From: Roger Pack <rogerpack2005@...>
Date: 2008-07-20T09:43:50+09:00
Subject: Re: JRuby performance questions answered

Rick Denatale wrote:

> Roger, you REALLY need to read the literature on GC which has been
> accumulating for the past 50 years.
> 
> Reference counting is pretty much an obsolete approach to GC.  It was
> probably the first approach taken for lisp back in the 1950s. Other
> language implementations usually started with reference counting (e.g.
> the first Smalltalk).
> 
> It's main advantage is that it's easy to understand. On the other hand
> it incurs a large overhead since counts need to be
> incremented/decremented on every assignment.  It can't detect circular
> lists of dead objects. In early Smalltalk programs when reference
> counting was used, you needed to explicitly nil out references to
> break such chains.  There's also the issue of the overhead for storing
> the reference count, and how many bits to allocate.  Most reference
> counting implementations punt when the reference count overflows, they
> treat a 'full' count as an infinite count and no longer decrement it,
> leading to more uncollectable objects.
> 
> Mark and sweep, such as is used in the Ruby 1.8 implementation quickly
> replaced reference counting as the simplest GC considered for real
> use.

This is somewhat confusing for me, because it seems that using a mark 
and sweep generational style GC slows down Ruby, as per

Matz's observations: http://www.ruby-forum.com/topic/123747

On my own I tried integrating Boehm's GC as a drop in replacement and it 
seemed to cause a serious slowdown [possibly because I didn't integrate 
it right :P ]

Also macruby reports that their runtime is slightly slower, I'll make a 
big assumption--because it uses a conservative GC so it can't allocate 
as cheaply [however their garbage collection is faster, and runs in a 
different thread so doesn't cause a program halt--so not all bad there]

Performance-wise, Python uses reference counting and seems to have 
'acceptable' performance.  They avoid the circular problems by running a 
full mark and sweep every so often.

I suppose I'm just naive, but it doesn't seem clear to me which is the 
'best' GC style, from the above observations.  There appears to be no 
clear answer.
Cheers!
-R


> Which particular GC approach is best for Ruby is subject to some study.
-- 
Posted via http://www.ruby-forum.com/.