Re: teraspace, user spaces, etc. -- MI400


Steve Richter asks:

>What apps need untagged ptrs?

Strictly speaking, none should.  If all applications were strictly ANSI C
compliant, none would.

But, there are some well-worn practices in the rest of the world where
"integers" passed back to callers in C turn out to, covertly, be
"pointers".

These programs' porting to iSeries is eased with an untagged strategy.

>would not a sls space or teraspace implicitly be a file mapped object?

There's all that code out there that lives with what I call the "poor man's
single level store" paradigms of Unix.  They expect to "map" things into a
process local storage (unaddressable by anyone else, without these poor
man's interfaces) where you and I would simply grab a tagged pointer and
run with it.

>If so, why not or was the teraspace made a full mbr of the single level
>store concept?

This is all about how the rest of the world runs, with no idea that there
could be a single level store, and bringing them into the single level
store fold, not how we run, having full visibility to it.  In Single Level
Store, a uniform address space is potentially shared by default.  In Unix,
a process local store is local and unshared by default.

What is therefore obvious to you is unobvious to millions of lines of C
code and their programmers.  Teraspace lets them code the way they always
have, yet achieve the blessings of the real thing.  It isn't that our
system of SLS "wouldn't work".  It is rather that the scheme used in the
Unix world has certain requirements which produce not-quite-identical
results though trying to do the same basic job.  It is easier to
accommodate their requirements with the teraspace scheme (allowing them to
use SLS in their own fashion) than to ask them to change app code to run by
our rules.  For instance, the Unix sharing scheme sometimes is nearly
identical, semantically, to single level store; two jobs access the same
storage with the same address.  But, sometimes, the same shared storage is
accessed with different local addresses.  That latter usage is not readily
implemented in single level store proper.  Teraspace allows them to get at
SLS with their own set of rules, as natural to them as SLS is to us.

So, if you don't need this stuff, you also don't need to bother with it.
Grab a pointer, just as you always have, and get on with life.


Larry W. Loen  -   Senior Java and iSeries Performance Analyst
                          Dept HP4, Rochester MN


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