Low perf with smp

[Mark Kirkwood]

Eric wrote:
> Thanks a lot Lucas.
> What's sound so strange to me is that  there we so much difference 
> 30sec=> 54 sec simply by activating smp in kernel.
> But I will search on this way and see if it don't come from mysql.
> 
> I will send  you issue.
> 
> Eric.
> 


ISTR there was some business with Mysql calling gettimeofday a lot, and 
therefore being particularly impacted by the sysctl 
'kern.timecounter.hardware'.

As I understand it, when you activate SMP a more accurate, but slower 
timecounter is chosen (typically 'i8254' instead of 'TSC' on intel HW).

It might be interesting to see what happens to your times by forcing TSC 
and re-running the test (ahem - better only do this on a test box, as 
there is bound to be a good reason for the more accurate timecounter 
being chosen in SMP mode....).

Cheers

Mark