WTF is up with memory usage nowadays?

[Eric Wong <e@80x24.org>]

<rant> Came across this in my feeds today:

https://about.gitlab.com/2016/12/11/proposed-server-purchase-for-gitlab-com/

... Yeah, they cite 0.5 GB of memory usage per unicorn worker.
I guess this is typical nowadays, but damn, it sucks :<

This is not the future I had in mind or ever wanted unicorn to
be associated with back in 2009 when I started.

I don't think it's the fault of unicorn itself; unicorn recycles
request buffers, uses pre-frozen hash keys, and even
uses String#clear nowadays to discard heap memory, and never
buffers more than it has to.

Since day one, unicorn was built to handle multi-gigabyte
uploads and responses; even from a crappy 256MB laptop.
"curl -T-" is my co-pilot :)


So... I guess the problem is up the stack in the app or
framework.  Maybe Rails?  *shrug*  I don't use that anymore...

I remember using Rails over a decade ago and being shocked at
50MB (yes, fifty megabytes) of RSS usage.  This was on 32-bit,
but even in the worst case on 64-bit, it would be 100MB.
Of course, nowadays Rails has grown to the point where I'm
afraid to go near it; instead I work directly off Rack.

And yes, I still freak out nowadays when my Rack processes
exceed 100MB...


So, what can and should we do about it?

* First step: Limit ourselves.

  Use slower, older hardware, slower Internet connection so you
  force yourself to eke out every bit of performance out of
  what you have.

  It's utterly hilarious for me to hear about people complain
  about laptops which can "only" have 16GB RAM.

  I've definitely made transgressions in the past, and the worst
  code I've written was on powerful hardware.


Disclaimer: Some of the following may not be very Ruby-ish :P
And everything else is optional and the result of the first step
above.

* Recycle.  Don't waste object slots: {Array,Hash,String}#clear
  can allow you to recycle heap memory for large objects
  and minimize GC pressure.  Using thread-local variables
  in your app helps maintain compatibility with multi-threaded
  Rack servers; or perhaps go Rack env-local for compatibility
  with single-threaded non-blocking servers.


* Can't recycle?  Discard objects you don't need, ASAP,
  and continue #clear-ing what you can. Take advantage
  of streaming built into Rack.

  The Rack response body only needs to respond to #each.
  There should be no reason to build giant response
  documents in memory before sending them to a client.

  unicorn can't do the following for you automatically since
  we don't know how/if a Rack app will reuse a string;
  but upstack authors can String#clear after yielding
  in #each to ensure any malloced heap memory is immediately
  available for future use (but beware of downstream middlewares
  which do not expect this, too(**)):

    def each
      # .. do something to generate a giant string
      yield giant_string
      giant_string.clear # String#clear
    end

  A Rack response body may also respond to #close; it can
  be used to explicitly release any response-local resources.
  Rack::TempfileReaper + Rack::BodyProxy is an example of
  this for Tempfiles.

  Smaller functions and smaller code helps keep this manageable.

* Avoid slurping.  Large datasets do not need everything up
  front.  For example, threading 10K messages entirely
  in memory is no problem: just don't load entire messages
  into memory up front, only what you need.
  JWZ's algorithm was doing this in the 90s:
  https://www.jwz.org/doc/threading.html

Disclaimer: Some of these things may hurt throughput and
performance in benchmarks, especially with smaller datasets;
but I consider predictable and consistent performance more
far more important than burst throughput.


** Know your entire stack; top to bottom.
   You ought to be able to track every single line of code
   in a high-level Rack app you maintain down through each
   and every layer of framework, middleware, Rack server,
   Ruby VM, C library, down to the OS kernel.

   Yes, this limits you to using smaller and simpler stacks :P


*** Why stick with Ruby if you care about memory usage?

I'm too impatient to wait on compilers, and don't like the extra
storage of binaries.  Scripting languages forces authors to
distribute (hopefully non-obfuscated) code; reducing network and
storage costs, and that also lowers the barrier from user to
hacker.  Fwiw, I actually prefer Perl5 with the predictability
(and caveats of) refcounting over a GC like Ruby's.

</rant>