~bzr-pqm/bzr/bzr.dev

Commit Performance Notes
========================

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.

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

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.


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.