Re: How are CPU Speed and Overall CPW Related?

["Nathan M. Andelin" <nathanma@xxxxxxxxx>]

"For CPU-intensive batch applications, run time scales inversely with
Relative Performance Rating (CPWs). This assumes that the number synchronous
disk I/Os are only a small factor."

AS/400 Performance Capabilities Reference, V4R5

Should I understand that quote to mean that as CPW goes up, elapsed CPU time
goes down proportionately?  If so, that's precisely what happened in the
following example:


d TheString       s             50
d TheShorter      s             50    inz('TEST STRING')
d Nbr             s              6s 0

d #Now            s               z
d #Then           s               z

c                   time                    #Then

c                   do        100000        Nbr
c                   evalr     TheString = %trim(TheShorter)
c                   enddo

c                   time                    #Now
c     #Now          subdur    #Then         ##Sec:*MS        15 0
c                   return

 ##SEC = 000000001266000
Program Example from Ken Slough, compiled with *FULL optimization.

Since the program uses the EVALR op code, I assume that both machines were
using the V4R4 compiler - that's when EVALR came out.  So, CPU speed seemed
to have little to do with it.  What seemed to count was CPW rating.  Go
figure.


----- Original -------------------------

From: "Alexei Pytel" <pytel@us.ibm.com>

I can suggest three possible explanations.
One is pretty straightforward - you may be mistaken in relation to cache
sizes.

Another is also quite simple.
RPG program always uses some runtime support - string manipulation BIFs
etc.
So if you have different OS releases, or PTF levels, then RPG program may
have different performance - due to software, not hardware differences. In
the same vein, different levels of compilers may produce different code
quality.

The last one is more far-fetched.
When you create a program, it is optimized for some target CPU (which is
different for different releases).
You then save and restore the program to a different box with different CPU
chips.
RISC processors are very sensitive to pipeline filling. Different chips
have different pipeline characteristics.
So the program highly optimized for Apache will not necessarily run
optimally on a newer chip.
I am not sure how to test it - try CHGPGM with forced program recreation.

Alexei Pytel



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