Learn approaches to implementing lightweight sparse branches to help reduce the database load on Perforce servers and avoid needless merges.

1. How to Reduce Database
Load with Sparse Branches
John LoVerso
Software Archeologist

2. 2
MathWorks
 We are a 3500+ person company dedicated to accelerating
the pace of engineering and science
 We have ~90 products based upon our core platforms
• MATLAB – The Language of Technical Computing
• Simulink – Simulation and Model-Based Design

3. 3
Technical Challenges
 Unified code base from which full product family is released
twice a year
 Managing an over 1 million file code base
• 5,000+ components, acyclic dependencies
 Integrating changes from ~1500 developers
on ~270 active branches (streams)

4. 4
Multilevel Streams Hierarchy
//mw/main
//mw/integ2//mw/integ1
//mw/product1 //mw/product5

5. 5
Merge Down and Merge Up
 Build and Test infrastructure blesses submitted changes
• Qualify changes already submitted to the branch
 Merge down from the last blessed change-level
• Almost never merge from the tip
 Cannot copy up; must merge

6. 6
Componentized Streams
 Our branches are built out of components
 We use pairs of stream specs for each branch
 For details, see
• Moving 1000 Users and 100 Branches Into Streams
- from MERGE2014
• Outsmarting Merge Edge Cases in Component Based Development
- from MERGE2016

7. 7
Pair of Streams
 Wide-open development stream specs
Stream: //mw/product
Parent: //mw/integ
Paths:
share …
 Virtual streams computed from component information
Stream: //mw/product~CTB
Parent: //mw/product
Paths:
share matlab/toolbox/prodA/...
share matlab/toolbox/prodB/...

8. 8
Average Week
Changes
Files
Min Average Max Total
New Work 3328 1 8 52 25641
Merges 610 1 1890 39305 1153239
Changes submitted over one week in our main product depot

9. 9
2015.1 Merge Meltdown
 Many regressions in integration engine
 Lost weeks of developer time
 All because we merge all the time

10. 10
What If We Didn’t Need to Merge?
 At any time, the number of files on a branch with unique
changes is small
 The rest of the files are the same as on the parent branch

11. 11
What is Sparse Branching?
 Sparse branching, a.k.a. lightweight or just-in-time
branching, is a strategy where files are only branched when
modified and are otherwise just a reference to a file on the
corresponding parent branch
 Initial creation of a sparse branch is an O(1) operation akin
to creating a clone or snapshot in a copy-on-write filesystem
like ZFS

12. 12
Why not simply use Task Streams?
 They are database expensive
• Order(n) rather than Order(1) to create
• They consume database space until deleted
 They require unnecessary merging in order to be kept up-to-
date with their parent
• Still exposes the user to the vagaries of complex merges
 They have limitations
• You can only merge from their parent
• Can’t recreate them if they are destroyed
• No virtual stream support

13. 13
Some Terms
 Winked-in file
• a file mapped to a revision on an ancestor branch (lazy copy)
 Active file
• a file with changes that have not yet propagated to the parent branch
 Make concrete
• the act of branching a winked-in file in order to make it active

14. 14
Our Approach to Sparse Branches
 A sparse branch is defined by the stream Paths:
• Winked-in files use “import path@change” to map the paths from an
ancestor branch
• Files are made active on the branch by inserting “share” lines for each file
after populating it on the sparse branch
 Moving ahead to a new parent change involves
• Advancing the change number in the “import path@change” directives
• Merging changes down from the parent into active files

15. 15
Pair of Streams
 Wide-open development stream specs
Stream: //mw/product
Parent: //mw/integ
Paths:
share …
 Virtual streams computed from component information
Stream: //mw/product~CTB
Parent: //mw/product
Paths:
share matlab/toolbox/prodA/...
share matlab/toolbox/prodB/...

16. 16
Pair of Streams – Sparse Branch
 Wide-open development stream specs
Stream: //mw/product
Parent: //mw/integ
Paths:
share …
 Virtual streams computed from component information
Stream: //mw/product~CTB
Parent: //mw/product
Paths:
import matlab/toolbox/prodA/... //mw/integ/matlab/toolbox/prodA/…@1000
import matlab/toolbox/prodB/... //mw/integ/matlab/toolbox/prodB/…@1000

17. 17
Pair of Streams – Sparse Branch
With Active Files
 Wide-open development stream specs
Stream: //mw/product
Parent: //mw/integ
Paths:
share …
 Virtual streams computed from component information
Stream: //mw/product~CTB
Parent: //mw/product
Paths:
import matlab/toolbox/prodA/... //mw/integ/matlab/toolbox/prodA/…@1000
import matlab/toolbox/prodB/... //mw/integ/matlab/toolbox/prodB/…@1000
share matlab/toolbox/prodA/file1

18. 18
Multi-level Sparse Hierarchies
 Multi-level sparse hierarchies
Stream: //mw/integ~CTB
Parent: //mw/integ
Paths:
import matlab/toolbox/prodA/... //mw/main/matlab/toolbox/prodA/…@980
import matlab/toolbox/prodB/... //mw/main/matlab/toolbox/prodB/…@980
share matlab/toolbox/prodB/file2
Stream: //mw/product~CTB
Parent: //mw/product
Paths:
import matlab/toolbox/prodA/... //mw/main/matlab/toolbox/prodA/…@980
import matlab/toolbox/prodB/... //mw/main/matlab/toolbox/prodB/…@980
import matlab/toolbox/prodB/file2 //mw/integ/matlab/toolbox/prodB/file2@1000
share matlab/toolbox/prodA/file1

19. 20
Transparent to the User
 Once a sparse branch is created, user commands should be
entirely agnostic to the nature of the branch
• add/edit/delete/move/unshelve/merge should all just work
 Updating to a newer change level of the parent is special
• merge + revert of newly branched files

20. 21
But what about new work?
We have explored two approaches:
 Branch On Edit
• Just-in-time branching of a file as soon as “p4 edit” happens
 Branch On Submit
• Files are opened on the parent branch and are only branched at submit
time

21. 22
Branch On Edit
 Has the benefit of being easier to implement
 Broker wrapper to intercept operations that open files
• edit, delete, move, open, unshelve, merge
• Compute all the files about to be operated on
• Invoke ‘p4 populate’ to make concrete revisions on the sparse branch of
winked-in files
• Invoke ‘p4 flush’ to switch have revision
• Let operation complete to affect opened revision
 Trigger (change-content) to add files to active list on submit

22. 23
How It Looks
$ p4 stream -t sparse_v1 -P //mw/robotics@1705498 //pdb/jloverso/robotics/demo
Stream //pdb/jloverso/robotics/demo created.
$ p4 stream -o //pdb/jloverso/robotics/demo~CTB | tail -2
import matlab/toolbox/robotics/... //mw/robotics/matlab/toolbox/robotics/...@1705498
$ p4 files //pdb/jloverso/robotics/demo/...
//pdb/jloverso/robotics/demo/... - no such file(s).

23. 24
How It Looks - Sync
$ p4 client -s -S //pdb/jloverso/robotics/demo
Client jloverso.demo switched.
$ p4 sync
…
//mw/robotics/matlab/toolbox/robotics/Makefile#3 -
/sandbox/jloverso.demo/matlab/toolbox/robotics/Makefile
//mw/robotics/matlab/toolbox/robotics/baseline.cpp#7 -
/sandbox/jloverso.demo/matlab/toolbox/robotics/baseline.cpp
…

24. 25
How It Looks - Edit
$ p4 have matlab/toolbox/robotics/Makefile
//mw/robotics/matlab/toolbox/robotics/Makefile#3 -
/sandbox/jloverso.demo/matlab/toolbox/robotics/Makefile
$ p4 edit matlab/toolbox/robotics/Makefile
//pdb/jloverso/robotics/demo/matlab/toolbox/robotics/Makefile#1 - opened for edit
$ p4 have matlab/toolbox/robotics/Makefile
//pdb/jloverso/robotics/demo/matlab/toolbox/robotics/Makefile#1 -
/sandbox/jloverso.demo/matlab/toolbox/robotics/Makefile

25. 26
How It Looks – Dynamically Branched
$ p4 files //pdb/jloverso/robotics/demo/...
//pdb/jloverso/robotics/demo/matlab/toolbox/robotics/Makefile#1 - branch change
1722340 (text)
$ p4 filelog //pdb/jloverso/robotics/demo/matlab/toolbox/robotics/Makefile
//pdb/jloverso/robotics/demo/matlab/toolbox/robotics/Makefile
... #1 change 1722340 branch on 2016/03/31 by jloverso@jloverso.demo (text)
'Dynamically branched'
... ... branch from //mw/robotics/matlab/toolbox/robotics/Makefile#1,#4

26. 27
How It Looks - Submit
$ p4 submit -d "new work"
Submitting change 1722341.
Locking 1 files ...
edit //pdb/jloverso/robotics/demo/matlab/toolbox/robotics/Makefile#2
Change 1722341 submitted.
$ p4 stream -o //pdb/jloverso/robotics/demo~CTB | tail -3
import matlab/toolbox/robotics/... //mw/robotics/matlab/toolbox/robotics/...@1705498
share matlab/toolbox/robotics/Makefile

27. 28
Branch On Submit
 Provides a truer version of copy-on-write semantics
 Pending changes are fully discardable with no remnants or
commit server impact
 Requires the ability to re-base (reopen) the files in a pending
change from one branch to another
• Can be done by creating journal entries in order to modify entries in
db.have, db.working, and db.locks tables
• This is known in 2015.2 as “p4 sync –r”

28. 29
Status
 Branch-on-edit in production use for
• private developer branches
• “fixes” branches that are created on-the-fly for each change that is
processed by our Build & Test automation
 We have a prototype of branch-on-submit
• Some limitations; being worked on
 Both versions support multilevel hierarchies

29. 30
Future Plans
 Our goal is to get all non-mainline branches to be sparse
• This is where we can truly reduce database sizes
 Possibility of open-source release of broker and trigger logic
• Some internal dependencies need to be eliminated

30. Thank you!
John.LoVerso@MathWorks.com