Re: Debian stock kernel config -- CONFIG_NR_CPUS=32?
>---- Original Message ----
>Subject: Re: Debian stock kernel config -- CONFIG_NR_CPUS=32?
>Date: Fri, 22 Oct 2010 02:00:45 -0500
>>On 10/22/2010 12:53 AM, Arthur Machlas wrote:
>>> On Thu, Oct 21, 2010 at 8:15 PM, Andrew
>>>> 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
>>> I roll my own, which on my puny systems is either 4 or 8. I seem
>>> recall reading that there is a slight performance hit for every
>>> 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.
In fact IIRC the additional overhead follows the square of the number
of CPUs. I seem to recall this was called Amdahl's Law after Gene
Amdahl of IBM (and later his own company)
>>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
>>Seek truth from facts.
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