[LBo] Performance of Multiprocessor Systems

[Allen]

This is very odd seen one way. If 2 cores/CPUs is only 1.5 as 
fast as a single core due to SNMP overhead, this would imply that 
a 4 core/CPU machine might not even reach 2x a single core/CPU. 
If the SNMP overhead is so high then how do super computers with 
thousands of cores/CPUs get up to teraflops?

I don't think this is accurate from some points of view as a 
company I know, Bivio, uses 7 dual core PPC chips and gets almost 
exactly 13 times the throughput on dnetc.org calcs as I get on a 
single core 3 GHz Intel.

Also, my next door neighbor has a dual core Intel at 2.8 GHz 
running dnetc client and get 13+ million calcs a second and I 
have a single core 3.0 Intel single core getting 7.1 million 
calcs per second.

Allen

Stefan Waidele wrote:
> 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