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<email@example.com> 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
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
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
Or was it memory hit? Or was that a bong hit I'm thinking
Seek truth from facts.