Feature #17261
closedSoftware transactional memory (STM) for Threads and Ractors
Description
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 runexpr
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 valueval
. -
Thread::TVar#increment(n=1)
: Just same asThread.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 withatomically
. We can define asThread.atomically{ tv.value = val }
, but it can lead misusing withoutatomically
. -
TVar#increment
is special case to allow setting withoutatomically
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¶
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 thattv1.value == tv2.value
. - the results
[3000000, 3000000]
shows consistency on+=1
.
Here is famous bank-account example:
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:
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:
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 withoutatomically
(dosync
in Clojure). - Clojure doesn't allow to set TVar value without
atomically
.
- Clojure allows to access TVar (
- The API is similar to Concurrent Haskell (
TVar
andatomically
.
- The model of STM is similar to Clojure.
- Concurrent-ruby has
Concurrent::TVar
.- But it allows to have an unshareable object.
- But is allows to set the value with
atomically
.
Updated by ioquatix (Samuel Williams) about 4 years ago
What does TVar mean?
Updated by hsbt (Hiroshi SHIBATA) about 4 years ago
See http://ruby-concurrency.github.io/concurrent-ruby/1.1.4/Concurrent/TVar.html
A TVar is a transactional variable
Updated by Eregon (Benoit Daloze) about 4 years ago
Interesting :)
Having it on Thread
sounds nice to me.
I wonder how limiting it is to restrict to only shareable values for TVar.
Is there actually any type usable for TVar except Integer?
A frozen String could only be swapped with .value=
, which seems rather uninteresting.
For a frozen Array of shareable elements, one would need to create a new frozen Array to modify any element, which seems rather expensive.
One could use Array.new(n) { TVar.new(0) }.freeze
I guess, but that would then need to create a new array to change the size of the Array (for <<,pop,shift,unshift,...).
That's probably still enough to build a STM hashtable with an Array of TVar of Entry and the Entry are frozen and have key: shareable, value: TVar
.
Not sure about other thread-safe collection using the STM to synchronize, maybe the shareable-only is too limiting for some structures like trees (or causes too much overhead).
Updated by ko1 (Koichi Sasada) about 4 years ago
At last dev-meeting, this proposal is not accepted to introduce it in core because the importance of this feature is not sure.
Maybe I'll introduce as ractor gem to study the API and usefulness of this feature.
Updated by chrisseaton (Chris Seaton) about 4 years ago
I wrote a long-form blog post to give people interested in this proposed feature some context.
https://chrisseaton.com/truffleruby/ruby-stm/
I've also suggested a benchmark that we can start to use for experimenting.
Updated by Eregon (Benoit Daloze) about 4 years ago
I think it would be nice to make it its own gem (tvar
maybe?) given it's under Thread
and not under Ractor
, and it could most likely work on other Ruby implementations (e.g., TruffleRuby).
That should also make it possible to e.g. improve performance independently of Ruby releases.
Updated by ko1 (Koichi Sasada) about 4 years ago
I think it would be nice to make it its own gem (tvar maybe?) given it's under Thread and not under Ractor, and it could most likely work on other Ruby implementations (e.g., TruffleRuby).
For this purpose, concurrent-ruby provides TVar for threads.
Updated by ko1 (Koichi Sasada) about 4 years ago
- Status changed from Open to Rejected
Updated by Eregon (Benoit Daloze) about 4 years ago
ko1 (Koichi Sasada) wrote in #note-8:
For this purpose, concurrent-ruby provides TVar for threads.
That does not work with Ractor though (at least currently).
Updated by ko1 (Koichi Sasada) about 4 years ago
Eregon (Benoit Daloze) wrote in #note-10:
That does not work with Ractor though (at least currently).
You wrote "under Thread and not under Ractor". What does it mean?
Updated by Eregon (Benoit Daloze) about 4 years ago
ko1 (Koichi Sasada) wrote in #note-11:
You wrote "under Thread and not under Ractor". What does it mean?
I was talking about the namespace, the proposal is Thread::TVar (and not Ractor::TVar), which I agree is a good namespace for it.
In any case, I think it would be valuable to have this work in a gem if it's not in core directly.
Updated by ko1 (Koichi Sasada) about 4 years ago
Updated by chrisseaton (Chris Seaton) about 4 years ago
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.
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.
Updated by ko1 (Koichi Sasada) about 4 years ago
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.