[LBo] Performance of Multiprocessor Systems

[Stefan Waidele]

Hi everybody,

Andrew Henry schrieb:
> mars mr wrote:
>> AND NO, if you have Dual CORE at 1,66 GHz you wont
>> have same power as 1 processor at 3,2 GHz. 
>>
>> a second processor at the same speed give around 50%
>> more processing power since it has to sincronize with
>> the first processor.
>>   
> Where did you get this information from?  I do not think it is correct
> but would like to read your source.  As far as I know, a dual core
> processor package runs at the stated GHz speed (1.66 GHz in this case)
> but there are 2 distinct cores within the package that split the
> workload transparently for the applications

According to "Amdahl's Law" 
(http://en.wikipedia.org/wiki/Amdahl%27s_Law) the performance of 
SMP-systems does not scale ideally with the number of processors.

The more processors there are, the more "bookkeeping" needs to be done 
to coordinate the processors.

While each "core" might run a different (set of) program(s), access to 
RAM and cache memory needs to be serialized. The same for harddisk or 
network access.

Doubling the number of cpus will not double the performance.

> --apps do not need to be
> multicore aware to be able to use the chips features (this is still
> true, despite what I write about processes and multi threading below). 

Yes. Even if we ran only one application, the second core could handle 
the drawinf of the windows and stuff. Today's systems are so 
multi-tasking oriented that a second core will not be idle for long :)

> You wrongly refer to core #2 as a second processor and imply that core
> #2 runs at 0.5x1.66GHz

No, he does not. He assumes the speed increase will be about 1.5 (Twice 
the raw power minus the overhead in SMP-systems)

> [Intel GHz vs. AMD Speedrating]
> [...]
> If you notice, server chips that are the
> best performers only run at 1.8GHz (Xeon/Opteron) whereas desktop
> processors are hitting 4GHz, but the server processors are obviously
> faster.

That shows nicely that "GHz ain't everything".
CPU speed also depends on such things as cache-size and strategy, 
branch-prediction, the number and granularity of the steps in the 
pipeline,...

But I want a dual-core, anyways :)

Stefan