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

[Eric Wong <e@yhbt.net>]

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.