Issue #13618 has been updated by ioquatix (Samuel Williams).


> Wouldn't having these abstractions allow building this by hand using existing Fiber?

Yes, it's feasible and already implemented here https://github.com/socketry/async and it's backwards compatible with older Rubies.

> Even just solving this problem is enough of a hornets nest prior to introduction of other complications.

I agree with this, but not for the reason stated. I think modern epoll/kqueue/select basically just work. Yes, there are some odd issues you have to deal with, but for the most part things work well and it's as efficient as it's going to get in a general sense.

What I think is a bigger issue is blocking system calls. An example of this would be system name lookup (e.g. DNS). The two main mitigations are using a threadpool (libuv and neverblock do this AFAIK), or having multiple reactors and migrating other Fibers if the reactor is blocked. Even just having a tight loop can cause problems, and even in the case where you have non-blocking IO, if it never actually blocks and yields back to the reactor.

> pool = ThreadPool.new(concurrency: 100, max_workers: 5 # optional)

It's a bit surprising to see this, but your example is almost exactly the same as using `Async::Reactor`, simply replace `ThreadPool` with `Async::Reactor` and the code will almost work. Semantically it's about the same as what I think is the ideal solution.

I think that abstracting around the `Reactor` pattern is a good idea. It provides strong guarantees about the state of the program.

Here is the main entry point for an `Async::DNS::Server` instance: https://github.com/socketry/async-dns/blob/5ec883c0dd3d69b766668e4e6811561aba847ac6/lib/async/dns/server.rb#L106-L120

`Async::Reactor#run` handles nesting: https://github.com/socketry/async/blob/4f695ed6e340031f27f6db5100ab86ba139ae3d9/lib/async/reactor.rb#L38-L61

If you call the run method inside an existing reactor, it returns an async task which you can use to stop the server and all async tasks started within the server. If you call it outside of a reactor, it will create a reactor and block forever. In both cases the life cycle is managed correctly.

Simply making a per-thread reactor and making read/write calls non-blocking only solves about 10% of the problem IMHO.

To compare some of the pseudo examples with real code, take a look at the C10k implemented here: https://github.com/socketry/async-io/blob/master/spec/async/io/c10k_spec.rb


----------------------------------------
Feature #13618: [PATCH] auto fiber schedule for rb_wait_for_single_fd and rb_waitpid
https://bugs.ruby-lang.org/issues/13618#change-70041

* Author: normalperson (Eric Wong)
* Status: Assigned
* Priority: Normal
* Assignee: normalperson (Eric Wong)
* Target version: 
----------------------------------------
```
auto fiber schedule for rb_wait_for_single_fd and rb_waitpid

Implement automatic Fiber yield and resume when running
rb_wait_for_single_fd and rb_waitpid.

The Ruby API changes for Fiber are named after existing Thread
methods.

main Ruby API:

    Fiber#start -> enable auto-scheduling and run Fiber until it
		   automatically yields (due to EAGAIN/EWOULDBLOCK)

The following behave like their Thread counterparts:

    Fiber.start - Fiber.new + Fiber#start (prelude.rb)
    Fiber#join - run internal scheduler until Fiber is terminated
    Fiber#value - ditto
    Fiber#run - like Fiber#start (prelude.rb)

Right now, it takes over rb_wait_for_single_fd() and
rb_waitpid() function if the running Fiber is auto-enabled
(cont.c::rb_fiber_auto_sched_p)

Changes to existing functions are minimal.

New files (all new structs and relations should be documented):

    iom.h - internal API for the rest of RubyVM (incomplete?)
    iom_internal.h - internal header for iom_(select|epoll|kqueue).h
    iom_epoll.h - epoll-specific pieces
    iom_kqueue.h - kqueue-specific pieces
    iom_select.h - select-specific pieces
    iom_pingable_common.h - common code for iom_(epoll|kqueue).h
    iom_common.h - common footer for iom_(select|epoll|kqueue).h

Changes to existing data structures:

    rb_thread_t.afrunq   - list of fibers to auto-resume
    rb_vm_t.iom          - Ruby I/O Manager (rb_iom_t) :)

Besides rb_iom_t, all the new structs are stack-only and relies
extensively on ccan/list for branch-less, O(1) insert/delete.

As usual, understanding the data structures first should help
you understand the code.

Right now, I reuse some static functions in thread.c,
so thread.c includes iom_(select|epoll|kqueue).h

TODO:

    Hijack other blocking functions (IO.select, ...)

I am using "double" for timeout since it is more convenient for
arithmetic like parts of thread.c.   Most platforms have good FP,
I think.  Also, all "blocking" functions (rb_iom_wait*) will
have timeout support.

./configure gains a new --with-iom=(select|epoll|kqueue) switch

libkqueue:

  libkqueue support is incomplete; corner cases are not handled well:

    1) multiple fibers waiting on the same FD
    2) waiting for both read and write events on the same FD

  Bugfixes to libkqueue may be necessary to support all corner cases.
  Supporting these corner cases for native kqueue was challenging,
  even.  See comments on iom_kqueue.h and iom_epoll.h for
  nuances.

Limitations

Test script I used to download a file from my server:
----8<---
require 'net/http'
require 'uri'
require 'digest/sha1'
require 'fiber'

url = 'http://80x24.org/git-i-forgot-to-pack/objects/pack/pack-97b25a76c03b489d4cbbd85b12d0e1ad28717e55.idx'

uri = URI(url)
use_ssl = "https" == uri.scheme
fibs = 10.times.map do
  Fiber.start do
    cur = Fiber.current.object_id
    # XXX getaddrinfo() and connect() are blocking
    # XXX resolv/replace + connect_nonblock
    Net::HTTP.start(uri.host, uri.port, use_ssl: use_ssl) do |http|
      req = Net::HTTP::Get.new(uri)
      http.request(req) do |res|
    dig = Digest::SHA1.new
    res.read_body do |buf|
      dig.update(buf)
      #warn "#{cur} #{buf.bytesize}\n"
    end
    warn "#{cur} #{dig.hexdigest}\n"
      end
    end
    warn "done\n"
    :done
  end
end

warn "joining #{Time.now}\n"
fibs[-1].join(4)
warn "joined #{Time.now}\n"
all = fibs.dup

warn "1 joined, wait for the rest\n"
until fibs.empty?
  fibs.each(&:join)
  fibs.keep_if(&:alive?)
  warn fibs.inspect
end

p all.map(&:value)

Fiber.new do
  puts 'HI'
end.run.join
```


---Files--------------------------------
0001-auto-fiber-schedule-for-rb_wait_for_single_fd-and-rb.patch (82.8 KB)


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