G5 and memory speeds

[winterlight]

Hi guys,

I just installed 16GB of PC2 6400 RAM into a Late 2005 G5 (11,2)

The memory is showing as PC2-3200U-288 in the system info.

QUESTION 1:  Would I benefit from installing actual PC2 4200 RAM into the machine? (Which is the specified maximum speed for the machine)

GeekBench2 in 32 bit mode gives a benchmark of 3370 which seems to be OK for this class of machine.

QUESTION 2: Would using non ECC vs ECC make a notable difference?

Thank you guys.

 

[ibmxt286]

My personal experience with RAM and computers has been that as long as it fits, and it works, you won't notice a difference.  That has been my personal experience.  

 

[Unknown_K]

Whatever faster RAM you use won't make a difference in speed. Get whatever is cheapest.

 

[winterlight]

OK, thanks for the reply guys.

So faster clearly won't happen but with PC2 6400 being clocked down to 3200 (and @ 288 CLs) by the G5 will that make things slower than PC2 4200? (which is 444 CLs)

 

[Unknown_K]

Speed is dictated by the RAM controller.

 

[rsolberg]

Do you still have the original RAM to check if it's running at the specified PC4200? With the new RAM, is it still clocked at PC3200 with only two sticks installed? ECC vs not shouldn't make any difference in from the rated timings.

My guess is that the new RAM doesn't supply timings for PC4200 via SPD. It might have PC6400 (800), PC5300 (667), and PC3200 (400), or perhaps it lacks anything at PC4200 or below and the Mac's RAM controller is just falling back to PC3200 by default.

Also, Apple has specified RAM at a higher speed than the machine actually runs it in the past-- I don't think this is the case here though.

 

[winterlight]

@ rsolberg

The System Profiler shows PC2-4400-444 when I have the original RAM in it.  With the new sticks (16GB of PC2 6400) the System Profiler shows PC2-3200-288.

The amounts are correct but I'm concerned that the CL timing of 288 on the de-clocked 6400 is going to make things slower than the 444 timing on the PC2 4200 RAM.

Subjectively I feel like things are slower with the 6400 but the G5 is more than a decade old so doing things like running all the ads on a web page are generally slow.

Maybe everything is CPU limited at this point and the CAS Latency won't make a difference?  I'm not sure.

I'm thinking about buying 16GB of ECC PC2-4200 if the CL is going to make a differences.  Otherwise I guess I should just live with it (?)  Maybe...

I'm using GeekBench2 to test memory speed but the info is very limited.  Any recommendations on a good memory benchmarking program for OS 10.4.11?

 

[rsolberg]

I'm of the opinion that the new modules you have don't contain timings for PC4200. This is not the case with all PC6400 DDR2 RAM, as I have some that runs at 533MHz (4200) without a hitch. If you have a PC that uses DDR2 RAM, you could install some of these modules and run CPU-Z in Windows. CPU-Z has an SPD tab that lists all of a DIMM's reported timings. I'm not sure how to do that on a PPC Mac, but maybe someone else here does?

 

[winterlight]

@rsolberg

I used Xbench 1.3 which is compatible with OS 10.4.11  (just found that utility)

The 6400 RAM is giving me faster speeds than the 4200 but I don't know if the difference is big enough.

The specific benchmarks are 10% to 15% faster with the 6400 modules which are registering as 3200.

So if I had to guess maybe OS 10.4.11 can't enumerate the higher RAM spec but the faster speeds do actually get exploited by the hardware???

Not sure how much of a performance leap one SHOULD be getting from a move from 4200 to 6400

(Also, I only have 512MB of 4200 to benchmark with, I benchmarked the 6400 with 4GB and 16GB)  Not sure if the different total memory affects the benchmark scores.

 

[rsolberg]

Xbench's RAM score is likely based on a test that performs poorly with smaller amounts of system RAM. If About This Mac-System Information says 3200, then that's how the memory controller is accessing the RAM. Memory bandwidth will be approximately 30% more with RAM operating at PC4200. Using a DDR2 based x86 PC and the right utility, you may be able to flash each DIMM's SPD EEPROM to include PC4200 with the appropriate timings instead of one of the other entries.

I've done this once before with some DDR2 PC6400 that lacked an entry for 3200. The PC I needed to install it in had an unusual 400MHz DDR2 memory interface and wouldn't even POST with said RAM installed. When I installed the RAM in another PC, its SPD table had values for 6400, 5300, and 4200. I was able to reflash the SPD EEPROM using SPDUtility and get it to work in the intended PC, but I learned that not all DIMMs have an erasable/writable SPD ROM. It seems like a lot of RAM is limited to three SPD entries, so picking RAM one speed faster than specified should be safe, but two or three speeds faster might not work.