RubyConf 2016

Ruby 3
Concurrency
Koichi Sasada
ko1@heroku.com

RubyKaigi 2016

A proposal of
new concurrency modelfor Ruby 3
Koichi Sasada
ko1@heroku.com

Motivation
Productivity• Thread programming is very difficult• Making correct concurrent programs easily

Performance by Parallel execution• Making parallel programs• Threads can make concurrent programs, but can’t run them inparallel• People want to utilize Multi/many CPU cores

RubyKaigi2016 Proposal
Guild: new concurrency abstraction for Ruby 3• Idea: DO NOT SHARE mutable objects between Guilds• → No data races, no race conditions

Replace Threads to Guilds

RubyKaigi2016 Proposal
Guild: new concurrency abstraction for Ruby 3• Idea: DO NOT SHARE mutable objects between Guilds• → No data races, no race conditions

Kill Threads

Today’s talkWhy is thread programming difficult?

Why does Guild solve this difficulty?
I’ll try to shrink this talk (but has 70 pages).
Long version talk at RubyKaigi2016 is available:
http://rubykaigi.org/2016/presentations/ko1.html

NOTE“Guild” is proposal for Ruby 3.
Specifications
and name of “Guild”
can be changed.

Koichi Sasada•A programmer living in Tokyo, Japan•Ruby core committer since 2007
•YARV, Fiber, … (Ruby 1.9)
•RGenGC, RincGC (Ruby 2…)

Koichi is an Employee

Koichi is an Employee

Visit Heroku booth and discuss more!

Difficulty ofMulti-threads programmingMuilti-threads programming is difficult• Introduce data race, race condition• Introduce deadlock, livelock
Difficult to make• Difficulty on debugging because of correct (bug-free)programs
nondeterministic behavior• difficult to reproduce same problem• Difficult to tune performance

Difficult to makefast programs

Data race and race condition• Traditional “Bank amount transfer” example• Quoted from Race Condition vs. Data Race
http://blog.regehr.org/archives/490
def transfer1 (amount, account_from, account_to)if (account_from.balance < amount) return NOPE
account_to.balance += amountaccount_from.balance -= amountreturn YEP
end

Data race and race condition
def transfer1 (amount, account_from, account_to)if (account_from.balance < amount) return NOPE
account_to.balance += amountaccount_from.balance -= amountreturn YEP
end

Can you find all bugs?

Data race and race condition
def transfer1 (amount, account_from, account_to)if (account_from.balance < amount) return NOPE
account_to.balance += amountaccount_from.balance -= amountreturn YEP
Data
Race
end

Data race and race condition
def transfer1 (amount, account_from, account_to)if (account_from.balance < amount) return NOPE
account_to.balance += amountaccount_from.balance -= amountreturn YEP
Race ConditionendData race and race condition• Solution: Lock (synchronize) all over the method
def transfer1 (amount, account_from, account_to)Thread.exclusive{if (account_from.balance < amount) return NOPE
account_to.balance += amountaccount_from.balance -= amountreturn YEP}
endDifficulty of multi-threads programs• We need to synchronize all sharing mutableobjects correctly• Easy to share objects, but difficult to recognize• We can track on a small program• Difficult to track on a big programs, especially on
programs using gems

• We need to check all of source codes, or believe
library documents (but documents should be correct)

Overcome thread difficultyKey idea
Problem of multi-thread programming:
Easy to share mutable objects
Idea:
Do not allow to share mutable objects
without any restriction

Study from other languages• Do not share mutable objects• Copy to send message (shell, druby, …)•  Copy everything is slow
• Prohibit mutable objects (functional lang, Erlang, Elxir)•  We can’t accept such big incompatibility• Share only immutable objects (Place (Racket))•  We want to share other kind of objects• Allow sharing with restriction• Allow mutation only with special protocol (Clojure)•  we can’t accept special protocol

NOTE: we do not list approaches using “type system” like Rust

Our goal for Ruby 3• We need to keep compatibility with Ruby 2.
• We can make parallel program.
• We shouldn’t consider about locks any more.
• We can share objects with copy, but copy
operation should be fast.
• We should share immutable objects if we can.
• We can provide special objects to share mutableobjects like Clojure if we really need speed.

“Guild”
New concurrency model for Ruby 3

Guild: New concurrency abstraction• Guild has at least one thread (and a thread has
at least one fiber)

Guild
ThreadFiber
FiberThread
Fiber

Guild
Guild
ThreadFiberThreadFiberThreads in different guilds can run in
Parallel• Threads in different guilds can run in parallel• Threads in a same guild can not run in parallelbecause of GVL (or GGL: Giant Guild Lock)
Acquire GGL

G1:T1
G1:T2
G2:T3

Acquire GGL

Important rule:
Mutable Objects have a membership•All of mutable objects should belong to only
one Guild exclusively•Because Guild is not “Community”
Guild 1
obj

obj
obj

NG!!
Can’t access(read/write)

Guild 2obj
obj

Object membership
Only one guild can access mutable object→ We don’t need to consider about locks(if Guild has only one thread)Inter-guild communication•“Guild::Channel” to communicate each guilds•Two communication methods
1. Copy
2. Move (transfer_membership)Copy using Channel
channel.transfer(o1)o1 = channel.receive

Guild1

Guild2o1channelo2
O2:Datao1o2o3
O3:DataO2:Data

COPY

o3
O3:DataMove using Channel
channel.transfer_membership(o1)o1 = channel.receive

Guild1

Guild2o1channelo2
O2:Datao3
O3:DataMOVE

Move using Channel
channel.transfer_membership(o1)o1 = channel.receive

Guild1

Guild2-o1channel-o2

From Guild1 perspective,
transferred objects are invalidated

O2:DataMOVE

o3
O3:DataMove using Channel• Prohibit accessing to left objects• Cause exceptions and so on• ex) obj = “foo”
ch.move (obj)
obj.upcase#=> Error!!
p(obj)#=> Error!!

Use cases for copy and move
• You can copy small objects (dRuby does)• Parameter array ([:do_foo, 1, 2, 3], like Erlang)• You can move small amount number of objects• Move a long string and modify them in parallelSharing immutable objects• Immutable objects can be shared with any
guilds• a1 = [1, 2, 3].freeze: a1 is Immutable object• a2 = [1, Object.new, 3].freeze: a2 is not immutable• We only need to send references• Very lightweight, like thread-programming• Numeric objects, symbols, true, false, nil are
immutable (from Ruby 2.0, 2.1, 2.2)

Sharing immutable objects
We can share reference to immutable objects
channel.transfer(o1)o1 = channel.receive

Guild1

Guild2

Ref too1
readchannelRef to
o1

o1

read

o2

o3
If o1 is immutable, any Guild can read o1
O2:Data

O3:DataUse-case 1: master – worker typedef fib(n) ... end
g_fib = Guild.new(script: %q{ch = Guild.default_channel
while n, return_ch = ch.receive
return_ch.transfer fib(n)end})
ch = Guild::Channel.new
g_fib.transfer([3, ch])p ch.receiven, return_ch

Main
Guild

ch

Fibonacci
Guild

return_ch

Answer of fib(n)NOTE: Making other Fibonacci guilds,
you can compute fib(n) in parallelUse-case 2: pipeline
result_ch = Guild::Channel.new
g_pipe3 = Guild.new(script: %q{
while obj = Guild.default_channel.receive
obj = modify_obj3(obj)
Guild.argv[0].transfer_membership(obj)end}, argv: [result_ch])g_pipe2 = Guild.new(script: %q{
while obj = Guild.default_channel.receive
obj = modify_obj2(obj)
Guild.argv[0].transfer_membership(obj)end}, argv: [g_pipe3])
g_pipe1 = Guild.new(script: %q{
while obj = Guild.default_channel.receive
obj = modify_obj1(obj)
Guild.argv[0].transfer_membership(obj)end}, argv: [g_pipe2])
obj = SomeClass.new
g_pipe1.transfer_membership(obj)obj = result_ch.receive

Main
Guild

Obj’’’
Move

Obj’’’

obj

Pipe 1
Guild

Pipe 3
Guild

Moveand modify

Obj’

Move
and modify

Moveand modify

Pipe 2
Guild
Obj’’

Use-case:
Bank example
g_bank = Guild.new(script: %q{
while account_from, account_to, amount,ch = Guild.default_channel.receive
if (Bank[account_from].balance < amount)ch.transfer :NOPE
else
Bank[account_to].balance += amount
Bank[account_from].balance -= amountch.transfer :YEP
end
end
})
…

Only bank guild maintains bank data

Bank
Guild
requests

Other
guildsOther
guildsUse-case:
Introduce special data structure• Ideas of special data
structure to sharemutable objects• Use external RDB• In process/external
Key/value store• Software transactional
memory
•…

??

Other
guildsOther
guildsCompare between threads and guilds• Threads:•  Inter threads communication is very fast•  We already know thread-programming•  Difficult to make correct thread-safe programs• Guilds:•  Inter guilds communication introduces overhead•  “Move” technique can reduce this kind of overheads•  We need to learn this model•  We need to make parallel programs from scratch•  We don’t need to care about synchronizations any more

Trade-off: Performance v.s. Safety/Easily
Which do you want to choose?

Discussion: The name of “Guild”• “Guild” is good metaphor for “object’s membership”• Check duplication• Nobody using as programming terminology (maybe)• There are no duplicating top-level classes and modules
in all of rubygems• First letter is not same as other similar abstractions• For variable names• P is for Processes, T is for Threads, F is for Fibers

Implementation of “Guild”• How to achieve “object membership”• How to implement “Inter Guilds communication”• How to design “shared mutable data”• How to isolate “process global data”

How to implement inter Guilds
communication• Copy• Move (transfer membership)Copy using Channel
channel.transfer(o1)o1 = channel.receive

Guild1

Guild2o1channelo2
O2:Datao1o2o3
O3:DataO2:Data

COPY

o3
O3:DataCopy using Channel
Implementation
channel.transfer(o1)o1 = channel.receive

Guild1

Guild2(1) Make
deep copy

o1

o1

o2
O2:Datachannelo2o3
O3:DataO2:Datao3
O3:DataCopy using Channel
Implementation

We can use CoWtechnique for datachannel.transfer(o1)o1 = channel.receive

Guild1

Guild2o1channelo2
O2:Datao1o2o3
O3:Data(2) Move/Join

O2:Datao3
O3:DataMove using Channel
channel.transfer_membership(o1)o1 = channel.receive

Guild1

Guild2o1channelo2
O2:Datao3
O3:DataMOVE

Move using Channel
channel.transfer_membership(o1)o1 = channel.receive

Guild1

Guild2-o1channel-o2

From Guild1 perspective,
transferred objects are invalidated

O2:DataMOVE

o3
O3:DataMove using Channel
Implementation
channel.transfer_membership(o1)o1 = channel.receive

Guild1

Guild2(1) Make
deep copy

o1
-

o1

channel(2) Invalidate originals

o2
-

o2

o3
O2:Data

O3:Datao3

Move using Channel
Implementation
channel.transfer_membership(o1)o1 = channel.receive

Guild2

Guild1o1(2) Invalidate originals

channel

o2
-

o2

o3
-

o3
(3) Move/Join

O2:Datao1

O3:DataMove using Channel
Implementation• “Move” is not a reference passing,but a copy object headers→ Objects don’t need to know own guild→ Interpreter doesn’t need to check guilds• Mutable objects live in same guild their entire life

Ruby global data
• Global variables ($foo)• Change them to Guild local variables• Class and module objects• Share between guilds• Class variables• Change them to guild local. So that it is guild/class local variables• Constants• Share between guilds• However if assigned object is not a immutable object, this constant is accessed only by setting guilds. If otherguilds try to access it, them cause error.

• Instance variables of class and module objects• Difficult. There are several approaches.

• Proc/Binding objects• Make it copy-able with env objects or env independent objects• ObjectSpace.each_object• OMG

Interpreter process global data
• GC/Heap• Share it. Do stop the world parallel marking- and lazy concurrent sweeping.
• Synchronize only at page acquire timing. No any synchronization at creation time.

• Inline method cache• To fill new entry, create an inline cache object and update atomically.

• Tables (such as method tables and constant tables)• Introduce mutual exclusions.

• Current working directory (cwd)• Each guild should have own cwd (using openat and so on).

• Signal• Design new signal delivery protocol and mechanism• C level global variables• Avoid them.
• Main guild can use C extensions depends on them

• Current thread• Use TLS (temporary), but we will change all of C APIs to receive context data as first parameter in the future.

Performance evaluation• On 2 core virtual machine• Linux on VirtualBox on Windows 7• Now, we can’t run Ruby program on other thanmain guild, so other guilds are implemented by C
code

Performance evaluation
Simple numeric task in parallel
Main
Guild

Fibonacci
Guild
Fibonacci
Guild
Fibonacci
Guild
Fibonacci
Guild

Total 50 requests to compute fib(40)Send 40 (integer) in each request

Execution
time (sec)Single-Guild
19.45
Multi-Guild

10.45

Performance evaluation
Copy/Move
sum

Main
Guild

sum

sum
sum
Guild

Execution
time (sec)Single-Guild
1.00
Multi/ref

0.64

Multi/move

4.29

Multi/copy

5.16

Total 100 requests to compute sum of array
Send (1..10_000_000).to_a in each request

Too slow!!
Because “move” need tocheck all of elementsPerformance evaluation
Copy/Move
sum

Main
Guild

sum

sum
sum
Guild

Execution
time (sec)Single-Guild
1.00

Multi/ref

0.64

Multi/move

0.64

If we know this array only has immutable objects,
we don’t need to check all elements => special data structureCheck our goal for Ruby 3• We need to keep compatibility with Ruby 2.
• OK: Only in main guild, it is compatible.

• We can make parallel program.
• OK: Guilds can run in parallel.

• We shouldn’t consider about locks any more.

• OK: Only using copy and move, we don’t need to care locks.

• We can share objects with copy, but copy operation should be fast.
• OK: Move (transfer membership) idea can reduce overhead.

• We should share objects if we can.

• OK: We can share immutable objects fast and easily.

• We can provide special objects to share mutable objects like Clojure
if we really need speed.
• OK: Yes, we can provide.

FAQ• Q: Can we try Guild now?
• A: No.
• Implementation on MRI is big project. Not yet.
• Supporting this project is welcome.

• Some guys are trying to implement it on JRuby.

FAQ• Q: Should we wait Guild for Ruby 3?
• A: Not sure.
• 2.6? 2.7? 2.8?
• I want to implement it next year.

FAQ• Q: Can Guild replace ALL of Thread programs?
• A: No.
• To utilize Guild, you need to rewrite your programs.
• I assume 90% of programs are easy to replace.
• For example, “moving” IO object is easy tounderstand, so that web application server is easy
to implement.

FAQ• Q: Membership seems “ownership”. Right?
• A: Yes.
• Actually, we call this idea “ownership” before.
• We named “membership” because “Guild” is notowner of members.

FAQ• Q: “Moving” cause huge overhead for big object
graph (like big Hash object). Right?
• A: Yes.
• We need to move all of objects (e.g. Hash entries).
• We need to introduce special data structures for such
big object graph (like Clojure).
• I believe people can change their mind to fit this model.

FAQ• Q: Can we share Proc object?
• A: No.
• Good question. I’m thinking several options:• Allow to copy local environment (variables)• Allow to move local environment (variables)• Introduce isolated Proc

Summary• Introduce “why threads are very difficult”• Propose new concurrency abstraction “Guild” forRuby 3• Not implemented everything yet, but I show key
ideas and preliminary evaluation

Thank you for your attention
Koichi Sasada<ko1@heroku.com>

Approach comparison
Process/MVM

Place (Racket)

Guild(copy/move)ThreadHeap (GC)

Separate

Separate

Share

Share

Communication
Mutable objects

Copy

Copy

Copy/Move

Share

Communication
Immutable object

Copy

Share (maybe)

Share

Share

Lock

Don’t need

Don’t need(mostly) Don’t need

Required

ISeq (bytecode)

Copy

Share

Share

Share

Class/Module(namespace)

Copy

Copy (fork)

Share

Share

Related work
• “Membership transfer” is proposed by[Nakagawa 2012], but not completed• Alias analysis with type systems• Ruby doesn’t support static type checking• Dynamic alias analysis with runtime checking• We need to reduce dynamic check overhead• We can’t insert dynamic checking completely (thisis why I found “membership transfer”)