> > > >---- Original Message ---- >From: ron.l.john...@cox.net >To: debian-user@lists.debian.org >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 >Reid<rei...@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.
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) Larry >> >>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 >>> of? >>> >> >> >> >>-- >>Seek truth from facts. >> >> >>-- >>To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org >>with a subject of "unsubscribe". Trouble? Contact >listmas...@lists.debian.org >>Archive: http://lists.debian.org/4cc1369d.8070...@cox.net >> >> -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: http://lists.debian.org/380-2201010522153419...@netptc.net
