Re: Garbage Collection

[tpmsph@ecsvax.uncecs.edu (Thomas P. Morris)]

In article <6713@june.cs.washington.edu>, klaiber@june.cs.washington.edu (Alexander Klaiber) writes:
> The method of using key searches is, I believe, an unnecessary hack to
> avoid multiple references. Why bother to include a full B-tree package
> or such when we can do without? 
> 
You mean to tell us that you =intend= to keep your "mailing" lists
=permanently in memory=? You're =never= going to write them to a permanent
file system/disk? Or that your never going to keep a list larger than can
be kept in memory? Really!  The method of using key searches is a useful
aid to designing a program to read from a database larger than you can
keep in memory, which needs to be permanent, and which ought to be well-
behaved in terms of no =clobbering= other processes' performance!

:-) Yeah, I know that the mailing list abstraction was only for an example.
But a fairly poor example if you wish to justify structure sharing, which
*has* its proper place...

> Clearly, there is a tradeoff between readability and efficiency. In my
> version, I reduce efficiency by requiring garbage collection (although
> there exist rather powerful and fast GC algorithms). Your proposal
> would definitely increase the complexity of the program and, due to the
> overhead of a DB-lookup, might actually run slower than the version with
> GC --- depending on circumstances such as frequency of lookups in the 
> database, size of the database (your program) vs. time required for GC, 
> amount of garbage produced etc. (my program).
> 
For something like a database system (i.e. mailing lists, again), I would
have to disagree that a keyed-search DB-lookup paradigm is necessarily
less readable or understandable or complex than having to pre-process
the databases (to create your structure sharing) to have direct access,
which might not be reasonable or possible with a *large* database.

Your points about speed of access are well taken, but consider the end
user aspects: how many folks are going to wait 10, or 20, or 30 minutes
for you to read in the external data, build your structure sharing
structures, and =then= process their mailing list (personnel records,
widget manufacturing data, etc), rather than using a relational database
paradigm, using keyed access to those external permanent files?

> In my opinion, people tend to put too much emphasis on plain efficiency of
> their programs and too little on issues such as readability, reliability
> and extensibility --- and I contend that (depending on the application)
> garbage collection *CAN* help to improve all of these.

  Yes and yes. The mailing list example is and unfortunate choice perhaps.
And some of us put also put an emphasis on how well a system works for
its intended end-users!
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Tom Morris                                 BITNET: TOM@UNCSPHVX
UNC School of Public Health                UUCP  : ...!mcnc!ecsvax!tpmsph
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Tom Morris                                 BITNET: TOM@UNCSPHVX
UNC School of Public Health                UUCP  : ...!mcnc!ecsvax!tpmsph
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