[PATCH] prevent single listener from monopolizing a worker

[Eric Wong <e@yhbt.net>]

Stan Hu <stanhu@gmail.com> wrote:
> That seems to work, thanks!

Thanks for confirming.  I'll push the patch below out.
(ugh, dealing with crazy packet loss all around)

Expect a v5.6.0 release within a few days or week at most.
(hopefully no regressions).

And... I wonder, are most deployments nowadays single listener?

I don't think I've used multiple listeners for this aside from
experiments in the early days.

---------8<----------
Subject: [PATCH] prevent single listener from monopolizing a worker

In setups with multiple listeners, it's possible for our greedy
select(2)-avoidance optimization to get pinned on a single, busy
listener and starve the other listener(s).

Prevent starvation by retrying the select(2)-avoidance
optimization if and only if all listeners were active.  This
should have no effect on the majority of deployments with only a
single listener.

Thanks for Stan Hu for reporting and testing.

Reported-by: Stan Hu <stanhu@gmail.com>
Tested-by: Stan Hu <stanhu@gmail.com>
Link: https://yhbt.net/unicorn-public/CAMBWrQ=Yh42MPtzJCEO7XryVknDNetRMuA87irWfqVuLdJmiBQ@mail.gmail.com/
---
 lib/unicorn/http_server.rb | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/unicorn/http_server.rb b/lib/unicorn/http_server.rb
index a52931a..45a2e97 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
@@ -708,7 +709,7 @@ 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 == nr_listeners
         tmp = ready.dup
         redo
       end