Re: need help making shell script use two CPUs/cores

[Stan Hoeppner <stan@hardwarefreak.com>]

Carl Johnson put forth on 1/13/2011 11:34 AM:

> Processors      Time (seconds)
> P1              66
> P2              36
> P3              25
> P4              20
> P5              20
> P6              20
> P7              20
> P8              20
> 
> I am sure the time would have increased if the system had run out of
> memory and had to start swapping.  The system is not completely idle
> since I am running a KDE 4.4.5 desktop and VirtualBox with two guest
> OSs (Debian and NetBSD).  I suspect it would have closer to linear
> scaling if the system had been completely idle.

Your numbers bear out exactly what I predicted.  Look at the decrease in run
time from 1 to 2, 2 to 3, and from 3 to 4 processes:

#CPUs	Decremental run	time	Fractional gain per CPU
2	30s  			1/2
3	11s			1/6th
4	 5s			1/13th

You can clearly see the effects of serious memory contention when 3 cores are
pegged.  Bringing the 4th core into the mix yields almost nothing compared to
three cores, cutting only 5 seconds from a 66 second run time.

I'm anxious to see someone's results for a Phenom II X2 with the 6MB L2 cache to
verify my prediction there.  That's a tougher prediction though as I haven't
modeled the cache behavior of Imagemagick's convert program.  And the data above
shows it seems to be very memory b/w heavy.  Such a test would definitely be
very revealing of the effectiveness of the Phenom II X2's L3 cache, given what
we've seen so far.

-- 
Stan