Re: Paging file

[Hans Boldt <boldt@xxxxxxxxxx>]

Don wrote:
> TOOOOOOO bad the AIX, linux and Windows boys haven't picked up on Single
> Level Storeage yet....
Fascinating debate!

I first learned about the concept of single level store in school,
circa 1979. While it is certainly interesting, you do have to wonder
why, after all this time, there's really only one computer commonly
available that implements it.

As others have pointed out, there are always trade-offs in system
design. All modern CPU's and operating systems implement virtual
memory. Most implement it by giving each process its own address
space, which minimizes the risk of one process adversely affecting
others. But since all processes in an SLS architecture share the
same (humongous) virtual address space, you need some means of
ensuring that a process really is allowed to access a particular
region of store. That puts more overhead on securing memory
accesses, which requires substantial capabilities in the CPU itself.
It also meant 16 byte pointers, which can increase memory
requirements. If you remember back to the early days of the S/38, it
had significant performance problems, which adversely affected the
way system programming was done. (There *were* some very good
reasons for rewriting the RPG compiler for RPG IV!)

In terms of what it means to the average application developer, I
don't think it really makes much difference. You still read files
one record at a time, much like you would do in a non-SLS system.
Even the idea of memory-mapped files (which would seem to be an
obvious feature of a SLS system) is available in several non-SLS
OS's. And even components like the RPG IV compiler front-end don't
have any dependencies on the SLS architecture, which has allowed the
code to be ported to other systems (such as the CODE program
verifier and VA/RPG).

Cheers!  Hans