~bzr-pqm/bzr/bzr.dev

"bzr update" performance analysis

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There are 5 different slightly different situations in which bzr update 

can be used:

* local only (no-op)

* lightweight checkout

* heavy checkout

* heavy checkout w/ local changes

* bzr update could work on "bound branch" w/no wt

No new revisions

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

Should be O(1) to determine

Tree base is up to date

wt.last-rev == wt.b.last-rev

No local changes, only new revisions

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

1) Need to move wt.last_rev (O(1))

2) apply delta from base to new rev (O(changes))

   applying changes to files is approx (O(lines-in-files ^ 2))

3) update meta-info (executable bits, etc) about modified files (O(changes))

2/3 could be concurrent (but that may not necessarily be faster)

potential issue w/ serialized is having 50k files in limbo/

the limbo/ directory could be avoided in some cases, for example when 

adding new files in new directories.

modifying in place: reduces fragmentation of fs, not atomic

w/ local modification, potential of data loss

w/o should be safe

"local mod" is diff between disk and last commit, not merge base

Detecting name conflicts should be O(siblings). Alternatively, conflicts 

with existing files can be detected using stat() and conflicts with new files 

can be detected by examining the pending transform. This changes 

complexity to O(changes).

out of date heavyweight checkout, out of date w/master

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1) open working tree, check latest revision

2) open working tree branch, check latest revision

3) mismatch => update wt => wt.b.lastrev

   apply delta to tree O(changed file size)

   ---- conflicts

   stop on conflicts

   stop always -> inform user they need to repeat (why not?, GFD)

4) pull new revs M => L O(newrevs)

5) apply delta to wt

   local committed changes become a pending merge

   local uncommitted stay uncommitted

   local pending merges are retained (should be gc'd)

offtopic:

should bzr update report where the source is ?

should bzr update handle both cases (local tree out-of-date w/local branch, checkout out-of-date w/master) ?

if updating would diverge, give opportuniuty to branch/unbind instead

local ahead, "push to master"

ideas:

1) can this be done as a single logical step?

2) can this be done w/o modifying working tree until end? possible performance improvements

3) if the pulling revision step could deliver full texts, that may help for the merge (same thing as "bzr pull")