Improved multiprocessor usage on amd64

[Stephen Montgomery-Smith]

cpghost wrote:
> On Sun, Sep 14, 2008 at 08:38:00PM -0500, Stephen Montgomery-Smith wrote:
>> I have a dual core amd64 on which I run a processor intensive numerical 
>> program.  I had been frustrated because it seemed to run 3 or 4 times 
>> faster under Linux.  But with a recent upgrade of FreeBSD-CURRENT, it 
>> now goes at about the same speed as Linux.
>>
>> The program takes about an hour.  For the first minute, the program runs 
>> rather slowly, but then it is as if the operating system finds its way, 
>> and suddenly it speeds up.  "top -H" suggests that for the first minute 
>> that one thread is going really slowly, and is perhaps being starved or 
>> something.
>>
>> My question is - why is this happening, and is this something I should 
>> expect?  Are there certain switches or sysctls I can set to make it go 
>> fast from the get go?
> 
> It looks like you're running powerd (see in /etc/rc.conf). It can take up
> to a minute for the load average of the machine to exceed a certain
> threshold where powerd would finally bump the cpu(s) to full speed.
> 
> As for sysctls, check the speed with something like:
> 
> # sysctl dev.cpu.0

Excellent idea!  I should have thought of that myself.  Unfortunately it
didn't help much when I switched powerd off.