[timmerk]

My MacBook Pro supports up to 3GB of RAM. However, I'm told that you get better performance if RAM is installed in pairs. So either I can have 2 GB (2x1GB modules) or 3GB (1x1GB & 12GB module). Which would be better?

Thanks!

[ghporter]

Compared to having only 2GB, having 3GB is better. Your video performance may suffer a tiny bit compared to having 2X 1.5GB RAM, but since that's not an option, go for the 1X1GB + 1X2GB.

[tooki]

You can put in 2x2GB, which will give you the memory performance, and give you 3.3ish GB of RAM. For how cheap it is, the wasted 700MB won't cost much at all.

[Simon]

If you install 4 GB OS X will only use exactly 3 GB of RAM. No extras (the chipset can address a bit more than 3 GB, but not OS X). IOW you are wasting a full GB.

Since you have a MBP you do not have integrated graphics sharing video memory with RAM. Therefore you will likely see zero benefit from having matched RAM (this is different for MB users with shared VRAM). There are very few exceptions. And if you had written code which would benefit from this kind of bandwidth increase you would know about it.

If you have money to waste you can go for 4 GB. There's certainly no harm done. But if money is of at least minor concern you should get 3 GB.

[mduell]

If you find a 2x2GB kit on sale (someone linked to one for $35 AMIR last week) for less than the price of 1GB+2GB it doesn't hurt to go that way, but the latter is probably a couple dollars cheaper at normal prices.

[tooki]

Originally Posted by Simon 

If you install 4 GB OS X will only use exactly 3 GB of RAM. No extras (the chipset can address a bit more than 3 GB, but not OS X). IOW you are wasting a full GB.

You sure? I've read reports that Mac OS X will recognize the full addressable amount, that little bit above 3GB. It's not as though Mac OS X has a general 3GB RAM limit.

[OreoCookie]

I think you have 32-bit Windows in mind: here the usable RAM is effectively at most 3.5 GB (the exact amount varies on your hardware, e. g. how much is reserved by your graphics card).

Simon is correct that the (stupid) 3GB limit which I also happen to suffer from is due to the chipset.

@OP
If you need more than 2 GB RAM, 3 GB will be faster. There is a slight performance advantage if you use two of the same RAM modules, but it's not going to matter if you need more than 2 GB. Your harddrive is a lot, lot slower

[Big Mac]

Originally Posted by OreoCookie 

Simon is correct that the (stupid) 3GB limit which I also happen to suffer from is due to the chipset.

We're talking about the MacBook line, here right? Not the MBP. Otherwise I'm very confused as to why a dedicated GPU chipset would ever be using regular RAM.

[Simon]

Originally Posted by tooki 

You sure? I've read reports that Mac OS X will recognize the full addressable amount, that little bit above 3GB. It's not as though Mac OS X has a general 3GB RAM limit.

Yes. The Calistoga chipset can address 3.something, but not OS X. If you put 4 GB into a Calistoga Mac the OS will address exactly 3.0 GB of memory. That's it.

You can easily verify this behavior with top or Activity Monitor.

[Simon]

Originally Posted by OreoCookie 

@OP
If you need more than 2 GB RAM, 3 GB will be faster. There is a slight performance advantage if you use two of the same RAM modules, but it's not going to matter if you need more than 2 GB. Your harddrive is a lot, lot slower

The HD is not so much the issue.

The point is that there are few mem bandwidth sensitive tasks that could benefit from mathed pairs. Personally, I know of a few 3D field solvers that can be used to demonstrate this quite nicely. But those are highly specialized codes. The people that write and use them are aware of this. The one commonly cited example that everybody can encounter in regular use is graphics performance with shared video memory. If you have a Mac with a GMA 950 or X3100 it will use regular RAM instead of dedicated VRAM to buffer graphics. Pushing this data across the memory bus (this can be a really big amount of data actually) requires time and it is one of the few things that can be used to easily demonstrate the improved bandwidth with matched pairs.

That all said, the MBP never had shared video memory. It has dedicated VRAM so all of this is a non-issue. As I already pointed out, if the OP was writing or using a highly memory bandwidth dependent code, he'd probably know about it. Since he didn't mention anything, I'm assuming he won't see a benefit from going matched 4 GB over the 3 GB addressing limit.

Right now these are Newegg's lowest prices:
2 GB SO-DIMM for $29.49.
1 GB SO-DIMM for $15.49.

Bottom line is going matched costs at least $14. If you don't mind spending $14 for something you won't notice, fine. Go for it, it will work just fine. But if you're on a budget it's definitely wiser to save those $14 and put them towards something you will notice. A nice big and fast HD for example.

[Simon]

Originally Posted by Big Mac 

We're talking about the MacBook line, here right? Not the MBP. Otherwise I'm very confused as to why a dedicated GPU chipset would ever be using regular RAM.

There are both MBs and MBPs with Calistoga. The MB went through three Calistoga generations, the MBP through two. So the 3GB addressing limitation can be an issue for both lines regardless of the graphics chipset.

The OP mentioned a MBP. That's why in his case shared video memory is not an issue. And hence matching pairs will not lead to any noticeable performance increase in everyday tasks.

The MB with its shared video memory is exactly the opposite. If you play 3D games on a MB (not that much a good idea though) you will want to match your DIMMs.

[OreoCookie]

Originally Posted by Simon 

The HD is not so much the issue.

Of course it is: as soon as your RAM requirements exceed what you have installed, the data is swapped onto the harddrive -- and any harddrive is an order of magnitude slower than RAM.

Originally Posted by Simon 

The point is that there are few mem bandwidth sensitive tasks that could benefit from mathed pairs. Personally, I know of a few 3D field solvers that can be used to demonstrate this quite nicely.

I'm aware of this. But it's not going to matter if you just need more RAM, then this speedup does not really matter, because data is swapped onto the harddrive -- which is slower than any RAM configuration.

[OreoCookie]

Originally Posted by Big Mac 

We're talking about the MacBook line, here right? Not the MBP. Otherwise I'm very confused as to why a dedicated GPU chipset would ever be using regular RAM.

No, it's a matter of addressing with Windows and has nothing to do with MacBook vs. ProBook. 32-bit Windows can at most use 3.5 GB of 4 GB that you have installed. And it's not a matter of RAM being taken by the GPU either: certain hardware (e. g. PCI cards, PCI Express cards, network and SCSI controllers) reserve memory addresses within Windows and they need that to function properly.

So even if the graphics card has built-in memory, Windows still needs to reserve the memory addresses and you effectively lose this amount of memory one way or another.

[P]

This needs to be a FAQ...

If you have a 945 chipset (Calistoga, from the Napa platform) your memory bandwidth will be cut in half in a 2+1 configuration. For Macs with integrated graphics, this can be a big deal. For Macs with discrete graphics, like your MBP, it has almost zero effect. This chipset also has a total limit of adressable memory (RAM + other hardware) of 4 GB. EFI then cuts the RAM bit to 3 GB (or 2 GB, for the Core Duo ones).

If you have 965 chipset (Crestline - the one introduced in the incorrectly named Santa Rosa Macs in 2007) it seems you get the double memory bandwidth for the largest possible area - in this case, the first 2 gigs. The max limit is also 4 GB, but for RAM only - other hardware is not included.

For 32-bit Windows after XP SP1, the total amount of addressable memory for RAM and all devices is 4 GB. This means that you usually end up at 3 and a bit. As was said above, most of that is for graphics cards. Most importantly, a 512 MB GPU takes up 512 MB of the addressing space. This is because of the way the driver model has been implemented by certain manufacturers (*cough*nVidia*cough*) and you can actually go over this limit in XP before SP1 if you have a BIOS that supports it and no drivers from inept manufacturers. As of SP1, MS gave up trying to get people to follow the rules and decided to focus on 64-bit Windows instead, since it needed to get the virtual memory limit of 3 gigs raised as well.

[OreoCookie]

AFAIK even with 64-bit Windows, you effectively `lose' this half a gigabyte's worth of memory.

[Simon]

Originally Posted by OreoCookie 

Of course it is: as soon as your RAM requirements exceed what you have installed, the data is swapped onto the harddrive -- and any harddrive is an order of magnitude slower than RAM.

Yes, obviously we are all well aware of that. But the OP never asked about that and it's not what we are discussing here, is it? The question at hand is if he should get as much as he can address or more than he can address in order to match. In that regard the only thing that matters is your mem bandwidth requirement and budget.

I'm aware of this. But it's not going to matter if you just need more RAM, then this speedup does not really matter, because data is swapped onto the harddrive -- which is slower than any RAM configuration.

Certainly. But the OP is already aware of that. He already knows he wants/needs more RAM. He specifically asked how much. And that has nothing to do with the disk as regardless of how much he buys the addressing limit is 3 GB. The question is what his bandwidth requirements are. If they are high (which they are probably not) he should match. If not he can go with the cheaper 2+1.

[Simon]

Originally Posted by P 

This needs to be a FAQ...

There already is one. A sticky one actually.

http://forums.macnn.com/69/mac-noteb...-ram-upgrades/

The Crestline vs. Calistoga and matched vs. unmatched as well as integrated vs. discrete graphics have all been covered in that thread. In some cases several times actually. What isn't covered there is Windows. But since the OP never even mentioned Windows I'm surprised we're discussing its memory management.

[OreoCookie]

Originally Posted by Simon 

Yes, obviously we are all well aware of that. But the OP never asked about that and it's not what we are discussing here, is it?

Of course it is!
The OP has asked which config will yield better performance, and the statement that if he needs more than 2 GB of RAM at one point, not installing RAM in pairs is faster

Originally Posted by Simon 

The question at hand is if he should get as much as he can address or more than he can address in order to match.

It's in the title: either 2x1 GB or 1x1 GB + 1x2 GB. In the latter case, he only needs to replace one module.

Originally Posted by Simon 

The question is what his bandwidth requirements are. If they are high (which they are probably not) he should match. If not he can go with the cheaper 2+1.

No, it's primarily a question of how much RAM you use.

[mduell]

Originally Posted by Big Mac 

Otherwise I'm very confused as to why a dedicated GPU chipset would ever be using regular RAM.

Even if the GPU (or any other device) isn't using the RAM, it may still be using the address space; this is known as aperture.

Slightly offtopic: ATi and nVidia have both released (relatively recently, not a decade ago) GPUs that use system memory as VRAM (usually in addition to a small amount of dedicated VRAM); they called this feature HyperMemory and TurboCache, respectively. None of these GPUs are OS X compatible AFAIK.

Originally Posted by OreoCookie 

AFAIK even with 64-bit Windows, you effectively `lose' this half a gigabyte's worth of memory.

If that's true, then you also save TLB flushes.

[DCJ001]

Originally Posted by tooki 

You can put in 2x2GB, which will give you the memory performance, and give you 3.3ish GB of RAM. For how cheap it is, the wasted 700MB won't cost much at all.

I agree. I installed 2x2GB in my 05/2007 MacBook based on the following info:

http://eshop.macsales.com/shop/Memor...Apple_MacBook/

and the performance improved greatly.

[OreoCookie]

Originally Posted by mduell 

If that's true, then you also save TLB flushes.

Well, the memory space between 3 and 4 GB is still reserved for devices (as much as needed of course, not necessarily all of it), so this part of the memory is inaccessible for applications.

[mduell]

Originally Posted by OreoCookie 

Well, the memory space between 3 and 4 GB is still reserved for devices (as much as needed of course, not necessarily all of it), so this part of the memory is inaccessible for applications.

Right, but with 64-bit Windows that physical memory should be mapped into a higher address space.

[Simon]

Originally Posted by DCJ001 

I agree. I installed 2x2GB in my 05/2007 MacBook based on the following info:

http://eshop.macsales.com/shop/Memor...Apple_MacBook/

and the performance improved greatly.

On a MB that is certainly the case. The OP here however has mentioned that he has a MBP. And due to the different graphics implementations of these two systems memory upgrades should be handled differently. This has already been pointed out several times in the course of this thread.

[Simon]

Originally Posted by OreoCookie 

It's in the title: either 2x1 GB or 1x1 GB + 1x2 GB. In the latter case, he only needs to replace one module.

You're absolutely right, I misread that. </blush>

In this case I'll gladly quote from the sticky thread on MB(P) RAM upgrades:
However, this doesn't mean that less matched memory is better than more unmatched memory! If your system is paging out to disk because not enough memory is available this will cause a huge performance decrease. This effect is orders of magnitude larger than the slight performance reduction due to unmatched memory pairs. Therefore: If you're paging out with 2 GB of matched memory, DO NOT hesitate to upgrade to an unmatched 3 GB!

[jjs357]

FWIW:

I have a Mac Pro 17 in, Core 2 Duo 2.33 ghz. When I got it, the supposed max memory was 3 GB and that is how I ordered it from Apple. Memory suppliers of add-on memory for this processor speed also report that the max RAM is also 3 GB.

This discussion prompted me to try and replace the 1 GB module with a matching 2 GB module. I did expect to see System Profiler reporting 4 GB total but that the OS would actually be using less.

But both Activity Monitor and the shell command top seem to indicate that the full 4 GBs are available to the OS. For example, top right now says:

PhysMem: 347M wired, 1237M active, 270M inactive, 2885M used, 1211M free.

The latter 2 numbers add up to 4096M which is I believe what 4 GB means in practice. Yesterday as I was trying to use several memory hungry programs like VMWare Fusion and running Java compiles, the free memory did get quite low: a small number of MB.

A colleague with a recent Macbook that does allow 2 2GB modules to be used reports the same kind of top numbers.

So it seems like I am enjoying a true 4 GB experience. The machine as I am using it also seems to be paging out a lot less frequently than I recall when I had 3 GB installed.

Your experience may vary!

[OreoCookie]

Originally Posted by Simon 

You're absolutely right, I misread that. </blush>

In this case I'll gladly quote from the sticky thread on MB(P) RAM upgrades:
However, this doesn't mean that less matched memory is better than more unmatched memory! If your system is paging out to disk because not enough memory is available this will cause a huge performance decrease. This effect is orders of magnitude larger than the slight performance reduction due to unmatched memory pairs. Therefore: If you're paging out with 2 GB of matched memory, DO NOT hesitate to upgrade to an unmatched 3 GB!

No prob.

[jjs357]

Originally Posted by jjs357 

FWIW:

But both Activity Monitor and the shell command top seem to indicate that the full 4 GBs are available to the OS. For example, top right now says:

PhysMem: 347M wired, 1237M active, 270M inactive, 2885M used, 1211M free.

The latter 2 numbers add up to 4096M which is I believe what 4 GB means in practice. Yesterday as I was trying to use several memory hungry programs like VMWare Fusion and running Java compiles, the free memory did get quite low: a small number of MB.

...

Your experience may vary!

Ah...

I did not see that the "Used" number is 1 GB greater than the sum of Wired, Active and Inactive.

So the extra 1 GB is put into the Used final total but is in fact NOT being used.

I knew it was too good to be true.

[Simon]

Originally Posted by jjs357 

Ah...

I did not see that the "Used" number is 1 GB greater than the sum of Wired, Active and Inactive.

So the extra 1 GB is put into the Used final total but is in fact NOT being used.

I knew it was too good to be true.

Exactly. It's easy to get this stuff mixed up. But if you add the numbers yourself you will notice that in the end there's no way to cheat around Calistoga's addressing limit.

really addressed RAM = wired + active + inactive + free