4x1GB verses 2x2GB

speekez · Apr 6, 2007

Greetings,

I'm looking to outfit a new Mac Pro with 4GB of RAM.

At places like OWC and Datamem, the difference between buying 4x1GB and 2x2GB is fairly minimal.

Would there be anything wrong with buying the 2x2gb (putting that in one riser) and sticking the stock 2x512MB from Apple in the other riser for now (for a total of 5GB)? The 2x2gb pair seems to have more longevity to it, because I can always hold onto it if I choose to max out the machine's RAM. Whereas with 4x1GB I would have to sell off that pair to max out the machine.

Any opinions? I've also heard it is ideal to have matching pairs in each riser (a pair of 1gb in one riser, and a pair of 1gb in the other)... but not essential.

Chrispy · Apr 6, 2007

I think I read somewhere that 4 equal pairs was better than two. I know with our work computer, we put in 2 additional sticks of 512 to take it to 2GB for now. I don't think it really makes a huge difference but maybe someone else can weigh in who knows more about this than I do.

aviationwiz · Apr 6, 2007

Go for 4x1GB, I'm pretty sure having quad-channel (4 equal pairs) in the Mac Pro will give you at least a minimal performance boost over dual channel (2 equal pairs.)

speekez · Apr 6, 2007

thanks guys.

just to clarify, don't you mean 2 equal pairs (aka 4 matching sticks, hence the 4x1GB recommendation)

when i hear "4 equal pairs" I think 8 sticks.

osx-linux · Apr 6, 2007

They mean 4x1gb. 4 sticks.

Why, you ask?

* Its all about bandwidth, how big of a pipe you have from your cpu to your ram. (bigger, more bits, is better, faster)

You'll get the 256bit addressing [the max in the mac pro] *if* you have 4 sticks of ram, 2 matched sticks on card A and 2 matched sticks on card B.

The caveat here being that if the 4 sticks aren't all matched, you'll only be able get that 256-bit bandwidth on "part" of your ram.

Examples:
1) 2x512mb+2x1gb will give you 3gb total ram, of which only 2g will be 256 bit addressable, the last 1gig will only be addressable in 128bit fashion.
2) 2x512mb+2x2gb will give you 5gb total ram, of which only 2g will be 256 bit addressable, the last 3gig(ack!) will only be addressable in 128bit fashion.
3) 2x1gb+2x1gb will give you 4gb total ram, of which all 4g will be 256 bit addressable.
etc etc....

So there ya go... how much of speed decrease/increase you'll see is completely dependent upon how often you will push your machine to use over the 256-bit addressable ram you have.

So 2x512mb+2x1gb is still better than 4x512mb cause at least you've got 1 gig of this 'half-bandwidth' ram instead of nothing. Moreover, anything is better than the 2x512mb that comes as the default with the mac pro.

Anandtech did some testing on this, and saw a near 20% boost on highly memory intensive operations, and (as expected) not much of a boost on operations where data isn't being loaded into and out of memory intensively. Some like to downplay this alot, and claim it gives you a 'minimal' boost. However, 20% is quite significant to me if you are going to be intensively utilizing your ram.

Link to full article:
Apple's Mac Pro

Chrispy · Apr 6, 2007

Great info OSX-Linux! Thanks for posting. I thought I had read what you posted somewhere awhile back but I could not remember. Thanks for giving us some numbers to go from

speekez · Apr 7, 2007

Very helpful, thanks. I will think it over in terms of what my processing needs will be for the immediate future and go from there!

Multimedia · Apr 7, 2007

Nothing To Gain From Ordering Four 1GB Sticks Instead Of Two 2GB While Loosing Slots

speekez said:
I'm looking to outfit a new Mac Pro with 4GB of RAM.

At places like OWC and Datamem, the difference between buying 4x1GB and 2x2GB is fairly minimal.

Would there be anything wrong with buying the 2x2gb (putting that in one riser) and sticking the stock 2x512MB from Apple in the other riser for now (for a total of 5GB)? The 2x2gb pair seems to have more longevity to it, because I can always hold onto it if I choose to max out the machine's RAM. Whereas with 4x1GB I would have to sell off that pair to max out the machine.

Any opinions? I've also heard it is ideal to have matching pairs in each riser (a pair of 1gb in one riser, and a pair of 1gb in the other)... but not essential.

My opinion is that it is extremely foolish to purchase anything but two 2GB sticks at a time for $430 at Omni Tech via Ramseeker.com. Everyone charges more and there is nothing wrong with what Omni Tech sells at all. The following posts are bogus splitting of hairs offset by having larger amounts of RAM in 4GB increments up to 13GB when you can pull the two 512 sticks and add your last 4GB for a total of 16GB for less than what Apple charges for 8GB.

Chrispy said:
I think I read somewhere that 4 equal pairs was better than two. I know with our work computer, we put in 2 additional sticks of 512 to take it to 2GB for now. I don't think it really makes a huge difference but maybe someone else can weigh in who knows more about this than I do.

This is not true unless you care about a less than 4% improvement. Yes it does not make a whit of difference.

aviationwiz said:
Go for 4x1GB, I'm pretty sure having quad-channel (4 equal pairs) in the Mac Pro will give you at least a minimal performance boost over dual channel (2 equal pairs.)

less than 4%. Buy 4GB at a time two x 2GB sticks for only $430 at Omni please. And no I am not affiliated with Omni in any way.

speekez said:
Very helpful, thanks. I will think it over in terms of what my processing needs will be for the immediate future and go from there!

Think about the slots you will be wasting if you buy 1GB sticks. any Quad needs a minimum of 4GB and any Oct needs a minimum 8GB of ram to operate properly. Don't make the mistake of buying 1GB sticks since 2GB sticks cost the same or less than two 1GB sticks.

brooker · Apr 7, 2007

Multimedia said:
My opinion is that it is extremely foolish to purchase anything but two 2GB sticks...

Wow, you really feel strongly about this don't you?

Personally, i think it will be a rare individual who will need 16GB of RAM in the next 2 years, even among Ocho buyers. There are certainly some out there, but i'd bet that most people who go for 8 cores won't need to max out RAM any time soon.

If this is indeed the case, then it does NOT make sense to buy 2GB sticks. I understand your concern for conserving slot space, but what do you think about the chart posted by osx-linux above? There are situations where being able to use quad-channel ram provides a significant improvement.

So if someone needs 16GB now or soon, then yes, 2GB sticks is the best option, and has the optimum upgrade path. But if not (as in the case of the OP who is configuring with 4GB), then using 1GB sticks makes fine sense. If and when an upgrade is needed, 2GB sticks can be purchased at that time, likely for much less than what they are going for now (esp after DDR3 comes out?). So if the next logical upgrade is to add another 4GB, 2x2GB sticks would make more sense, so as not to fill every slot.

I guess what i'm saying is, why bother keeping empty slots open now, when there is a potential performance improvment to be had (even if it is marginal) if you KNOW you won't be using all slots soon? Those empty slots will have some performance cost from day one until you actually need to upgrade. Why not fill the slots now, and upgrade as you need to, so as to keep total RAM expenditures down. you are not "wasting" slots if the computer is taking advantage of them. On the contrary, you are "wasting" performance by not filling them.

AS ALWAYS, this is all largely dependant on what type of applications you use, and your usage patterns.

In summary, we all agree that 4x2GB would be the best!

AVID · Apr 7, 2007

Multimedia said:
any Quad needs a minimum of 4GB and any Oct needs a minimum 8GB of ram to operate properly. Don't make the mistake of buying 1GB sticks since 2GB sticks cost the same or less than two 1GB sticks.

Sorry if this is stupid question, but why is it that quad needs a minimum of 4GB of ram? i'm in the process of ram upgrade on my pro as well

and if i use parallel (for CAD), how much should i allocate to it? Does parallel start to take up ram only when it is opened?

bearbo · Apr 7, 2007

AVID said:
Sorry if this is stupid question, but why is it that quad needs a minimum of 4GB of ram? i'm in the process of ram upgrade on my pro as well

and if i use parallel (for CAD), how much should i allocate to it? Does parallel start to take up ram only when it is opened?

i think that was the rule of thumb that "you shouldn't have more cores than GB of RAM"... aka you should have at least 1 GB of RAM/core (i know RAM is not assigned to core that way)

i'm not sure how much memory CAD will take usually, but parallel only uses ram when it's open... i would say at least 2GB for parallel if you do anything intensive(?)

AVID · Apr 7, 2007

thanks bearbo, i'll do some experiments in the coming months and post the results, i'm sure there're a lot of people who'd be interested.

brooker · Apr 7, 2007

Multimedia said:
two 2GB sticks at a time for $430 at Omni Tech via Ramseeker.com.

I have a question about this ram... Looking at it compared to the stock RAM from apple, and this ram from OWC, there is an obvious difference.

Apple and OWC chips have huge heatsinks on them, the Omni Tech does not. It does have a heat spreader, but according to some tests (again, done by anandtech), having the better heatsink really makes a difference. They installed standard ram wiht the heat spreader but no sink, and reported significantly more CRC errors. Others reported system lockups.

Here is the difference in pictures. I have long been a fan of cheap RAM, but everything i've read researching what to get for the MP points to using full sized heatsinks. The results are better airflow, less fan time, longer lifetime, lower power consumption, and a more stable system. And you get all this for only $50 more at OWC, in addition to their best-in-class advanced replacement if anything goes wrong.

Now, with that said, i could definitely be swayed if someone could show rigoures testing of those modules in a Mac Pro kept temps and errors to the same level as stock or any heatsunk(?) ram.

Any such anecdotal evidence out there? any professional comparisons? I found this page (lots of user feedback), which mentions that both Kingston and Crucial initially offered flat heatshielded ram for Mac Pros, then pulled it and now sells large heatsink ram as recommended for the MP systems.

more on Apple's specifications here.

Multimedia · Apr 7, 2007

MacSink Is Only $499/4GB Pair Of Two 2GB Sticks

brooker said:
I have a question about this ram... Looking at it compared to the stock RAM from apple, and this ram from OWC, there is an obvious difference.

Apple and OWC chips have huge heatsinks on them, the Omni Tech does not. It does have a heat spreader, but according to some tests (again, done by anandtech), having the better heatsink really makes a difference. They installed standard ram wiht the heat spreader but no sink, and reported significantly more CRC errors. Others reported system lockups.

Here is the difference in pictures. I have long been a fan of cheap RAM, but everything i've read researching what to get for the MP points to using full sized heatsinks. The results are better airflow, less fan time, longer lifetime, lower power consumption, and a more stable system. And you get all this for only $50 more at OWC, in addition to their best-in-class advanced replacement if anything goes wrong.

Now, with that said, i could definitely be swayed if someone could show rigoures testing of those modules in a Mac Pro kept temps and errors to the same level as stock or any heatsunk(?) ram.

Any such anecdotal evidence out there? any professional comparisons? I found this page (lots of user feedback), which mentions that both Kingston and Crucial initially offered flat heatshielded ram for Mac Pros, then pulled it and now sells large heatsink ram as recommended for the MP systems.

more on Apple's specifications here.

Then try MaxSink then for $499/4GB pair of two 2GB sticks. But I would phone Omni and complain you don't think they are using the right sink before judging them without challenging them. Omni Tech's quality page says they do everything right. Have you phoned them? The photos at Omni look to me like they have good heat sinks. Why do you judge them without talking to them first?

osx-linux · Apr 7, 2007

Multimedia said:
The following posts are bogus splitting of hairs offset by having larger amounts of RAM in 4GB increments up to 13GB when you can pull the two 512 sticks and add your last 4GB for a total of 16GB for less than what Apple charges for 8GB.This is not true unless you care about a less than 4% improvement. Yes it does not make a whit of difference.less than 4%.

I respectfully disagree. Moreover, I will say that for certain applications it is much more than 4%, closer to 20% according to anandtech. Anecdotally, it can even be over 20%.

Apple charges people, what is it $799? to upgrade from a quad 2.66 to a quad 3.00 for what is an absolute maximum of ~12% difference in 'theoretical' performance, assuming your applications is CPU bound.

Just by having the quad channeled ram you have doubled the bandwidth between cpu and ram. So if thats where your personal bottleneck happens to be it will most definitely speed up your system, even more so than upgrading to the 3.0G cpu!

All of that being said, if your application is CPU bound, having quad channel RAM wont help you a bit. 0%.

Anyway, argue what you will, but those are the facts.

Copying data around in memory is by definition RAM intensive. It will always benefit significantly from the wider channel. Always.

Alpinism · Apr 7, 2007

Multimedia said:
I would phone Omni and complain you don't think they are using the right sink before judging them without challenging them.

Actually if you want to buy from Omni for your mac pro, you have to call them and tell them you want the BIG APPLE heatsink. They have standard and Extra Large heatsinks for their memory stick.

sblasl · Apr 7, 2007

Do you know what is the difference in pricing is, if any?

Alpinism said:
Actually if you want to buy from Omni for your mac pro, you have to call them and tell them you want the BIG APPLE heatsink. They have standard and Extra Large heatsinks for their memory stick.

speekez · Apr 7, 2007

Multimedia,

Thanks for chiming in on my thread. Actually, I had read another one of your posts a while back about advocating buying 2x2GB RAM packages. You are the one that got me thinking along those lines.

As someone who has been wanting to buy a Mac Pro since Decemeber (but waited for the anticipated upgrade of the product line), I've seen RAM prices take quite a dip. I think back in January 2x2gb could be had for about $700-800. Now, the price is about $430-500 depending upon where you all look. If you folks aren't liking the Omni Tech option, you can get 2x2GB for $498 at either TransIntl (Via their ad on xlr8yourmac) or at Data Memory System (via a link on dealram.com) Both companies have had good word of mouth.

I've been using my blue & white G3 since 1998. I upgraded it with a G4 ZIF. I'm the type who likes to hold onto something if it works and can be upgraded to today's demands. I guess that's why I was trying to think ahead on future RAM upgrade options for the Mac Pro. I'll be using Aperture and Photoshop.

Regarding all this "quad channel" stuff, I pulled this from an Anandtech page:

"Here are a couple of things you can do to maximize performance and minimize the cost of additional memory on your Mac Pro, and it starts with the number of FB-DIMMs you configure your system with. The Mac Pro ships with a default configuration of 2 x 512MB FB-DIMMs, unfortunately that means that you're only using two of the four available memory channels, cutting your peak theoretical memory bandwidth in half. You'll want to upgrade to at least four FB-DIMMs so that you can run in quad-channel mode, in the coming weeks we'll be running some tests to figure out exactly how much additional performance you'll gain by doing that and if it's noticeable or not { this links to the chart osx-linux shared with us}. If you do find yourself filling all 8 memory slots on the Mac Pro, we would suggest trying to move to 4 higher density modules instead. Remember that you gain an additional 3 - 5ns of latency (at minimum) with each FB-DIMM hop, so the fewer FB-DIMMs you have the lower your worst case scenario memory latency will be. But since you still want to be running in quad-channel mode you don't want to drop below four FB-DIMMs, making four the magic number with the Mac Pro."

Here's what I get out of that:
1. Populate at least four slots to run in quad channel mode.
2. If I drop 2x2GB in two slots on riser A and the stock 2x512MB in the other two slots on Riser B, I have populated at least four slots. I guess I'm not sure why this wouldn't trigger quad channel mode. Anandtech, at least from what I saw, never mentioned anything about having to match all four sticks. If someone can find a reference for having to match all four sticks, please share. osx-linux, I know you gave a bit of explanation.
3. The fewer RAM sticks you have, the better (for latency reasons) -- so make the most of those four slots.

Like Brooker said, I think the "best" set up is 4x2GB. All four sticks match, you have filled at least four slots, and you have achieved 8GB of RAM using only four sticks (less latency than 8 sticks of 1GB). Does everyone need 8GB of RAM? No. But I might.

If 4x2GB (8GB) is my ultimate goal, I guess I'll just need to ask myself if I can live with the minor/alleged performance hit in the meantime of using 2x2GB and 2x512MB in my machine.

speekez · Apr 7, 2007

osx-linux said:
They mean 4x1gb. 4 sticks.

Examples:
1) 2x512mb+2x1gb will give you 3gb total ram, of which only 2g will be 256 bit addressable, the last 1gig will only be addressable in 128bit fashion.
2) 2x512mb+2x2gb will give you 5gb total ram, of which only 2g will be 256 bit addressable, the last 3gig(ack!) will only be addressable in 128bit fashion.
3) 2x1gb+2x1gb will give you 4gb total ram, of which all 4g will be 256 bit addressable.
etc etc....

osx-linux, can you clarify how you determined how much will work in 256 vs how much will default to 128.

osx-linux · Apr 7, 2007

speekez said:
2. If I drop 2x2GB in two slots on riser A and the stock 2x512MB in the other two slots on Riser B, I have populated at least four slots. I guess I'm not sure why this wouldn't trigger quad channel mode. Anandtech, at least from what I saw, never mentioned anything about having to match all four sticks. If someone can find a reference for having to match all four sticks, please share. osx-linux, I know you gave a bit of explanation.

You will still get quad channel with 2x2GB + 2x512MB, just not on all of the installed memory. You will only get quad channel access to your lower 2 gigs of ram, the other 3 gigs will have dual channel access.

speekez said:
If 4x2GB (8GB) is my ultimate goal, I guess I'll just need to ask myself if I can live with the minor/alleged performance hit in the meantime of using 2x2GB and 2x512MB in my machine.

Indeed if 4x2GB is your end goal, I'd do the same if budget were constrained to only being able to purchase 2x2GB at this time.

Anything else I didn't quote from you, I probably agree with.

osx-linux · Apr 7, 2007

speekez said:
osx-linux, can you clarify how you determined how much will work in 256 vs how much will default to 128.

Certianly, I read the mac developer reference page on ram expansion.

Specifically you want to take a look at this:

Ok, so now I'll try to explain that in non-techno-speak.

The memory controller (yellowish box) connects your cpu and ram. The yellow box does this via the branches (blueish boxes). Each branch has a 128-bit data path to one of the risers. These 'risers' (A and B) are where you install your ram chips. Each branch has a 128-bit datapath to one and only one of the risers. Ok, that should explain the picture well enough for our purposes here.

So, if you stick all of your RAM on one riser, you only have a 128-bit path from your cpu to your RAM. You need to have RAM installed on both risers to have any 256-bit datapaths. That means you need at least 4 sticks, 2 sticks on each riser for the full datapath, thats why anandtech says 4 sticks for quad channel.

However, its not quite that simple, lets take the case where you have 2x512mb and 2x2gb.

Now fill that up with 5G total worth of data. We can load the first 2G via 256 bit access via both branches, you'd get 1G of data in the ram in riser A via branch 0(128 bit datapath) and you get 1G of data in the ram in riser B via branch 1(128 bit datapath).

Ok, great, we've got quad channel here!. We are using both branches simultaneously to fill the RAM, thus a 256 bit datapath!

Now try to load the final 3G worth of data into RAM, well if riser B has the 2x512mb sticks, its full and cannot accept anymore data, thus all 3G of data go into riser A via branch 0 (we cant use branch 1, as all of the RAM linked to branch 1 is full!). Thus we utilize only one 128 bit datapath, to access the final 3G of data.

Note: quad channel = 256 bit datapath, dual channel = 128 bit datapath

Hope this is clear. If not perhaps some pictures will help, but I'm no graphics guru and any pictures I draw will likely remind you of elementary school drawings (and not the talented kids either, more like the kid in the back finger painting the walls).

brooker · Apr 7, 2007

wow, great summary, osx-linux. Thanks a lot!

BiikeMike · Apr 7, 2007

OSX-Linux, thanks for explaining that. My question is, if the hangup is the 2x512s making Riser B full, why not put 1x2GB and 1x512MB in each riser?

what kind of conflicts would that cause, and where would the bottleneck be?

osx-linux · Apr 7, 2007

BiikeMike said:
OSX-Linux, thanks for explaining that. My question is, if the hangup is the 2x512s making Riser B full, why not put 1x2GB and 1x512MB in each riser?

what kind of conflicts would that cause, and where would the bottleneck be?

Excellent question. The same argument that applies to the branches, also applies to the channels(see 'channels' on the diagram). Each channel is 64-bit and connects to 2 of the ram slots on a single riser. However they connect to the 2 slot in a serial rather than parallel fashion(see apple docs linked above). Thus, this why apple has you install ram in matched pairs-- to avoid really nasty 64-bit (single-channel) datapaths.

I left this bit out earlier for clarity, as well as a whole slew of other stuff.

So anyway, now all that "single-channel" , "dual-channel", and "quad-channel" jargon should make sense?

BiikeMike · Apr 7, 2007

So, the moral of this story is, according to your diagram and the info you posted, 4x1GB can perform better than 2x2GB+2x512MB? Eventhough you have an extra gig of RAM, most of it is not acting to it's full capacity?

4x1GB verses 2x2GB

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