From: Michael Selig <michael.selig@...>
Date: 2006-11-08T14:53:38+09:00
Subject: Ruby performanmce improvements

I know you guys are in the middle of YARV stuff, but I thought you might be
interested in some performance improvements (around 20%) I made to Ruby in
my spare time recently.

One afternoon when I had a bit of spare time I decided to compile Ruby with
profiling (C profiling that is), to try to see where it was spending most of
its time. Also I needed some benchmarks to test with, so, for want of
anything better, I took some ruby scripts from the "Computer Language
Shootout" (http://shootout.alioth.debian.org/).

I found 3 main areas which I then focused on:
1) rb_call() especially rb_call0()
2) Loops & rb_yield(), especially rb_yield_0()
3) The large number of recursive calls to rb_eval()

So addressing these one at a time:
1) rb_call()
I found that many of these calls are to CFUNC (internal C) methods such as
Fixnum#+, most of which are simple, and don't access anything other than
their parameters. However, in order to execute these methods, ruby is still
doing quite a bit of work in rb_call0(), such as setting up a stack frame
etc, which is usually not used. So I made an optimisation that if a CFUNC
method is being called, and no block is being passed to it, then it bypasses
stack frame creation. (I also had to make a small hack to "backtrace()" to
handle errors in this case).

2) Loops & rb_yield()
Most of Ruby's loops end up calling rb_yield() for each iteration. However
much of the work that rb_yield_0 does each time can be often be done once at
the start of the loop:
    - Pushing frame, block, vars, class etc
    - Freeing/creation of dynamic variables (instead they can be initialized
to NIL each time, unless the context has been saved)
Also loops done by rb_iterate() (via an IFUNC stub) can be simplified
further to reduce the overhead of "double-yield".

3) rb_eval() calls
I simply inlined the code for common, simple nodes such as: NODE_LIT,
NODE_LVAR etc in certain places within rb_eval(), which resulted in a large
reduction in the number of calls to rb_eval().

After applying these mods, I was able to achieve a performance improvement
averaging 20% (ie: 20% less User CPU time) under Linux/386 in the "shootout"
benchmarks. I know these are not representative of Ruby programs in general,
but I had to use something that was simple to time.

My mods are based on Ruby 1.8.4.

I am not 100% sure that my mods haven't broken something - especially in the
Garbage Collector (which I haven't really tried to understand), but it seems
to work fine here. I did not touch the parser at all.

Please let me know if you'd like me to post a patch. If I get some more
spare time, I'd be happy to look at the Ruby 2.0 code to see if similar
optimizations can be made.

Mike