Re: How are CPU Speed and Overall CPW Related?

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

It's great to get a response from you, Alexei.  I think we all appreciate
your insider insight.  I won't bother adding more memory now, because at
this point I'm interested in raw CPU speed.

I'm aware that CPW is a measure pertaining to a mixed workload under load
testing and is affected by overall system design, but I still question
AS/400 CPU performance.

I wondered about a CPU governor for several reasons.  The program we were
benchmarking was entirely CPU bound.  The program simply repeated a few
String functions, in memory, maybe 100,000 times within an RPG do while
loop.  Both machines had about the same RAM.  Neither had L2 Cache.  I'm
talking about a model 170-2160 and a model 170-2290.  Yet the 100 Mhz
machine executed the code faster than the 200 Mhz machine.  If I were
running the same logic on a pair of Intel boxes, identical in every way
except Mhz, then I'm fairly certain the higher Mhz machine would be faster.

What makes the AS/400 CPU different, in this limited scenario?

Thanks,

Nathan.


> From: "Alexei Pytel" <pytel@us.ibm.com>
> Subject: Re: How are CPU Speed and Overall CPW Related?
>
> I see that now you suspect that IBM is putting a horde of governors all
> over the place.
> This is not the case.
>
> If your program uses little memory - it is not memory constrained - so
> adding more memory will not change anything for it.
> But if it requires lots of memory, and real memory is not big enough, and
> parts of a program or data get paged in and out - then your application is
> memory constrained. Adding more memory will reduce or eliminate paging
> overhead and make your program run faster.
>
> Same logic applies to disk constrained - 100 disk drives can perform more
> disk accesses per second than 10 disk drives.
> So if your application needs 1000000 disk accesses to complete, then on a
> 100 drive system it will finish faster than on 10 drive system.
>
> That simple. And no need for governors...
>
>
> Best regards
>     Alexei Pytel
> System Performance III
> Dept XQK/006-2    Rochester, MN
> (507) 253- 2867 or T/L 553-2867
> Internet:  pytel@us.ibm.com  VM mail: IBMUSM07(PYTEL)


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