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"bzr update" performance analysis
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=================================
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There are 5 different slightly different situations in which bzr update
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* heavy checkout w/ local changes
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* bzr update could work on "bound branch" w/no wt
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Should be O(1) to determine
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Tree base is up to date
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wt.last-rev == wt.b.last-rev
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No local changes, only new revisions
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====================================
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1) Need to move wt.last_rev (O(1))
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2) apply delta from base to new rev (O(changes))
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applying changes to files is approx (O(lines-in-files ^ 2))
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3) update meta-info (executable bits, etc) about modified files (O(changes))
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2/3 could be concurrent (but that may not necessarily be faster)
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potential issue w/ serialized is having 50k files in limbo/
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the limbo/ directory could be avoided in some cases, for example when
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adding new files in new directories.
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modifying in place: reduces fragmentation of fs, not atomic
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w/ local modification, potential of data loss
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"local mod" is diff between disk and last commit, not merge base
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Detecting name conflicts should be O(siblings). Alternatively, conflicts
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with existing files can be detected using stat() and conflicts with new files
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can be detected by examining the pending transform. This changes
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complexity to O(changes).
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out of date heavyweight checkout, out of date w/master
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=======================================================
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1) open working tree, check latest revision
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2) open working tree branch, check latest revision
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3) mismatch => update wt => wt.b.lastrev
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apply delta to tree O(changed file size)
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stop always -> inform user they need to repeat (why not?, GFD)
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4) pull new revs M => L O(newrevs)
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local committed changes become a pending merge
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local uncommitted stay uncommitted
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local pending merges are retained (should be gc'd)
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should bzr update report where the source is ?
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should bzr update handle both cases (local tree out-of-date w/local branch, checkout out-of-date w/master) ?
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if updating would diverge, give opportuniuty to branch/unbind instead
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local ahead, "push to master"
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1) can this be done as a single logical step?
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2) can this be done w/o modifying working tree until end? possible performance improvements
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3) if the pulling revision step could deliver full texts, that may help for the merge (same thing as "bzr pull")
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doc/developers/update.txt
