Tom Harper wrote:
Tracy,
I believe you are seeing the effects of queuing. Consider the opposite
issue. Suppose you are having response time problems and you wish to
resolve them. Sometimes adding just a small amount of resource will
reduce your response time dramatically. The converse is true: removing a
small amount of resource can increase your response time dramatically.
The reason the simple math model does not reflect reality is because
each of your transactions response times consist of the sum of times
needed of each resource, plus the sum of the times waiting for that
resource. So even though the resource usage is unchanged, the wait time
for the resource (in this case, the CPU), goes up. Sometimes you can see
this in queues at the grocery store or bank: adding a single checker or
teller can quickly reduce your time in line. The time to process your
transaction is unchanged, but the time you spend waiting got the process
to occur is reduced, and you are happy.
Some basic queuing theory very well explained, Tom!
IBM has a solution for this: Capacity on Demand. You configure your
system similar to what you are proposing, but when you need additional
capacity, it is provided. It is not free, however.
I would also suggest the use of soft capping rather than hard capping if
the hardware supports it. Spikes to handle sudden demand will be allowed
so long as the rolling four-hour average doesn't exceed 50% of the
processor. (LPAR Capacity Groups are a great way to manage this.)
And, if the processor is an older model, switching to a newer model at
the same "software MSU" rating can provide considerably more compute
power--without increasing software costs--due to the 10% Technology
Dividend. Example: a z10BC rated at 30 MSU delivers as much capacity
(based on LSPR measurements) as a z800 rated at 41 MSU or 37% more capacity.
--
Edward E Jaffe
Phoenix Software International, Inc
5200 W Century Blvd, Suite 800
Los Angeles, CA 90045
310-338-0400 x318
[email protected]
http://www.phoenixsoftware.com/
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