Re: Sustained queuing on one listener can block requests from other listeners

[Stan Hu <stanhu@gmail.com>]

Thanks, Eric. That patch didn't work; it spun the CPU. I think this worked?

diff --git a/lib/unicorn/http_server.rb b/lib/unicorn/http_server.rb
index a52931a..aaa4955 100644
--- a/lib/unicorn/http_server.rb
+++ b/lib/unicorn/http_server.rb
@@ -708,7 +708,7 @@ def worker_loop(worker)
       # we're probably reasonably busy, so avoid calling select()
       # and do a speculative non-blocking accept() on ready listeners
       # before we sleep again in select().
-      unless nr == 0
+      if nr == readers.size
         tmp = ready.dup
         redo
       end

On Tue, Apr 14, 2020 at 10:26 PM Eric Wong <e@yhbt.net> wrote:
>
> Stan Hu <stanhu@gmail.com> wrote:
> > My unicorn.rb has two listeners:
> >
> > listen "127.0.0.1:8080", :tcp_nopush => false
> > listen "/var/run/unicorn.socket", :backlog => 1024
>
> Fwiw, lowering :backlog may make sense if you got other
> hosts/instances.  More below..
>
> > We found that because of the greedy attempt to accept new connections
> > before calling select() in
> > https://github.com/defunkt/unicorn/blob/981f561a726bb4307d01e4a09a308edba8d69fe3/lib/unicorn/http_server.rb#L707-L714,
> > listeners on another socket stall out until the first listener is
> > drained. We would expect Unicorn to round-robin between the two
> > listeners, but that doesn't happen as long as there is work to be done
> > for the first listener. We've verified that deleting that `redo` block
> > fixes the problem.
> >
> > What do you think about the various options?
> >
> > 1. Only running that redo block if there is one listener
>
> That seems reasonable, or if ready.size == nr_listeners
> (proposed patch below)
>
> > 2. Removing the redo block entirely
>
> From what I recall ages ago, select() entry cost is pretty high
> and I remember that redo helping a fair bit even in 2009 with
> simple apps.  Syscall cost is even higher now with CPU
> vulnerability mitigations, and Ruby 1.9+ GVL release+reacquire
> is also a penalty I didn't have when developing this on 1.8.
>
> Do you have time+hardware to benchmark either approach on a
> simple app?  I no longer have stable/reliable hardware for
> benchmarking.  Thanks.
>
> Totally untested patch to try approach #1
>
> diff --git a/lib/unicorn/http_server.rb b/lib/unicorn/http_server.rb
> index a52931a..69f1f60 100644
> --- a/lib/unicorn/http_server.rb
> +++ b/lib/unicorn/http_server.rb
> @@ -686,6 +686,7 @@ def worker_loop(worker)
>      trap(:USR1) { nr = -65536 }
>
>      ready = readers.dup
> +    nr_listeners = readers.size
>      @after_worker_ready.call(self, worker)
>
>      begin
> @@ -698,7 +699,6 @@ def worker_loop(worker)
>          # but that will return false
>          if client = sock.kgio_tryaccept
>            process_client(client)
> -          nr += 1
>            worker.tick = time_now.to_i
>          end
>          break if nr < 0
> @@ -708,7 +708,7 @@ def worker_loop(worker)
>        # we're probably reasonably busy, so avoid calling select()
>        # and do a speculative non-blocking accept() on ready listeners
>        # before we sleep again in select().
> -      unless nr == 0
> +      if ready.size == nr_listeners
>          tmp = ready.dup
>          redo
>        end
>
>
>
> And `nr' can probably just be a boolean `reopen' flag if we're
> not overloading it as a counter.