make -j$n buildworld : use of -j investigated

[Rob]

David Schwartz wrote:
>>According to my formula:
>>
>>   time(minutes) = 1e5 / ( speed(MHz) * nproc )
>>
>>and taking nproc = 1, this results in
>>
>>   time = 1e5 / 2798.66 = 36 minutes
>>
>>Quite accurate for your system as well. At least this formula gives a
>>resonable estimate about the compile time.
>>
>>Apparently HT does not help much, since with SMP kernel, times do not
>>get any close to 36/2 = 18 minutes. However, there is a slight improvement
>>from -j1 to -j2 with SMP.
> 
> 
> 	You're saying that HT does not help because it doesn't double the speed?
> That makes no sense. HT helps if it brings the time down. If it brings it
> down to 34 minutes, it helps a little. If it brings it down to 24 minutes,
> it helps quite a bit.	It's unrealistic to even imagine that HT will bring
> anything close to the benefit of a second physical CPU.

OK, sorry for me rushing into conclusions. All I tried to say was that in
cases when 'nproc' plays are role, then nproc=2 with a second CPU, but it
is somewhere between 1 and 2 for HT systems; possibly closer to 1 than to
2 (but indeed still larger than 1 !)

Rob.