-to-date, etc. So

we really need to have a "create_an_up_to_date_inventory()" function.

But because we are accessing every object on disk, we want to be working

in tuples rather than Inventory objects. And because DirState already

has the parent records next to the current working inventory, it can do

all the work to do really fast comparison and throw-away of unimportant

records.

The way I made "bzr status" fast is by moving the 'ignore this record'

ability as deep into the stack as I could get. Status has the property

that you don't care about most of the records, just like commit. So the

sooner you can stop evaluating the 99% that you don't care about, the

less work you do.

Avoiding work: avoiding reading parent data

-------------------------------------------

We would like to avoid the work of reading any data about the parent

revisions.  We should at least try to avoid reading anything from the

repository; we can also consider whether it is possible or useful to hold

less parent information in the working tree.

When a commit of selected files is requested, the committed snapshot is a

composite of some directories from the parent revision and some from the

working tree.  In this case it is logically necessary to have the parent

inventory information.

If file last-change information or per-file graph information is stored

then it must be available from the parent trees.

If the Branch's storage method does delta compression at commit time it

may need to retrieve file or inventory texts from the repository.

It is desirable to avoid roundtrips to the Repository during commit,

particularly because it may be remote.  If the WorkingTree can determine

by itself that a text was in the parent and therefore should be in the

Repository that avoids one roundtrip per file.

There is a possibility here that the parent revision is not stored, or not

correctly stored, in the repository the tree is being committed into, and

so the committed tree would not be reconstructable.  We could check that

the parent revision is present in the inventory and rely on the invariant

that if a revision is present, everything to reconstruct it will be

present too.

Code structure

--------------

Caller starts a commit

>>> Branch.commit(from_tree, options)

This creates a CommitBuilder object matched to the Branch, Repository and

Tree.  It can vary depending on model differences or by knowledge of what

is efficient with the Repository and Tree.  Model differences might

include whether no-text-change merges need to be reported, and whether the

The basic CommitBuilder.commit structure can be

1. Ask the branch if it is ready to commit (up to date with master if

   any.)

2. Ask the tree if it is ready to commit to the branch (up to date with

   branch?), no conflicts, etc

3. Commit changed files; prototype implementation:

   a. Ask the working tree for all committable files; for each it should

      return the per-file parents, stat information, kind, etc.

   b. Ask the repository to store the new file text; the repository should

      return the stored sha1 and new revision id.

4. Commit changed inventory

5. Commit revision object

Complications of commit

-----------------------

Bazaar (as of 0.17) does not support selective-file commit of a merge;

this could be done if we decide how it should be recorded - is this to be

stored as an overall merge revision; as a preliminary non-merge revisions;

or will the per-file graph diverge from the revision graph.

There are several checks that may cause the commit to be refused, which

may be activated or deactivated by options.

* presence of conflicts in the tree

* presence of unknown files

* the working tree basis is up to date with the branch tip

* the local branch is up to date with the master branch, if there

  is one and --local is not specified

* an empty commit message is given,

* a hook flags an error

* a "pointless" commit, with no inventory changes

Most of these require walking the tree and can be easily done while

recording the tree shape.  This does require that it be possible to abort

the commit after the tree changes have been recorded.  It could be ok to

either leave the unreachable partly-committed records in the repository,

or to roll back.

Other complications:

* when automatically adding new files or deleting missing files during

  commit, they must be noted during commit and written into the working

  tree at some point

* refuse "pointless" commits with no file changes - should be easy by

  just refusing to do the final step of storing a new overall inventory

  and revision object

* heuristic detection of renames between add and delete (out of scope for

  this change)

* pushing changes to a master branch if any

* running hooks, pre and post commit

* prompting for a commit message if necessary, including a list of the

  changes that have already been observed

* if there are tree references and recursing into them is enabled, then

  do so

Commit needs to protect against duplicated file ids

Updates that need to be made in the working tree, either on conclusion

of commit or during the scan, include

* Changes made to the tree shape, including automatic adds, renames or

  deletes

* For trees (eg dirstate) that cache parent inventories, the old parent

  information must be removed and the new one inserted

* The tree hashcache information should be updated to reflect the stat

  value at which the file was the same as the committed version, and the

  content hash it was observed to have.  This needs to be done carefully to

  prevent inconsistencies if the file is modified during or shortly after

  the commit.  Perhaps it would work to read the mtime of the file before we

  read its text to commit.

Interface stack

---------------

The commit api is invoked by the command interface, and copies information

from the tree into the branch and its repository, possibly updating the

WorkingTree afterwards.

The command interface passes:

* a commit message (from an option, if any),

* or an indication that it should be read interactively from the ui object;

* a list of files to commit

* an option for a dry-run commit

* verbose option, or callback to indicate

* timestamp, timezone, committer, chosen revision id

* config (for what?)

* option for local-only commit on a bound branch

* option for strict commits (fail if there are unknown or missing files)

* option to allow "pointless" commits (with no tree changes)

(This is rather a lot of options to pass individually and just for code tidyness maybe some of them should be combine into objects.)

>>> Branch.commit(from_tree, message, files_to_commit, ...)

There will be different implementations of this for different Branch

classes, whether for foreign branches or Bazaar repositories using

different storage methods.

Most of the commit should occur during a single lockstep iteration across

the workingtree and parent trees.  The WorkingTree interface needs to

provide methods that give commit all it needs.  Some of these methods

(such as answering the file's last change revision) may be deprecated in

newer working trees and there we have a choice of either calculating the

value from the data that is present, or refusing to support commit to

newer repositories.

For a dirstate tree the iteration of changes from the parent can easily be

done within its own iter_changes.

Dirstate inventories may be most easily updated in a single operation at

the end; however it may be best to accumulate data as we proceed through

the tree rather than revisiting it at the end.

Showing a progress bar for commit may not be necessary if we report files

as they are committed.  Alternatively we could transiently show a progress

bar for each directory that's scanned, even if no changes are observed.

This needs to collect a list of added/changed/removed files, each of which

must have its text stored (if any) and containing directory updated.  This

can be done by calling Tree._iter_changes on the source tree, asking for

changes

In the 0.17 model the commit operation needs to know the per-file parents

and per-file last-changed revision.

(In this and other operations we must avoid having multiple layers walk

over the tree separately.  For example, it is no good to have the Command

layer walk the tree to generate a list of all file ids to commit, because

the tree will also be walked later.  The layers that do need to operate

per-file should probably be bound together in a per-dirblock iterator,

rather than each iterating independently.)

Branch->Tree interface

----------------------

The Branch commit code needs to ask the Tree what should be committed, in

terms of changes from the parent revisions.  If the Tree holds all the

necessary parent tree information itself it can do it single handed;

otherwise it may need to ask the Repository for parent information.

This should be a streaming interface, probably like iter_changes returning

information per directory block.

The interface should not return a block for directories that are

recursively unchanged.

The tree's idea of what is possibly changed may be more conservative than

that of the branch.  For example the tree may report on merges of files

where the text is identical to the parents: this must be recorded for

Bazaar branches that record per-file ancestry but is not necessary for all

branches.  If the tree is responsible for determining when directories

have been recursively modified then it will report on all the parents of

such files.  There are several implementation options:

1. Return all files and directories the branch might want to commit, even

if the branch ends up taking no action on them.

2. When starting the iteration, the branch can specify what type of change

is considered interesting.

Since these types of changes are probably (??) rare compared to files that

are either completely unmodified or substantially modified, the first may

be the best and simplest option.

The branch needs to build an inventory to commit, which must include

unchanged files within changed directories.  This should be returned from

the working tree too.  Repositories that store per-directory inventories

will want to build and store these from the lowest directories up.

For 0.17 format repositories with an all-in-one inventory it may be

easiest to accumulate inventory entries in arbitrary order into an

in-memory Inventory and then serialize it.

It ought to be possible to commit any Tree into a Branch, without

requiring a WorkingTree; the commit code should cope if the tree is not

interested in updating hashcache information or does not have a

``last_revision``.

Information from the tree to repository

---------------------------------------

The main things the tree needs to tell the Branch about are:

* A file is modified from its parent revision (in text, permissions,

  other), and so its text may need to be stored.

  Files should also be reported if they have more than one unique parent

  revision, for repositories that store per-file graphs or last-change

  revisions.  Perhaps this behaviour should be optional.

  **XXX:** are renames/deletions reported here too?

* The complete contents of a modified directory, so that its inventory

  text may be stored.  This should be done after all the contained files

  and directories have been reported.  If there are unmodified files,

  or unselected files carried through from

  XXX: Actually perhaps not grouped by directory, but rather grouped

  appropriately for the shape of inventory storage in the repository.

  In a zoomed-in checkout the workingtree may not have all the shape data

  for the entire tree.

* A file is missing -- could cause either automatic removal or an aborted

  commit.

* Any unknown files -- can cause automatic addition, abortion of a strict

  commit, or just reporting.

Information from the repository to the tree

-------------------------------------------

After the commit the tree needs to be updated to the new revision.  Some

information which was accumulated during the commit must be made available

to the workingtree.  It's probably reasonable to hold it all in memory and

allow the workingtree to get it in whatever order it wants.

* A list of modified entries, and for each one:

  * The stat values observed when the file was first read.

  * The hash of the committed file text.

  * The file's last-change revision, if appropriate.

  This should include any entries automatically added or removed.

This might be construed as an enhanced version of ``set_parent_trees``.

We can avoid a stat on each file by using the value that was observed when

it was first read.

Selective commit

----------------

For a partial commit the directory contents may need to contain a mix of

entries from the working tree and parent trees.  This code probably

shouldn't live in a specific tree implementation; maybe there should be a

general filter that selects paths from one tree into another?

However, the tree walking code does probably need to know about selected

paths to avoid examining unselected files or directories.

We never refuse selective file commits (except of merges).

Common commit code

------------------

What is common to all commit implementations, regardless of workingtree or

repository format?

* Prompting for a commit message?

* Strictness/conflict checks?

* Auto add/remove?

How should this be separated?

Order of traversal

------------------

For current and contemplated Bazaar storage formats, we can only finally

commit a directory after its contained files and directories have been

committed.

The dirstate workingtree format naturally iterates by directory in order

by path, yielding directories before their contents.  This may also be the

most efficient order in which to stat and read the files.

One option would be to construe the interface as a visitor which reports

when files are detected to be changed, and also when directories are

finished.

Open question: per-file graphs

------------------------------

**XXX:** If we want to retain explicitly stored per-file graphs, it would

seem that we do need to record per-file parents.  We have not yet finally

settled that we do want to remove them or treat them as a cache.  This api

stack is still ok whether we do or not, but the internals of it may

change.