RE: More slower CPUs vs. fewer faster CPUs

From: Brian D. Silverio <bsilverio_at_necc.mass.edu>
Date: Fri, 23 Feb 2001 09:00:14 -0800
Message-ID: <F001.002BC009.20010223084142@fatcity.com>

The following discussion assumes "big buss SMP" not MPP or any of its derivations. Your mileage may vary.

If you have one CPU then all of the multiple CPU code in the kernel is not used. There is a penalty that is paid for the second and subsequent cpu's.

So, the answer is....... it depends!

If your particular version of Unix scales well WITH YOUR WORKLOAD, you might expect to get something like 95% of the second CPU. If it does not scale well, then you might get 80% of the CPU. Subsequent CPU's give even less value.

If CPU #2 gave 90% of CPU #1 then CPU #3 can be expected to give 90% of CPU #2! So with 3 CPU's you get a total of 2.71 times the power of the one CPU system.
CPU #1 = 1.0, CPU #2 = 0.9, CPU #3 = 0.81 1.0 + 0.9 + 0.81 = 2.71

So all other things being equal, fewer CPU's is better. One is best. Unfortunately, all other things are never equal.

Brian

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Author: Brian D. Silverio
  INET: bsilverio_at_necc.mass.edu

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