Re: WTF is up with memory usage nowadays?

[Sam Saffron <sam.saffron@gmail.com>]

As to who is at fault here, it is a little bit of "everyone" in a big bucket.

- The new GC is far more memory hungry than 1.9 line, even with
`RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR=1.5` it is still a lot more space
than it used to

- Ruby have been very slow at dealing with the "elephant in the room"
which is larger processes, stuff like
https://bugs.ruby-lang.org/issues/12967 goes ignored and is sadly
unlikely to happen for upcoming release

- The current focus for "Ruby" is 3x3 (ruby 3 being 3 times faster)
... there is no focus on reducing memory usage

- Most people use default c memory allocator, despite tcmalloc
offering quite a decent win
https://github.com/SamSaffron/allocator_bench

- mime types gem is a pariah... apparently EVERY install of rails
needs to hold 46 megabytes of mime types in memory cause ... I don't
know why ... (even with 'mime/types/columnar')

- Booting a rails app eats up half a million slots in your ruby heaps,
clearly the vast majority of this is unneeded waste. Stuff like
keeping the string "MIT" in memory 125 times forever cause "rubygems",
jumps out.  There is zero focus anywhere to fix this issue.

- Ruby heaps grow too fast, even if you put on the breaks, it is hard
to diagnose how you reached 7000 heaps at runtime when 6000 of them
are empty.

Overall there is tons that can be done, but unfortunately there is no
focus anywhere to fix stuff.

On Mon, Dec 12, 2016 at 1:10 PM, Eric Wong <e@80x24.org> wrote:
> <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>
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