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On 09/02/2009, at 4:23 AM, Lukas Beeler wrote:
As far as i have understood the OS/400 architecture (and i'm wrong:
please correct me here - there are people on this list that know far
more about this than me), memory is never used explicitly - this is
because of the single level storage system. Instead, the bits adressed
are always on-disk somewhere (or not yet, if not explicitly
synchronised). Memory just acts a giant disk cache, if compared to
other architectures like Windows, Linux or AIX, where memory is
explicitly adressed and allocated (and the "hack" of swapping was
invented to use more memory than is physically installed).
Generally correct. There is no "swap file" and main storage can be
thought of as a cache for disk but data must be moved from disk to
main storage before it can be processed. The way this happens is a
sort of "sleight of hand". Objects have a single virtual address.
That virtual address is mapped to a real address. The real address
will be either an auxiliary storage address or a main storage
address. This is why we say there is only ever a single copy of an
object. Part of it may be addressable in main storage while the
remainder is on disk but ALL of it is in the same virtual address
space. The part that is found in main storage is no longer
addressable on disk even though it still occupies space on disk.
Compare this with other systems that rely on a swap file. They have a
copy of an object (say word.exe) on disk, the user runs that program
and the entire thing is loaded into virtual storage (i.e., copied
into the swap file), and now you have two copies of word.exe, one on
disk and one in virtual storage, delete the object on disk and you
can continue to run the copy in virtual storage. Try deleting a
program object that is in use on XPF and see what happens. A learning
experience will ensue.
This will allow the IBM i to work with even extremely low amounts of
memory compared to other architectures, but obviously with a brutal
degradation of speed.
Speed degradation only occurs when main storage is over-committed and
you'll get that happening on "swap file" based systems too. Watch
what happens when WinDOS runs out of swap space. Unix isn't pretty
either when that happens. OS/400 will only fail when auxiliary
storage is full although if main storage is over-committed you'll get
"thrashing" as the system spends more and more of its time paging
stuff out of and in to main storage in an attempt to accomplish work.
However, as long as 5250 is used this can be
negligible as the system does not have to directly interprete data
entered on the 5250 terminal, instead the data is sent to the IBM i
screen-by-screen instead of key-by-key like vt100 terminals did.
It's true that the host system does not have to concern itself with
echoing individual keystrokes back to the terminal (who thought that
would be efficient) but each screen of data results in some further
processing--usually database related--and it is this activity that
can kill performance of even 5250 workload if there is insufficient
main storage to adequately service the number of 5250 sessions. In
addition we have issues caused by insufficient disk arms and gutless
processors.
I suspect the root cause of your upgrade problem was two-fold: 2 disk
arms simply isn't enough, CD/DVD drives are notoriously slow, and all
I/O is probably going through the same controller. The system is
probably spending a lot of time waiting for I/O to occur.
Regards,
Simon Coulter.
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