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

On 10/22/2010 12:53 AM, Arthur Machlas wrote:
On Thu, Oct 21, 2010 at 8:15 PM, Andrew Reid<reidac@bellatlantic.net>  wrote:
  But I'm curious if anyone on the list knows the rationale for
distributing kernels with this set to 32.  Is that just a
reasonable number that's never been updated?  Or is there some
complication that arises after 32 cores, and should I be more
careful about tuning other parameters?

I've always set the number of cores to exactly how many I have x2 when
I roll my own, which on my puny systems is either 4 or 8. I seem to
recall reading that there is a slight performance hit for every core
you support.

Correct. The amount of effort needed for cross-CPU communication, cache coherency and OS process coordination increases much more than linearly as you add CPUs.

Crossbar communication (introduced first, I think, by DEC/Compaq in 2001) eliminated a lot of the latency in multi-CPU communications which plagues bus-based systems.

AMD used a similar mesh in it's dual-core CPUs (not surprising, since many DEC engineer went to AMD). Harder to design, but much faster.

Intel's first (and 2nd?) gen multi-core machines were bus-based; easier to design, quicker to get to market, but a lot slower.

(OP's machine is certainly NUMA, where communication between cores on a chip is much faster than communication with cores on a different chip.)

             Or was it memory hit? Or was that a bong hit I'm thinking

Seek truth from facts.

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