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


Thanks for the detailed information.

So, it seems like your design has unavoidable contention (and therefore latency) because you need to send events between threads, which is what I expected. However, you argue this overhead should be small. I'd like to see actual numbers TBH.

And as you state, it's not possible (nor desirable IMHO) to move fibers between threads. Yes, head-of-line blocking might be an issue. Moving stacks between CPU cores is not without it's own set of overheads. If you have serious issues with head-of-line blocking it's more likely to be a problem with your code (I've directly experienced this and the result was: https://github.com/socketry/async-http/blob/ca655aa190ed7a89b601e267906359793271ec8a/lib/async/http/protocol/http11.rb#L93).

It would be interesting to see exactly how much overhead is incurred using a shared epoll. I know from my testing that 

I remember in my tests, the latency of yahns was a lot higher than async-http:

```
async-http

koyoko% wrk -c 16 -t 16 -d 10 http://localhost:9292/wiki/index
Running 10s test @ http://localhost:9292/wiki/index
  16 threads and 16 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     9.18ms    3.23ms  86.46ms   98.51%
    Req/Sec   109.47     17.70   121.00     95.49%
  8954 requests in 10.02s, 29.99MB read
  Socket errors: connect 0, read 0, write 0, timeout 4
Requests/sec:    893.68
Transfer/sec:      2.99MB

yahns

koyoko% wrk -c 16 -t 16 -d 10 http://localhost:9292/wiki/index
Running 10s test @ http://localhost:9292/wiki/index
  16 threads and 16 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    20.51ms   16.04ms 190.57ms   85.79%
    Req/Sec    54.43     32.59   191.00     65.56%
  8702 requests in 10.10s, 29.68MB read
Requests/sec:    861.61
Transfer/sec:      2.94MB
```

This was a long time ago, async-http performance has also improved and the issue regarding timeouts was resolved. When I have some time I can repeat these tests.

Tangentially related:

In my own IO scheduler/reactor, I choose to use `EPOLLET` and manually add/remove the handlers. The basic implementation looks something like this:

Implementation of Readable which is what manages adding events to epoll/kqueue: https://github.com/kurocha/async/blob/master/source/Async/Readable.cpp

It's prepared but not added to the reactor here (it's lazy): https://github.com/kurocha/async-http/blob/eff77f61f7a85a3ac21f7a8f51ba07f069063cbe/source/Async/HTTP/V1/Protocol.cpp#L34

By calling wait, the fd is inserted into the reactor/selector: https://github.com/kurocha/async/blob/2edef4d6990259cc60cc307b6de2ab35b97560f1/source/Async/Protocol/Buffer.cpp#L254

The cost of adding/removing FDs is effectively constant time given an arbitrary number of reads or writes. We shouldn't preclude implementing this model in Ruby if it makes sense. As you say, the overhead of the system call is pretty minimal.

Now that you mention it, I'd like to compare EPOLLET vs EPOLLONESHOT. It's an interesting design choice and it makes a lot of sense if you are doing only one read in the context of a blocking operation.

----------------------------------------
Feature #13618: [PATCH] auto fiber schedule for rb_wait_for_single_fd and rb_waitpid
https://bugs.ruby-lang.org/issues/13618#change-71784

* 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)


-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>