[Mongrel] Random(?) action hanging

Zed Shaw zedshaw at zedshaw.com
Mon Sep 18 21:20:18 EDT 2006

On Mon, 2006-09-18 at 13:22 -0700, Steven Lumos wrote:
> Is this the behavior that supposedly indicates a problem with a
> particular action? Mongrel in development mode randomly hangs on
> random actions.  Further requests cause USR1 logs like:
> Mon Sep 18 13:04:03 PDT 2006: 0 threads sync_waiting for /, 2 still active in mongrel.
> Mon Sep 18 13:07:52 PDT 2006: 1 threads sync_waiting for /public/stylesheets/table.css, 3 still active in mongrel.
> Mon Sep 18 13:08:21 PDT 2006: 2 threads sync_waiting for /, 4 still active in mongrel.

Looks like / is blocking.  Is this a Rails action?  Also, you should
probably run this in production mode as in development mode it's kind of
slow and won't give you reasonable results.

> 0 threads sync_waiting? Isn't that good? Sync waiting for a file?
> Is that supposed to happen?
Yep, 0 is what you want, but you really want to start this up in
production mode and then thrash the suspect action with something

As for the file, this means that your apache instance isn't serving
files but mongrel is.  I suspect since you were running in development
mode that you weren't behind a web server though.

> I thought that the USR1 output was supposed to help point out which
> action(s) had a problem, but the (random) action that actually caused
> the hang is never displayed, just whatever was the latest request
> after the hang.

The log doesn't display whichever is stuck, it just periodically lists
how many requests are waiting for which actions.  You gotta pound it or
run it in heavy production to get a better sense.

> This doesn't seem to happen in production mode, at least it hasn't so
> far.  It happens every few minutes in development, and sending INT to
> the process causes this:

Yep, that's something blocking your threads.  See what happens is you
have a Rails action that's got things locked with Sync.  A bunch of
Mongrel threads pile up behind this sync lock waiting for rails to
finish.  Eventually when you shutdown with INT mongrel tries to do a
graceful shutdown.  It goes through all the active threads, sees that
you've got a bunch that are really old, and then it starts killing them
so you can get a stack trace.

Hope that helps.

Zed A. Shaw, MUDCRAP-CE Master Black Belt Sifu
http://www.lingr.com/room/3yXhqKbfPy8 -- Come get help.

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