Newsgroups: comp.lang.dylan,comp.lang.lisp,comp.lang.java
Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!oitnews.harvard.edu!newsfeed.rice.edu!bcm.tmc.edu!cs.utexas.edu!venus.sun.com!wnoc-sfc-news!kogwy!math-keio!mad
From: mad@math.keio.ac.jp (MAEDA Atusi)
Subject: Re: Garbage collection cost (was Re: Parenthesized syntax challenge)
In-Reply-To: Simon Kinahan's message of Mon, 16 Oct 1995 14:21:59 GMT
Message-ID: <MAD.95Oct18040436@tanzanite.math.keio.ac.jp>
Sender: news@math.keio.ac.jp
Nntp-Posting-Host: tanzanite
Reply-To: mad@math.keio.ac.jp
Organization: Faculty of Sci. and Tech., Keio Univ., Yokohama, Japan.
References: <44aa9a$j5h@miso.cs.uq.edu.au> <LUDEMANN.95Oct6140930@expernet26.expernet.com>
	<DGApp8.J41@undergrad.math.uwaterloo.ca>
	<MAD.95Oct13123618@tanzanite.math.keio.ac.jp>
	<45ksdk$7gr@jive.cs.utexas.edu> <DGJp8o.7nF@Cadence.COM>
Date: Tue, 17 Oct 1995 19:04:35 GMT
Lines: 43
Xref: glinda.oz.cs.cmu.edu comp.lang.dylan:5438 comp.lang.lisp:19552 comp.lang.java:1892

In article <DGJp8o.7nF@Cadence.COM> Simon Kinahan <simonk> writes:

    simonk> wilson@cs.utexas.edu (Paul Wilson) wrote:
    >> For more info, you might see followups that made it to some of the three
    >> newsgroups the original post went to (but not all), and a recent thread
    >> in comp.lang.scheme.  Hans Boehm has clearly argued (again) that the
    >> asymptotic arguments about GC cost (and about relative costs of copying
    >> vs. non-copying GC) just don't wash.  He (and I) have repeatedly tried
    >> to kill all of these myths, but they're very hardy memes.  There are
    >> some similar misunderstandings of generational GC issues.

    simonk> I agree that 'in practice' GC is not faster. However mathematically
    simonk> a copying GC program is faster asymptoptically than its malloc/free
    simonk> equivalent. It is not a myth jsut people mistaking maths for reality :-)

He pointed out that my reasoning about why GC is faster is a myth and
also mentioned about "asymptotic complexity myth".

My statement (gc is faster than free because cost of free is
proportional to the amount of objects died during computation while gc
cost isn't) came from Kenneth Anderson's article "Courage in Profiles"
in Lisp Pointers Vol.8, No.1.  (He is also the author of the article I
quoted in my previous post.)

When I read that excellent article I accepted the reasoning without
thinking much since it sounds logical at first glance.  (A question
came to mind was "What about reference counting?".  But I, er,
deferred thinking about that.)

All the references shown by Paul Wilson say that things are not that
simple.  Hans Boem's web page ftp://parcftp.xerox.com/pub/gc/complexity.html
was especially clear and convincing.  Now I understand why "copying
collector is faster than mark and sweep" is a myth.

Also, in his survey on memory allocator Wilson warned that it is
dangerous to relying too much on simplified mathematical model.
(Although I haven't thoroughly read it yet; it's a *long* survey.)

;;;  Keio University
;;;    Faculty of Science and Technology
;;;      Department of Math
;;;		MAEDA Atusi (In Japan we write our family names first.)
;;;		mad@math.keio.ac.jp
