[ruby-core:100662] [Ruby master Feature#17261] Software transactional memory (STM) for Threads and Ractors

[ko1@atdot.net]

Issue #17261 has been updated by ko1 (Koichi Sasada).


chrisseaton (Chris Seaton) wrote in #note-14:
> I think there's benefits to building STM into the language (if we decided we want STM at all) rather than it being a library.

I think so, especially with Ractors, but now we (*I*) don't have enough evidence to persuade, so we can ask later with use cases.

> When you look at more advanced features like conflict resolution and conflict mitigation you may want to do more lower level things like control scheduling.

Yeah it will be fun work.

----------------------------------------
Feature #17261: Software transactional memory (STM) for Threads and Ractors
https://bugs.ruby-lang.org/issues/17261#change-88302

* Author: ko1 (Koichi Sasada)
* Status: Rejected
* Priority: Normal
----------------------------------------
## Abstract

I propose Software transactional memory (STM) for threads and ractors.

Implementation is here: https://github.com/ruby/ruby/pull/3652

The interface is similar to concurrent-ruby, but not the same.
http://ruby-concurrency.github.io/concurrent-ruby/1.1.4/Concurrent/TVar.html

## Basic concept

https://en.wikipedia.org/wiki/Software_transactional_memory
Transaction is popular idea on data base systems to keep state consistency.

STM is similar idea to implement optimistic synchronization strategy.

There are several advantages compare with traditional synchronization techniques like Mutex and so on:

* Performance: in some cases, it is faster because of optimistic nature.
* Composability: multiple locks can introduce dead-lock. STM allows nested transaction. In other words, (some kind of) STM can guarantee the progressiveness.

The disadvantages is, it can lead slow down on high-contention cases.

## API

* `Thread::atomically do expr end`: make a new transaction and run `expr` in it. `expr` can be retried if the conflict is detected.
* `Thread::TVar.new(default_value)`
* `Thread::TVar#value`: get current value of TVar
* `Thread::TVar#value = val`: set TVar value `val`.
* `Thread::TVar#increment(n=1)`: Just same as `Thread.atomically{ tv.value += 1 }`.

Note that `expr` for `Thread.atomically` can retries and all `TVar#value=` (set TVar values) are reverted before retries. Another operations such as other memory modification, IO operations includes network operations etc are not reverted.

The very difference between `Concurrent::TVar` is:

* TVar only refer to shareable objects to support Ractor.
* `TVar#value=` should be used with `atomically`. We can define as `Thread.atomically{ tv.value = val }`, but it can lead misusing without `atomically`.
* `TVar#increment` is special case to allow setting without `atomically` to support typical single counter cases.

## Implementation

https://github.com/ruby/ruby/pull/3652

The implementation is almost same as TL2, lock-based STM with global version clock with pthread/win32 threads.
We can use atomic operations but not supported yet (but only a few performance benefit on my measuremnets).

## Example

```ruby
N = 1_000_000

tv1 = Thread::TVar.new(0)
tv2 = Thread::TVar.new(0)

r1 = Ractor.new tv1, tv2 do |tv1, tv2|
  loop do
    Thread.atomically do
      v1, v2 = tv1.value, tv2.value
      raise if v1 != v2
    end
  end
end

rs = 3.times.map do
  Ractor.new tv1, tv2 do |tv1, tv2|
    N.times do
      Thread.atomically do
        tv1.value += 1
        tv2.value += 1
      end
    end
  end
end

rs.each{|r| r.take}
p [tv1.value, tv2.value] #=> [3000000, 3000000]
```

In this case, 

* all `atomically` blocks keep consistency that `tv1.value == tv2.value`.
* the results `[3000000, 3000000]` shows consistency on `+=1`.

Here is famous bank-account example:


```ruby
class Account
  COUNT = Thread::TVar.new 0

  def initialize deposit = 0
    @i = COUNT.increment
    @balance = Thread::TVar.new(deposit)
  end

  def transfer_from acc, n
    Thread::atomically do
      acc.withdraw n
      self.deposit n
    end
  end

  def transfer_to acc, n
    Thread::atomically do
      self.withdraw n
      acc.deposit n
    end
  end

  def withdraw n
    @balance.value -= n
  end

  def deposit n
    @balance.value += n
  end

  def balance
    @balance.value
  end
end

AN =  1_0000
N = 10_000_000
RN = 10
iter = 0
accs = AN.times.map{Account.new.freeze}.freeze

require 'benchmark'

# :forward
#   two ractors operate N times: a[i].transfer(a[i+1])
#     R1: a1->a2, a2->a3, ...
#     R2: a1->a2, a2->a3, ...

# :reverse
#   two ractors operate N times: a[i].transfer(a[i+1]),
#   but the oroder of accounts are reversed.
#     R1: a1->a2, a2->a3, ...
#     R2: a1->aN-1, a2->aN-2, ...

# :shuffle
#   RN ractors operate N times: a[rand].transfer(a[rand])
#   It simulates normal bank-operation

mode = :shuffle

loop do
  iter += 1

  btime = Time.now

  case mode
  when :forward
    rs = []

    rs << Ractor.new(accs) do |accs|
      N.times{|i|
        a1, a2 = accs[i%accs.size], accs[(i+1)%accs.size]
        a1.transfer_to(a2, 1)
      }
    end

    rs << Ractor.new(accs) do |accs|
      N.times{|i|
        a1, a2 = accs[i%accs.size], accs[(i+1)%accs.size]
        a1.transfer_from(a2, 1)
      }
    end

    rs.each{|r| r.take}

  when :reverse
    rs = []

    rs << Ractor.new(accs) do |accs|
      N.times{|i|
        a1, a2 = accs[i%accs.size], accs[(i+1)%accs.size]
        a1.transfer_to(a2, 1)
      }
    end

    rs << Ractor.new(accs.reverse.freeze) do |accs|
      N.times{|i|
        a1, a2 = accs[i%accs.size], accs[(i+1)%accs.size]
        a1.transfer_from(a2, 1)
      }
    end

    rs.each{|r| r.take}

  when :shuffle
    RN.times.map{
      Ractor.new(accs) do |accs|
        rnd = Random.new
        N.times{
          a1 = accs.sample random: rnd
          a2 = accs.sample random: rnd
          redo if a1 == a2
          a1.transfer_to(a2, rnd.rand(1000))
        }
      end
    }.each{|r| r.take}

  else
    raise
  end

  sum = accs.inject(0){|r, acc| acc.balance + r}
  if sum != 0
    pp accs
    raise "iter #{iter} sum:#{sum}"
  end

  etime = Time.now
  p time: etime - btime

  # break
end

```

This program create AN bank accounts and repeat N transafer operations.
You can observe that huge AN reduces conflicts and the execution time is low. Small AN reduces conflicts -> many retries and the execution time is high.

```
     AN    Execution time (s)  Retry counts
    100                6.914        958,969
  1_000                3.107        186,267
 10_000                2.549         26,183
100_000                2.627          2,458
```

Now x10 retries doesn't affect execution time x10, this is because the current Ractor implementation (acquiring a global lock to raise an exception, and it reduces the retry counts). If we improve the Ractor's implementation, the result would be more worse.


## Consideration

### `Thread.atomically` in ractors

At first, I implemented this feature with `Ractor::atomically` and `Ractor::TVar`.
However, this STM feature will help the thread programming.
This is why I moved from `Ractor::atomically` to `Thread::atomically`.

Introduce `Concurrent` namespace what concurrent-ruby are using. However, there are small differences so that I'm not sure is is feasible.

Another idea is to support alias: `Thread.atomically` and `Ractor.atomically`.

### `Thread::TVar` can refer only shareable objects

Threads can access all objects so we don't need to restrict by such rule.
However, to support ractors, this restriction is needed.

One idea is separate `Thread::TVar` and `Ractor::TVar`, but it can introduce confusion.
Only with shareable objects, thread programs become more thread-safe, so I think it is good choice to have current restriction.

### Bug detection

Similar to locking, we can forget to use a `atomically` like that:

```ruby
class C
  def initialize
    @tv1 = Thread::TVar.new(0)
    @tv2 = Thread::TVar.new(0)
  end
  def tv1() = @tv1.value
  def tv2() = @tv2.value
  def tv1 = (v)
    Thread.atomically{ @tv1.value = v }
  end
  def tv2 = (v)
    Thread.atomically{ @tv2.value = v }
  end
end

obj = C.new
obj.tv1 += 1
obj.tv2 += 2
```

It works but it can introduce inconsistency if tv1 and tv2 are tightly coupled with because tv1 and tv2 are not accessed in the same transaction.
If tv1 and tv2 need to be modified consistently, we need to write like the following:

```ruby
Thread.atomically do
  obj.tv1 += 1
  obj.tv2 += 1
end
```

and `tv1/tv2/tv1=/tv2=` methods should not be defined.

I mean we can write bad programs easily.

It is same situation with traditional locking (we need to use `Mutex` appropriately). The duty to use it correctly is for programmer.

There are some advantages compared with traditional locking:

* We can concentrate on TVars. On traditional thread programming we need to check all memory state.
* We can introduce logging mechanism and we can find wrong usage (for example: tv1 and tv2 are set within independent transactions). I think we can make some checker based on the log. On traditional thread programming, there are several similar works, but it is difficult to check it because the target of state is most of memory operations.

## Related works

* There are many STM implementation techniques. https://www.morganclaypool.com/doi/abs/10.2200/S00070ED1V01Y200611CAC002
* Concurrent Haskell and Clojure are famous to support STM in language (I think).
  * The model of STM is similar to Clojure.
    * Clojure allows to access TVar (`ref` in Clojure) value without `atomically` (`dosync` in Clojure).
    * Clojure doesn't allow to set TVar value without `atomically`.
  * The API is similar to Concurrent Haskell (`TVar` and `atomically`.
* Concurrent-ruby has `Concurrent::TVar`.
  * But it allows to have an unshareable object.
  * But is allows to set the value with `atomically`.





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