From: ko1@... Date: 2020-05-13T06:58:21+00:00 Subject: [ruby-core:98314] [Ruby master Feature#16786] Light-weight scheduler for improved concurrency. Issue #16786 has been updated by ko1 (Koichi Sasada). ioquatix (Samuel Williams) wrote in #note-26: > > non-blocking fiber creation API > > It was voted by community, strongly in favour of `Fiber do ... end`. If you think your suggestion is better, we should confirm with community. I don't think we should refer this kind of result because the voted people does not know concerns. > > Scheduler class > > We can introduce `Fiber::Scheduler` however I don't think it's necessary. The benefit would be adding default functionality to existing schedulers. > > In fact, such a default implementation could be provided by a gem or some other code which can be shared between implementations. It can be, Maybe we should discuss later after more trials. > > What's happen when a non-blocking fiber creates a blocking fiber > > Resuming a blocking fiber is a blocking operation. This is good behaviour and ensures things like Enumerator won't be broken by this proposal. oK. > > For example, if a fiber F1 is waiting for a network IO and a fiber F2 resume F1, then it will be blocking until the IO is ready. > > Scheduler will require access to resume/transfer/yield operations, so removing them is not realistic. In addition, Async uses these operations in `Async::Semaphore` & `Async::Queue` implementations, as well as other places. Not sure it should be implemented independently (it should be a scheduler's duty IMO). But I agree it is more flexible. > Scheduler should be robust against spurious wake-ups (example using `Async` given below). However, user who calls `#resume` without any care will suffer the consequences if the code is not robust. ok. > > BTW, hooks are called on root fiber, if a fiber F1 calls blocking IO operations, back to the root fiber and call the hook? > > My interpretation of this is you are asking if the root fiber (which is blocking) will enter a recursive loop when invoking non-blocking operations. The answer is no. My question is which fiber context is used for `wait_xxx` method? i asked Samuel and the answer is a fiber which called the blocking IO. > > However, `wait_any` is not clear for me. > > `wait_any` is modelled after https://ruby-doc.org/stdlib-2.7.1/libdoc/io/wait/rdoc/IO.html#method-i-wait > > We can change name to something else, do you have better idea? I have no knowledge about it... > > There is a `wait_sleep`, but I'm not sure the hooks are enough or not. > > What do you mean it is enough or not? Do you mean there are other ways to sleep? Sorry, my question is, if we recognized other hooks are needed after release it, what happens? the answer was: #28 > > `enter_blocking_region`/`leave_blocking_region` are strongly connected to the MRI. > > Yes, agreed. These hooks were added as a result of our meeting in Japan. > > I'd be happy to remove it but I think it provides very valuable insight into blocking operations within MRI. Maybe other implementations can comment on whether it's useful or not. Even if it's not supported, not much functionality is lost, so I don't see the point in removing it - it's not critical, but it's very useful. > > > user should know the details to utilize the information > > To get the method name, you can use `caller`, which is shown in `Async::Scheduler` implementation: https://github.com/socketry/async/blob/c173f5880c566724f104855941f9af12fbf4d7e7/lib/async/scheduler.rb#L100-L112 > > I think it's best to avoid preparing the arguments (e.g. method name) when it may not be used, to avoid the overhead on critical path. I doubt it is performance issue. > > I'm negative yet about this proposal because we can not predict the context switch completely. > > To me, this is actually a major feature of the proposal. We provide the tools to make a concurrent context which enables us to progressively improve concurrency. e.g. > > ```ruby > Fiber do > # Any operation in here may be scheduling point. > connect(resolve("ruby-lang.org")) > end > ``` > > This proposal doesn't include concurrent DNS resolution. But with Ruby 3.1, we can introduce such a feature without impacting user code. That means, `resolve("ruby-lang.org")` can become switching point. The same change in Node.js requires rewriting the code, which we want to avoid. In the future I want to introduce non-blocking DNS, File, other system calls, etc. > > So users should not rely on blocking operations for synchronisation. > > To retain compatibility with Mutex, when a Mutex is locked on a thread, that entire thread becomes blocking w.r.t. non-blocking operations. This ensures existing code continues to work correctly, at the cost of reduced concurrency when holding a Mutex. > > The next step, as proposed by @eregon, is to make Mutex fiber aware. This improves the opportunity for concurrency but does not change the semantics of user code. > > Regarding some of the other options you list, one you have not considered is this: > > ``` > Fiber do > # Non-blocking fiber. > # io operations, etc. > > # This is effectively the same as `Fiber.exclusive`. > Fiber.new do > # Modify shared mutable state, any operation here is blocking so is guaranteed to be sequential. > end.resume > end > ``` > > So no new construct is required to force sequential execution. creating blocking fiber is interesting idea, but the backtrace is separated so it shouldn't be used IMO. ---------------------------------------- Feature #16786: Light-weight scheduler for improved concurrency. https://bugs.ruby-lang.org/issues/16786#change-85553 * Author: ioquatix (Samuel Williams) * Status: Open * Priority: Normal ---------------------------------------- # Abstract We propose to introduce a light weight fiber scheduler, to improve the concurrency of Ruby code with minimal changes. # Background We have been discussing and considering options to improve Ruby scalability for several years. More context can be provided by the following discussions: - https://bugs.ruby-lang.org/issues/14736 - https://bugs.ruby-lang.org/issues/13618 The final Ruby Concurrency report provides some background on the various issues considered in the latest iteration: https://www.codeotaku.com/journal/2020-04/ruby-concurrency-final-report/index # Proposal We propose to introduce the following concepts: - A `Scheduler` interface which provides hooks for user-supplied event loops. - Non-blocking `Fiber` which can invoke the scheduler when it would otherwise block. ## Scheduler The per-thread fiber scheduler interface is used to intercept blocking operations. A typical implementation would be a wrapper for a gem like EventMachine or Async. This design provides separation of concerns between the event loop implementation and application code. It also allows for layered schedulers which can perform instrumentation, enforce constraints (e.g. during testing) and provide additional logging. You can see a [sample implementation here](https://github.com/socketry/async/pull/56). ```ruby class Scheduler # Wait for the given file descriptor to become readable. def wait_readable(io) end # Wait for the given file descriptor to become writable. def wait_writable(io) end # Wait for the given file descriptor to match the specified events within # the specified timeout. # @param event [Integer] a bit mask of +IO::WAIT_READABLE+, # `IO::WAIT_WRITABLE` and `IO::WAIT_PRIORITY`. # @param timeout [#to_f] the amount of time to wait for the event. def wait_any(io, events, timeout) end # Sleep the current task for the specified duration, or forever if not # specified. # @param duration [#to_f] the amount of time to sleep. def wait_sleep(duration = nil) end # The Ruby virtual machine is going to enter a system level blocking # operation. def enter_blocking_region end # The Ruby virtual machine has completed the system level blocking # operation. def exit_blocking_region end # Intercept the creation of a non-blocking fiber. def fiber(&block) Fiber.new(blocking: false, &block) end # Invoked when the thread exits. def run # Implement event loop here. end end ``` A thread has a non-blocking fiber scheduler. All blocking operations on non-blocking fibers are hooked by the scheduler and the scheduler can switch to another fiber. If any mutex is acquired by a fiber, then a scheduler is not called; the same behaviour as blocking Fiber. Schedulers can be written in Ruby. This is a desirable property as it allows them to be used in different implementations of Ruby easily. To enable non-blocking fiber switching on blocking operations: - Specify a scheduler: `Thread.current.scheduler = Scheduler.new`. - Create several non-blocking fibers: `Fiber.new(blocking:false) {...}`. - As the main fiber exits, `Thread.current.scheduler.run` is invoked which begins executing the event loop until all fibers are finished. ### Time/Duration Arguments Tony Arcieri suggested against using floating point values for time/durations, because they can accumulate rounding errors and other issues. He has a wealth of experience in this area so his advice should be considered carefully. However, I have yet to see these issues happen in an event loop. That being said, round tripping between `struct timeval` and `double`/`VALUE` seems a bit inefficient. One option is to have an opaque argument that responds to `to_f` as well as potentially `seconds` and `microseconds` or some other such interface (could be opaque argument supported by `IO.select` for example). ### File Descriptor Arguments Because of the public C interface we may need to support a specific set of wrappers for CRuby. ```c int rb_io_wait_readable(int); int rb_io_wait_writable(int); int rb_wait_for_single_fd(int fd, int events, struct timeval *tv); ``` One option is to introduce hooks specific to CRuby: ```ruby class Scheduler # Wrapper for rb_io_wait_readable(int) C function. def wait_readable_fd(fd) wait_readable(::IO.from_fd(fd, autoclose: false)) end # Wrapper for rb_io_wait_readable(int) C function. def wait_writable_fd(fd) wait_writable(::IO.from_fd(fd, autoclose: false)) end # Wrapper for rb_wait_for_single_fd(int) C function. def wait_for_single_fd(fd, events, duration) wait_any(::IO.from_fd(fd, autoclose: false), events, duration) end end ``` Alternatively, in CRuby, it may be possible to map from `fd` -> `IO` instance. Most C schedulers only care about file descriptor, so such a mapping will introduce a small performance penalty. In addition, most C level schedulers will not care about `IO` instance. ## Non-blocking Fiber We propose to introduce per-fiber flag `blocking: true/false`. A fiber created by `Fiber.new(blocking: true)` (the default `Fiber.new`) becomes a "blocking Fiber" and has no changes from current Fiber implementation. This includes the root fiber. A fiber created by `Fiber.new(blocking: false)` becomes a "non-blocking Fiber" and it will be scheduled by the per-thread scheduler when the blocking operations (blocking I/O, sleep, and so on) occurs. ```ruby Fiber.new(blocking: false) do puts Fiber.current.blocking? # false # May invoke `Thread.scheduler&.wait_readable`. io.read(...) # May invoke `Thread.scheduler&.wait_writable`. io.write(...) # Will invoke `Thread.scheduler&.wait_sleep`. sleep(n) end.resume ``` Non-blocking fibers also supports `Fiber#resume`, `Fiber#transfer` and `Fiber.yield` which are necessary to create a scheduler. ### Fiber Method We also introduce a new method which simplifes the creation of these non-blocking fibers: ```ruby Fiber do puts Fiber.current.blocking? # false end ``` This method invokes `Scheduler#fiber(...)`. The purpose of this method is to allow the scheduler to internally decide the policy for when to start the fiber, and whether to use symmetric or asymmetric fibers. If no scheduler is specified, it is a error: `RuntimeError.new("No scheduler is available")`. In the future we may expand this to support some kind of default scheduler. ## Non-blocking I/O `IO#nonblock` is an existing interface to control whether I/O uses blocking or non-blocking system calls. We can take advantage of this: - `IO#nonblock = false` prevents that particular IO from utilising the scheduler. This should be the default for `stderr`. - `IO#nonblock = true` enables that particular IO to utilise the scheduler. We should enable this where possible. As proposed by Eric Wong, we believe that making I/O non-blocking by default is the right approach. We have expanded his work in the current implementation. By doing this, when the user writes `Fiber do ... end` they are guaranteed the best possible concurrency possible, without any further changes to code. As an example, one of the tests shows `Net::HTTP.get` being used in this way with no further modifications required. To support this further, consider the counterpoint, that `Net::HTTP.get(..., blocking: false)` is required for concurrent requests. Library code may not expose the relevant options, sevearly limiting the user's ability to improve concurrency, even if that is what they desire. # Implementation We have an evolving implementation here: https://github.com/ruby/ruby/pull/3032 which we will continue to update as the proposal changes. # Evaluation This proposal provides the hooks for scheduling fibers. With regards to performance, there are several things to consider: - The impact of the scheduler design on non-concurrent workloads. We believe it's acceptable. - The impact of the scheduler design on concurrent workloads. Our results are promising. - The impact of different event loops on throughput and latency. We have independent tests which confirm the scalability of the approach. We can control for the first two in this proposal, and depending on the design we may help or hinder the wrapper implementation. In the tests, we provide a basic implementation using `IO.select`. As this proposal is finalised, we will introduce some basic benchmarks using this approach. # Discussion The following points are good ones for discussion: - Handling of file descriptors vs `IO` instances. - Handling of time/duration arguments. - General design and naming conventions. - Potential platform issues (e.g. CRuby vs JRuby vs TruffleRuby, etc). The following is planned to be described by @eregon in another design document: - Semantics of non-blocking mutex (e.g. `Mutex.new(blocking: false)` or some other approach). In the future we hope to extend the scheduler to handle other blocking operations, including name resolution, file I/O (by `io_uring`) and others. We may need to introduce additional hooks. If these hooks are not defined on the scheduler implementation, we will revert back to the blocking implementation where possible. -- https://bugs.ruby-lang.org/ Unsubscribe: