Mail Archives: cygwin/1998/06/26/08:55:36
On Thursday, June 25, 1998 06:24 AM, Edward Avis wrote:
> AFAIK, Windows loads a _separate copy_ of each DLL for every program
> that uses it. That's one of the reasons it uses so much memory.
>
> (Please tell me I'm wrong... :-( )
You're wrong. ;-)
In fact, under both Win9x and NT each process runs in its own
virtual machine, isolated from all the other processes (except
16-bit programs, which run all together in a single VM).
Under each virtual machine, the DLLs required by the program are
loaded at some virtual address. The same DLL can be loaded in
more than one VM, maybe at different addresses. But it _is_ the
same DLL, and even if it looks like it has been loaded multiple
times, all the "copies" are actually sharing the same physical
memory. This, of course, only applies to read-only code and data.
Writeable data is actually replicated as many times as necessary.
The main reason Windows 9x and NT uses so much memory is
due to Microsoft having program size reduction way down in its
priority list. After all, they work under the assumption that RAM,
disk and CPU prices are always lower than the salaries of expert
programmers able to trim down programs to reasonable sizes
(not counting the time and expenses involved in the extra time
it takes to optimize complex, code). To them it would always
make more sense to sell the bloatware produced by automated
code wizards (a.k.a. program generators to us old-timers) than
to hire experts to write good, small code.
Time/size tradeoffs have been known in CS for a very long time.
But time used to mean run time, not time to market!
Leo Mauro
Principal Scientist
TeleSys Technologies, Inc.
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