Bumping MAXCPU on amd64?

Curtis Penner curtis.penner2 at gmail.com
Wed Sep 22 17:33:46 UTC 2010

MAXCPU at 32 has been good in the 32bit days.  Soon there will be (if 
not already) systems that will have 16cores/socket or more, and 
motherboards that have 4 sockets or more.  Combining this with 
hyper-threading, you have gone significantly beyond the limits of 
feasible server.

Bumping the number now is not feasible.  But in release 9+ or 10, this 
number could be bumped to something in the order of 1024 or more.  This 
will not be easy as there are considerable performance and compatibility 
problems.  But with Moore's law it will happen, and freeBSD will need to 
adapt to stay relevant.

Curtis Penner

On 09/22/2010 06:37 AM, John Baldwin wrote:
> On Wednesday, September 22, 2010 6:36:56 am Maxim Sobolev wrote:
>> Hi,
>> Is there any reason to keep MAXCPU at 16 in the default kernel config?
>> There are quite few servers on the market today that have 24 or even 32
>> physical cores. With hyper-threading this can even go as high as 48 or
>> 64 virtual cpus. People who buy such hardware might get very
>> disappointed finding out that the FreeBSD is not going to use such
>> hardware to its full potential.
>> Does anybody object if I'd bump MAXCPU to 32, which is still low but
>> might me more reasonable default these days, or at least make it an
>> kernel configuration option documented in the NOTES?
> ?
> % grep MAXCPU ~/work/freebsd/svn/head/sys/amd64/include/param.h
> #define MAXCPU          32
> #define MAXCPU          1
> In fact:
> % grep MAXCPU ~/work/freebsd/svn/stable/8/sys/amd64/include/param.h
> #define MAXCPU          32
> #define MAXCPU          1
> Unfortunately this can't be MFC'd to 7 as it would destroy the ABI for
> existing klds.

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