OT: RAM (DDR) speed

[Ron Smith]

Well, as with many performance oriented questions, there is no straight 
answer that I can think of to give you on this one....

It is true that in terms of physical performance there is not a *lot* 
to be gained from DDR 333 vs. DDR 266.

Having said that, in many opinions the choking point of Macs has been 
the front side bus speed of the Mac (PPC) processors and truthfully 
their speed has lagged significantly behind that of the Intel 
processors in raw speed. That significantly changed with the G5 
processor. Now the question remains on whether the DDR 333 (or possibly 
the 266) can "feed" the processors pipeline at a fast and efficient 
rate to keep it operating at an optimum performance level. On a 
theoretical level, this could be easy to figure out mathematically, but 
in real world terms where cache misses and branch prediction errors 
happen, only a real world test will be able to show if DDR 333 is 
sufficient, insufficient,  or overkill.

This is purely conjecture on my part:
Your question really struck my interest and although I haven't had the 
time to do the math for theoretical performance, getting the hardware 
to test is at least 9 to 12 months away for me, so hopefully someone 
else will do that work. =)   My suspicion is that since the core speed 
and the front side bus speed of the G5 are so closely matched that this 
is really just "growth room" so that the core speed can be scaled up 
over time without worrying as much about how to feed data to it. I also 
suspect that (theoretically) the DDR 333 wouldn't be able to keep up 
with the FSB speed without pipeline stalls, but I doubt the core is 
going to be able to consume data at a greater rate that the DDR 333 can 
supply right now.

As far as sticking DDR 266 into a new G5 I would say is doubtful. At 
best case the hardware would be able to "compensate" for it although I 
suspect there would be a lot of stalls and errors during high loads, or 
possibly the hardware would not recognize it at all. Yeah, that is a 
really vague cop-out answer, but my understanding of Mac firmware is 
not very deep. =(

I look forward to hearing other comments on this topic and please feel 
free to correct me if I have said anything blatantly wrong.

-Ron

On Saturday, June 28, 2003, at 05:41 AM, Thierry de Coulon wrote:

> Hello!
>
> This is quite off topic but since there should be Mac specialists 
> around....
> I've seen the new G5 Macs come with 333Mhz DDR. I've also come across 
> infos
> that DDR 333 only brings 5-8% speed incrase. Now the question is: if 
> you have
> DDR 266 modules, will a Mac G5 accept them (PC Bios should provide a
> possibility to set the speed, but I'm not aware of any such 
> possibility on a
> Mac)?
>
> Thierry
>
> -- 
> Smile . . . tomorrow will be worse.
>
> _______________________________________________
> yellowdog-general mailing list
> yellowdog-general@lists.terrasoftsolutions.com
> http://lists.terrasoftsolutions.com/mailman/listinfo/yellowdog-general
>