[Mongrel] Memory leaks in my site

Alexey Verkhovsky alexey.verkhovsky at gmail.com
Thu Mar 8 01:35:36 EST 2007

Some further findings:

"Hello World" Rails application on my rather humble rig (Dell D620 laptop
running Ubuntu 6.10, Ruby 1.8.4 and Rails 1.2.2)  can handle over 500 hits
per second on the following action:

def say_hi
  render :text => 'Hi!'

It also doesn't leak any memory at all *when it is not overloaded*. E.g.,
under maximum non-concurent load (single-threaded test client that fires the
next request immediately upon receiving a response to the previous one), it
stays up forever.

When Mongrel + "Hello, World" is overloaded, there is a memory leak to the
tune of 6 Mb per hour. I have yet to figure out where is it coming from.

On 3/7/07 Zed A. Shaw <zedshaw at zedshaw.com> wrote:
> I'd say first off the solution is:  just quit doing that.

This is something I can wholeheartedly agree with :)

maxing out your Mongrel servers then you're seriously screwed anyway
> and there's nothing but -n to help you.

After a couple of hours quietly pumping iron in a gym, I came to the same
conclusion. Let me explain myself, however.

The situation that I am concerned about is 20 to 50 apps clustered on the
same set of boxes, written by one group of people and supervised by another.
Think "large corporate IT", or a shared hosting a la TextDrive.  I want to
maximize throughput under heavy load, but a more important problem is to
reduce the ability of one app screwing up other apps on the same box(es).

> It's as simple as you make threads, threads take ram, threads
> don't go away fast enough.

What I was thinking is that by uncoupling the request from its thread, you
can probably max out all capabilities (CPU, I/O, Rails) of a 4 cores
commodity box with only 15-30 threads. 10-20 request handlers (that will
either serve static stuff or carry the request to the Rails queue), one
rails handler that loops over requests queue, takes requests to Rails and
drops responses off in the response queue, 5-10 response handlers (whose job
is simply to copy Rails response from responses queue to originating

Right now, as far as I understand the code, request is carried all the way
through by the same thread.

On second thoughts, this is asynchronous communications between threads
within the process. Far too much design and maintenance overhead for the
marginal benefits it may (or may not) bring. Basically, just me being stupid
by trying to be too smart. :)

> Until Ruby's IO, GC, and threads improve drastically you'll keep hitting
> these problems.

Yes. Meantime, the recipe apparently is "serve static stuff through an
upstream web server, and use smaller values of --num--procs". Mongrel that
only receives dynamic requests is, essentialy, a single-threaded process,
anyway. The only reason to have more than one (1) thread is so that other
requests can queue up while it's doing something that takes time. Cool.

By the way, is Ruby 1.9 solving all of these issues?

> No one who is sane is trying to really run a Rails app on a 64 meg VPS - -
thats just asking for a lot of pain.
Well, entry-level slices on most Rails VPS services are 64 Mb.
My poking around so far seems to say "it's doable, but you need to tune it".

Best regards,
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