Multiprocessor system VS one processor system

[Erich Dollansky]

Hi,

Artem Koutchine wrote:

> twice as fast than on dual 500Mhz, however, to box will
> become a bit less responsive, because in a dual box

Yes.

> when one CPU is loaded, another one may have some
> spare time to respond. However, in case of very loaded
> box both CPUs a extremely loaded and the box will
> be as responsive as single 1000Mhz CPU  box 100% loaded.

No. It depends very much on the application.

Two CPUs have two caches. As long as the data still fits into the 
caches of both CPU but does not fit into the cache of a single 
CPU, the dual CPU machine is much much faster. As soon as the data 
do not fit into the cache anymore the degration on a SMP machine 
are much more noticeable.

> So, according to what i just said there is not real reason to
> setup a dual CPU box under FREEBSD unless you want to
> have 5000Mhz box, but there is no CPU with such speed, so

I would always go for as many CPUs as possible even if the clock 
speed of the CPUs is lower. It still makes sense to run a dual 
Pentium III machine instead of a single CPU Pentium IV. Maybe not 
with 500 MHz, but with 1 GHZ and above.

> you put two 2500Mhz CPU in it and if you are running a lot

You never get a higher speed as the speed of a single CPUs. What 
you get is a higher through-put. The time to do a certain task 
depends on the clock speed of a CPU and not on the number of CPUs 
as long the task is just one process with one thread.

> of processes with short run periods you will get overal
> perfomance somewhere near 5000Mhz. However, when i 
> subsituted two 500Mhz CPUs with 1500Mhz one (even with
> less 2nd level cache) on a heavy loaded web server i notice
> that sites started to load faster. So, it seems as 
> one 3X Mhz  CPU is faster that two X MHz CPUs, at least
> for web server with sql base and many perl scripts.
> 
This is true as long there is no load until the CPU cache comes 
into the game. It also depends on how the CPUs are connected to 
the main memory.

Erich