Re: Fwd: Support for Soft Timeout in Unicorn

[Eric Wong <normalperson@yhbt.net>]

Pierre Baillet wrote:
> We use Unicorn at fotopedia since yesterday in production. We switched from
> Passenger due to an issue in the way Passenger was handling some error in
> our main application. Things run very well on Unicorn.
> 
> We have also modified Unicorn to handle a soft timeout for its workers. The
> Unicorn timeout was killing the workers without any chance for us to catch
> the Rails stack trace and identify the issue. I've forked and branched
> Unicorn from github and added a soft_timeout configuration value that is
> used for long running workers.
> 
> The workers now handle SIGABRT and will raise an exception. This will crash
> the Rails application if it can be crashed and force the framework to dump
> the stack trace in the logs. Let me know if this might be useful for other
> people and, why not, integrate that in the main Unicorn code !

Hi Pierre,

I'm thinking there's a better way to do this without involving the
master process.

The current timeout implementation[1] is really the last resort,
point-of-no-return situations when the workers are completely
stuck/blocked and cannot respond to other signals[2].  If the worker can
respond to SIGABRT (especially going through the interpreter and raising
an exception), then that means it could technically respond to
Thread#raise, too...

Unfortunately, the current core Ruby timeout implementation is extremely
naive and inefficient.  It shouldn't be hard to write a better timeout
implementation that reuses a single timer Thread which can be rearmed
with every request.

This would be doable as middleware, too, and if done carefully, even
safely reusable in multi-threaded web servers.  This would be a good
addition to rack-contrib, even.  I might consider doing it myself if I
had time.

[1] - I'm not completely happy with "needing" the current timeout
      implementation in the first place.  I will at least redo it
      at some point (after Unicorn 1.x) to gain some
      scalability/performance (and perhaps lose some portability).

[2] - SIGKILL and SIGSTOP are special, userspace has no mechanism
      to block/catch/ignore those signals, so we rely on the master
      process to deliver them.

-- 
Eric Wong
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