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From: John Pignata <john@pignata.com>
To: raindrops@librelist.org
Subject: Re: Fwd: Queued vs. Active in tcp_stats_listener
Date: Sun, 3 Mar 2013 17:26:38 -0500
Message-ID: <CAHzRVjMqKD6TgRsna1Mpc+ayLBRv4OPJAUp3-fDj5jUbLV5Qww@mail.gmail.com> (raw)
In-Reply-To: <20130303214613.GA7200@dcvr.yhbt.net>

Hi Eric,

Thanks for your super-fast and thorough response.

On Sun, Mar 3, 2013 at 4:46 PM, Eric Wong <normalperson@yhbt.net> wrote:

> I'm not sure what Heroku does, but I think they have some fancy load
> balancer/router.  Maybe that limits the number of connection it opens to
> the (virtual?) machine unicorn runs on.
>
> You should ask Heroku if they do this.


Right. I've been talking to Heroku about their overall router behavior. My
current understanding is that (on the Cedar stack) the router does not do
any intelligent queuing and doesn't apply back-pressure. As requests come
in they are passed off to a dyno where they may or may not be enqueued by
the application server.
https://blog.heroku.com/archives/2013/2/16/routing_performance_update has
some good details for any future readers of this thread trying to tune on
Heroku.


> By backlog, this is the :backlog parameter in the unicorn "listen"
> directive, correct?
>

Yes.


> This :backlog parameter is only a hint to the kernel to limit the maximum
> queue size.  If the kernel never sees many connections in the first place,
> the limit will never be hit.


Understood. Though, I'd expect I'd be able to move the queued number with
Apache Bench considering the default backlog of 1,024.

The "queued" in raindrops is the connections the kernel TCP stack knows
> about.  If your load balancer doesn't send requests, the TCP stack in
> the kernel won't see them.
>

Understood.


> Can you get around the load balancer and throw traffic at the box
> directly?  You should see queued counts go up.


I cannot on Heroku but I did prop up a Rack test application on an EC2
system and raindrops is behaving the way you're explaining.


> unicorn does not accept() connections unless a worker is idle[1], so
> what you're seeing with active == unicorn worker_processes is correct.
>

Understood. So the queued number should represent connections that (under
TCP) have been responded to with an ACK but are enqueued waiting for a
worker.

I think my issue is with how I'm conducting my test. I may deploy our
change to production and monitor it there. We made a similar change to Thin
running on Heroku to dump a number to statsd every time a connection
occurred and we saw a significant non-zero backlog of connections. I expect
to see a smaller but non-zero number under Unicorn.

Thanks again for your time and thoughts.

-jp


      reply index

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAHzRVjNOLb+UeNx4EGrUPbjdLkpku0Ppkbo4VU8c1260N-QhiA@mail.gmail.com>
2013-03-03 21:09 ` John Pignata
2013-03-03 21:46   ` Eric Wong
2013-03-03 22:26     ` John Pignata [this message]

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