From: "M. Edward (Ed) Borasky" <znmeb@...>
Date: 2009-02-02T10:47:00+09:00
Subject: Re: Minor 1.8 <-> 1.9 performance comparison gotcha

On Sun, Feb 1, 2009 at 5:29 PM, Gregory Brown <gregory.t.brown@gmail.com> wrote:
> Excited and inspired by the Ruby 1.9.1 release, I wrote a blog post
> the other day talking about how much faster Prawn's examples and tests
> run on Ruby 1.9 out of the box.  Of course, this isn't scientific
> benchmarking but it seemed like a reasonable way to say "Across the
> board, things run much faster".
>
> As it turns out, there was a fatal flaw in my (admittedly
> unscientific) analysis.  It turned out that my Ruby 1.8.6 was built
> without any compile time optimization, but my Ruby 1.9 build had been
> set to -O2 .  The difference was HUGE and completely skewed my
> results.
>
> This is almost certainly a silly mistake that most people who have
> experience comparing manually compiled software would not make, but
> I'm sure it'll trip up some folks who don't know what to look for. So
> the lesson I learned was: If you're doing performance comparisons,
> either to share with the community or for your own needs, be sure to
> check your CFLAGS.
>
>  $ ruby -e "require 'rbconfig'; puts Config::CONFIG['CFLAGS']"
>  $ ruby19 -e "require 'rbconfig'; puts Config::CONFIG['CFLAGS']"
>
> If the optimization levels don't match, all bets are off :)
>
> You can see the before[0] and after[1] if you wish, otherwise, sorry
> to spam those in the know.
> Hopefully this quick warning will be helpful to those who might not
> have considered it otherwise.
>
> -greg
>
> PS: Jamis Buck helped me track this issue down, so if he's reading: Thanks!
>
> [0] http://blog.majesticseacreature.com/archives/2009.01/prawn_and_ruby19.html
> [1] http://blog.majesticseacreature.com/archives/2009.02/lies_and_statistics.html
>
>

It's been a while since I recompiled either of them, but as long as
you're going to recompile:

1. As far as I know, '-O3' is safe for both 1.8.x and 1.9.x. I haven't
measured the performance improvement recently, but someone did a while
back and it was at least measurable.

2. You should also take advantage of the GCC '-march=' and '-mtune='
options and specify exactly what kind of processor(s) you have. You
may have to do a Google search of the GCC documentation on their web
site to find the exact list of processor types, but it is usually
worth it. Note that most Linux distros, with the happy exception of
openSUSE, provide binaries compiled with only i386 processor features
on 32-bit x86 machines. openSUSE compiles with i586 on 32-bit x86
machines. I'm not even sure you can get enough RAM in an i386 to even
run the 2.6 kernel, let alone a desktop or a Ruby interpreter. :)

-- 
M. Edward (Ed) Borasky

I've never met a happy clam. In fact, most of them were pretty steamed.