[G4] Single or Dual??

[Philip J Robar]

On Dec 13, 2005, at 5:10 AM, Aaron wrote:

>> Date: Thu, 08 Dec 2005 16:48:19 -0500
>> From: Mike Tweedie <mac at springerlabs.com>
>> To: "A place to discuss Apple's G4 computers."  
>> <g4 at listserver.themacintoshguy.com>
>> Subject: [G4] Single or Dual??
>> Reply-To: "A place to discuss Apple's G4 computers."  
>> <g4 at listserver.themacintoshguy.com>
>>
>> I just picked up a G4 Sawtooth 500Mhz ... it's time for a new  
>> processor.
>>
>> [snip]
>>
>> Single or Dual?
>>
>> Looks like I can get a dual for just a bit more than a single, see  
>> here;
>> https://eshop.macsales.com/item/PowerLogix/PF47D1200DA/
>>
>> What are the groups thoughts? 1.2Ghz Dual or 1.8 Single?
>
> Having observed CPU usage in my Dual 867 MDD using the CPU Monitor  
> part of Activity Monitor, I would say that, presuming you're  
> running OS X (Panther or Tiger, and probably even earlier  
> versions), you'll get close to 100% usage of the dual processors,  
> so that a 1.2 GHz Dual will be almost as fast as a 2.4 GHz single. ...

Wouldn't it be nice if it were this simple. A single processor will  
always be as fast or faster overall than multiple processors that sum  
up to the same speed because it doesn't have the overhead associated  
with distributing a task between the multiple CPUs. To take advantage  
of multiple processors an application has to be properly multi- 
threaded, and there's no guarantee that the applications that you  
care about or might care about in the future will be. Put another  
way, multi-threaded applications always run measurably, and often  
noticeably, slower on multiple processors than they do on a single  
fast-as-the-sum-of-the-multiple-processors machine.

So why would you want multiple CPUs? There are a couple of scenarios  
where multiple CPUs make sense.

The most obvious case is where the fastest CPU available isn't fast  
enough for your needs. In this case you have no choice but to figure  
out how to divide the task in question up between multiple processors.

Two, for whatever reason sequential processing of applications is  
unacceptable. For example it may be required that you get a constant  
flow of data from multiple applications and the OS's scheduler can't  
or doesn't divvy up processor time acceptably. Or it may not matter  
how fast a set of multiple applications run, but the applications are  
time sensitive enough that they need dedicated resources such that  
they are always ready to run. Multiple processors make it much more  
unlikely that any single application will take over a machine and  
make it unresponsive - particularly to interactive use.

I would always prefer to have a single processor that is twice as  
fast as 2 dual processors for a desktop machine. I rarely run more  
than one important task at a time, I want that particular task to run  
as fast as possible, and I'm willing to trust that the OS's scheduler  
will keep the machine reasonably responsive for interactive use.

Mike's case isn't as simple as the one that Aaron posited. As Aaron  
pointed out there may be significant differences between the CPUs in  
question. This makes it impossible to choose between them without  
knowing what Mike intends to do with the machine and how the  
performance of the CPUs compare. Note that some of the recent faster  
clocked G4 processors have smaller caches and different enough  
designs that they are not as fast as you might expect them to be, but  
they run cooler which may be important to some situations.

Also not that early Sawtooths are not multi-processor compatible.


Phil
--
We cannot defend freedom abroad by deserting it at home. -- Edward R.  
Murrow