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Re: Debian stock kernel config -- CONFIG_NR_CPUS=32?

owens@netptc.net put forth on 10/22/2010 5:18 PM:

> Ron et al
> See the following:
> http://en.wikipedia.org/wiki/Amdahl's_law
> Larry

Amdahl's law doesn't apply to capacity systems, only capability systems.
 Capacity systems are limited almost exclusively by memory,
IPC/coherence, and I/O bandwidth, most often the last of the three.

I think many folks forget about this "3rd rail" of system performance
when they go shopping for the latest/greatest high frequency multi-core
processor.  Even mainstream desktops all ship with at minimum a dual
core CPU today, and a single 7.2K RPM disk.  Those two cores combined
have I/O bandwidth a few thousand times greater than the disk.

Until the average system has I/O bandwidth that's much closer to parity
with the CPU bandwidth, we don't really need to worry about Amdahl's
law, or any other scalability laws.


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