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morlium · Jun 24, 2003, 01:07 PM

Does the new G5 REQUIRE RAM to be installed in pairs (i.e., i have to buy two 512 MB chips for a GB; not a single stick)?

It seems to imply that in the tech specs; plus the only BTO options involve even sets.

If that's the case, RAM could get very expensive, very quickly, with all of the swapping and doubling ...

Scotttheking · Jun 24, 2003, 01:09 PM

Yes, it requires pairs. Matched pairs, in fact.

morlium · Jun 24, 2003, 01:12 PM

can you explain why?

Eug Wanker · Jun 24, 2003, 01:13 PM

Originally posted by Scotttheking:
Yes, it requires pairs. Matched pairs, in fact.

You sure about that? It would make sense for the computer to simply run in single-channel half bandwidth mode if there isn't matched RAM installed. This is exactly what happens on the PC side. Mind you, it could very well be that Apple has removed this option.

Scotttheking · Jun 24, 2003, 01:14 PM

dual channel ram.
Think raid0 for ram.
It uses 2 64bit ram sticks and make them into one 128bit pair.
I'll have a more technical explanation added to my G5 article that I'm writing, hopefully tonight.

Scotttheking · Jun 24, 2003, 01:19 PM

Originally posted by Eug Wanker:
You sure about that? It would make sense for the computer to simply run in single-channel half bandwidth mode if there isn't matched RAM installed. This is exactly what happens on the PC side. Mind you, it could very well be that Apple has removed this option.

I'll check, but usually when the ram says 128bit, that means it must be paired. It also would make no sense for apple to not require pairs, as that would really kill performance.
For athlons, (nforce2) the dual channel doesn't help the processor, just the onboard video. I'm not sure if intel requires pairs, I don't follow them.

Edit: Here Apple says it requires pairs.

Eug Wanker · Jun 24, 2003, 01:23 PM

Originally posted by Scotttheking:
I'll check, but usually when the ram says 128bit, that means it must be paired.

AFAIK, there is no such thing as true "128 bit DDR400 RAM" for desktops. It's simply paired 64-bit RAM.

ie. You don't have to buy special dual channel RAM. Just buy 2 standard RAM DIMMs.

Edit: Here Apple says it requires pairs.

Yeah but is the issue:

1) If you don't run in pairs, the machine won't boot.

vs.

2) If you don't run in pairs, the machine will be much slower.

??? For Intels, it's the latter.

morlium · Jun 24, 2003, 01:26 PM

cool. i get it.

thanks everyone.

this board rocks ... and with a new G5 coming my way, i'll be frequenting it a whole lot more.

Scotttheking · Jun 24, 2003, 01:27 PM

Originally posted by Eug Wanker:
AFAIK, there is no such thing as true "128 bit DDR400 RAM" for desktops. It's simply paired 64-bit RAM.

ie. You don't have to buy special dual channel RAM. Just buy 2 standard RAM DIMMs.

That's what I said

128bit ram = 2 64bit chips, which is why it must be paired.

OreoCookie · Jun 24, 2003, 01:40 PM

Originally posted by Scotttheking:
I'll check, but usually when the ram says 128bit, that means it must be paired. It also would make no sense for apple to not require pairs, as that would really kill performance.
For athlons, (nforce2) the dual channel doesn't help the processor, just the onboard video. I'm not sure if intel requires pairs, I don't follow them.

Edit: Here Apple says it requires pairs.

About the Athlon, you are clearly wrong. My little brother has gotten the good Asus N8X. With identical CPU etc. He has gotten performance gains of about one level in CPU solely due to the DualChannel DDR.

Eriamjh · Jun 24, 2003, 01:48 PM

Pairs is because one is for each processor memory bus. They must be matched.

Apple is not compromising speed in this case. What a change!

Scotttheking · Jun 24, 2003, 01:49 PM

Originally posted by OreoCookie:
About the Athlon, you are clearly wrong. My little brother has gotten the good Asus N8X. With identical CPU etc. He has gotten performance gains of about one level in CPU solely due to the DualChannel DDR.

Sigh. I'm talking about significant changes here.
If you have an athlon with a 333FSB, and PC2700 ram, and nothing else is accessing the ram, then adding another channel will give no gain.
What your brother is seeing is DMA in action, and the extra channel is handling that.
Compare this to the G5, where if you removed half the ram you'd halve the performance.

thunderous_funker · Jun 24, 2003, 02:43 PM

Originally posted by Eug Wanker:
Yeah but is the issue:

1) If you don't run in pairs, the machine won't boot.

vs.

2) If you don't run in pairs, the machine will be much slower.

??? For Intels, it's the latter.

Nope. If you have a dual channel mobo, installing a single DIMM will result in the machine not booting. It will beep at you just as if no RAM was installed.

The RAM chips themselves are not different. DDR RAM is DDR RAM. The difference is the memory controller/mobo design.

tooki · Jun 24, 2003, 03:43 PM

As I posted in the WWDC tech specs thread, this is the first time since the NuBus Power Macs that Apple has made a machine that requires paired RAM.

The 73/75/76/85/86/95/9600 Macs (and all the clones bases on those architectures) have an optional "interleaving" mode where, if you install RAM in matched pairs, it will run it as 128-bit and give you a roughly 10% performance increase. But again, that was optional.

tooki

Eug Wanker · Jun 24, 2003, 04:13 PM

Originally posted by thunderous_funker:
Nope. If you have a dual channel mobo, installing a single DIMM will result in the machine not booting. It will beep at you just as if no RAM was installed.

The RAM chips themselves are not different. DDR RAM is DDR RAM. The difference is the memory controller/mobo design.

Dual channel chipsets do support single DIMMs.

See here and here.

thunderous_funker · Jun 24, 2003, 04:16 PM

I'm not sure if you're just musing or complaining. You make it sound like a bad thing.

Top of the line x86 motherboards are also dual channel. That architecture offers obvious benefits (as opposed to the minimal benefits you noted on nuBus interleaving memory) to the system.

It's refreshing to see Apple not just play catch up, but actually adopt a cutting edge approach to the system architecture. They could have gone with straight DDR, but once again found themselves a step behind as the x86 world moves to Dual Channel.

Eug Wanker · Jun 24, 2003, 04:21 PM

Originally posted by thunderous_funker:
I'm not sure if you're just musing or complaining. You make it sound like a bad thing.

Top of the line x86 motherboards are also dual channel. That architecture offers obvious benefits (as opposed to the minimal benefits you noted on nuBus interleaving memory) to the system.

It's refreshing to see Apple not just play catch up, but actually adopt a cutting edge approach to the system architecture. They could have gone with straight DDR, but once again found themselves a step behind as the x86 world moves to Dual Channel.

I'm just saying that all the statements that dual channel chipsets absolutely require pairs are false. They can greatly benefit from pairs of course, but technicially you can run unpaired RAM in a pinch.

I wouldn't complain if Apple DID make it a requirement, but chipsets can be designed to be flexible with regards to single vs. paired RAM.

Despite Apple's specs, I'm not 100% convinced they removed this flexibility, but if they did it'd be a design choice (to ensure maximum speed), and not because of technical restrictions.

thunderous_funker · Jun 24, 2003, 04:30 PM

Originally posted by Eug Wanker:
Dual channel chipsets do support single DIMMs.

See here and here.

Some chipsets do support single DIMMs. As we have this conversation, I am setting up some new 1U servers from SuperMicro (dual Xeon board) that will not boot without a matching pair of DIMMs. It's Intel's E7500 chipset if i'm not mistaken.

Dual Channel DDR offers RDRAM performace with cheaper SDRAM chips.

I suspect Apple's chipset doesn't offer the luxory of going both ways for obvious reasons:

cost
performance
designed for dual CPU configurations