Re: [Squeak-VMdev] re: version 4 changes

[Avi B. <av...@be...>]

On Apr 7, 2004, at 12:22 AM, Craig Latta wrote:

> 	I added a few things to the swiki page, I'll repeat them here =
for the
> sake of discussion. Under the heading "Facilitate a number of
> anticipated extensions to Squeak":

In a similar vein: since I added something to the swiki about an=20
immutability bit, I'll bring it up here as well.  As I think G=F6ran=20
already mentioned, the most interesting use is in transactional=20
systems: these need to be able to find out, for some group of objects,=20=

which of them has changed state since some point in the past. =20
Currently, the usual pattern is to store a copy of every object in the=20=

group when the transaction begins, and then scan through them when the=20=

transaction ends looking for changes.  This is, of course, generally=20
pretty slow, especially if you're trying to manage large chunks of the=20=

image transactionally.  Having an immutability bit allows a for much=20
more efficient approach, and has some other nice uses too (I very much=20=

like, for example, that objects in the literal frame of a VisualWorks=20
method get marked as immutable).  The real question, I guess, is=20
whether transactional systems are considered an important enough set of=20=

applications to use a whole header bit to optimize them.  Since I=20
mostly work on business applications, which usually need transactional=20=

behavior, my bias is that they are important enough, but obviously not=20=

everyone will think so.

Avi