Just rambling about optimization...

[Richard Harter]

In article <13089 at dog.ee.lbl.gov>, torek at elf.ee.lbl.gov (Chris Torek) writes:

> Indeed.  It has been observed (sorry, no references, but this is not
> only popularly known but also true :-) ) that many computers spend most
> of their time copying data, rather than `computing'.  This is largely
> why a fast bcopy/memcpy/memmove is important.  A fast block copy is
> a wonderful thing, but better yet is to eliminate the copies altogether.

True enough.  One of the traps in construction of large software is that
when one breaks up into components there is a real tendency copy data
from component to component as a matter of convenience.  For example
suppose we have components A, B, and C with data flowing from A to B to
C.  The easy way to deal with this is to make each component its own source
and sink.  I.e. A creates a data set, passes it to B, and then disposes
of it after B is completed.  B copies its input into its own area, passes
its processed data to C, and then disposes of it, and C does the same
thing.  Stated this baldly, it is clear that A should be the source, C
the sink, and nobody should copy.  However, if we look at the general
case, A, B, C, etc will be defined and implemented independently, i.e.
modularly.  If each module has responsibility for creating and disposing
of its own data then inter-module coupling is minimized and there are
no nasty questions about who disposes of what.  

Which, of course, is why it pays to use object-oriented techniques even
in non OO languages.  In this case it would pay to package the data set
with a reference count and a module of data-set methods.  Each user ups
the count upon access and decrements upon dispose.  When the count goes
to 0 the actual physical disposition is done.  [Operations hidden behind
macro or function calls, of course.]
-- 
Richard Harter, Software Maintenance and Development Systems, Inc.
Net address: jjmhome!smds!rh Phone: 508-369-7398 
US Mail: SMDS Inc., PO Box 555, Concord MA 01742
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