On Jul 14, 2007, at 9:01 AM, John S. Giltner, Jr. wrote:
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The "Mhz" rating of a zSeries CPU is slow than most of modern
processors. However that does not mean a computer using a slower
processor is slower than a computer using a faster process. That
is like saying just be cause the engine in one car runs at a higher
RPM makes it faster than a car whose engine that runs at a lower
RPM. The problem is that ignores the transmission and differential
gearing.
In this case it ignores the CPU's various instruction sets. Say
you have two CPU that have different instruction sets. During a
test it was found that CPU#1 can execute a mixed set of it
instruction at the rate of 100 MIPS, and CPU#2 tested at 200 MIPS.
Which is faster? You can't tell. The problem is that CPU#2 may
have a less efficient set of instructions and you may have to
execute 3 instructions to do the same amount of work as CPU#1 does
with 1 instruction.
I remember reading a article about the FLEX/ES software when it
first came out. They were taking about the difference in the two
instruction sets. The lowest instruction ratio was 1:2, zSeries to
Intel, the maximum was 1:1200 and the average was 1:17. So on
average for every 1 zSeries instruction they had to execute 17
Intel instructions to do the same work.
You also have the fact that some functions on zSeries are handled
by the hardware. Memory protection for one, zSeries hardware
prevents one task from getting to another tasks memory. I know on
the Intel platform this must be handled by the OS, so this is more
instructions that the OS must execute and more work done by the CPU.
IBM published a paper on the differences between zSeries and
PowerPC and the fact that zSeries CPU's are much better for SMP
environments that run varied workloads on a single computer. This
deals with the way that L2 cache works and what happens when you
have a context switch. On the distributed platforms L2 is not
shared (pre-multi-core).
Some distributed OS's will actually hold up work if the CPU it was
last dispatched on is not available right now, this is to prevent
the context switch on another CPU.
John,
Any chance this is online & where?
Ed
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