The Unofficial Apple Weblog (TUAW)

http://www.barefeats.com/mbcd6.html

DO MATCHING PAIRS OF MEMORY HELP?
Though we ran the tests above using matching 1GB SODIMMs in both MacBook Pros (for a total of 2GB), we also ran the same tests in the 15" MacBook Pro Core 2 Duo with one 1GB SODIMM and one 2GB SODIMM for a total of 3GB of memory. We wanted to see if non-matched modules would cause the MacBook to slow down due to the loss of interleaving.

The answer is "no, it didn't slow down." In some cases we saw a gain in speed. An example is Aperture where the "lift and stamp" ran 11% faster with 3GB of RAM. But that's probably due to the fact that Aperture + OS X = more than 2GB of total memory usage.

In a few days, when we have a second 2GB SODIMM, we plan to run two of them. Even though only 3GB of the 4GB will be available, we want to see if a matched pair of 2GB SODIMMs will show anything different.

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Is 1.25 in a MacBook ok?

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Just found this: http://docs.info.apple.com/article.html?artnum=303553

The MacBook Pro will run fine with either a single memory slot filled or with both memory slots filled with two unmatched memory sizes (such as a 512MB DIMM in one slot and a 1GB DIMM in the other).

However, if both slots are loaded with an equal amount of RAM (such as two 512MB DIMMs of the same type), you can take advantage of the system's dual-channel memory architecture for an additional performance boost. With a dual-channel memory interface, both banks of SDRAM can be addressed at the same time, enabling MacBook Pro to reach a memory throughput of up to 10.7 GBps.

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Extra ram speeds up a system way more than a little extra bandwidth. Few apps are memory bandwidth bound, while almost all benefit from less swapping.

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well... the memoryspeed of the first 2GB will be at dual-channel and the last 1GB will be at single-channel speed... It would surprise me (and I don't believe it either) if the third stick would drop the whole 3GB to single-channel speed.

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As Mark stated, having 3GB (2+1) will prevent the CPU from taking advantage of the dual-channel memory architecture, yielding to slower memory access times compared to a 2GB (1+1) configuration.

So, if you don't use more than 2GB most of the time, your machine will be slower with 3GB. However, as soon as you start using more than 2 GB of RAM, OS X won't have to swap memory to disk and will show a real performance gain, probably way more than the 'loss' on memory access.

In other words, if you get 3GB, you better use that 3rd one !

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...adding to this. So will it drop in performance? No. The first 2GB will be as quick as before. The third GB is slower than the first two, but a whole lot quicker than a swapping harddrive... :-)

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If I read the kingston paper correctly, it seems like the answer lies in how apple wires the memory controller, for example, instructing it to send more intensive tasks thorugh the 2GB channel, and relying on the remaining 1GB channel for background tasks.

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Chaps, I'm confused. We're talking about TWO (2) sticks of RAM here, a 2Gb stick and a 1Gb stick, not three 1Gb sticks, yet people are talking about a third stick.
In a 2 stick config (2+1) is the first 2Gb faster than the third gig, or does that require a pair of 1Gb sticks (obviously not in a Macbook - does the iMac take more than two sticks?)
Anyway, my 3Gb Macbook Pro shipped yesterday so I guess I'll know for sure in a few days time.

Woz

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Just in from Barefeats.com...

November 15th, 2006 (update) -- More on 3GB Memory Limit on MacBook Pro (Core 2 Duo): Apple states on their memory expansion page that "If you install a 2 GB SO-DIMM in both the bottom and top memory slots of the computer, the 'About This Mac' window and Apple System Profiler will both show that you have 4 GB of SDRAM installed. However, Activity Monitor and other similar applications will reveal that only 3 GB of SDRAM has been addressed for use by the computer." (This also applies to the iMac Core 2 Duo.)

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The most important part of the update

...Of more concern to "speed freaks" is the potential loss of interleaving when you install unmatched pairs (as in the Apple factory's 1GB + 2GB config). Based on our testing, the MacBook Pro goes just as fast with unmatched memory as it does with matched. And apps like Aperture and Photoshop can certainly use the extra memory afforded by the 3GB config.

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does this make a difference on the MBP CD or just the C2D

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Just wondering, can u put a 2 gig stick in a CD

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I think the core duo can only handle 2 Gb of RAM.

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Yeah, I have a core duo MacBook Pro, and I am wondering if I can install a 2GB stick of ram alongside my factory 1GB. Also, could I potentially install 2 2GB sticks? What about on a Core 2 Duo?

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While perhaps the performance gain from matched memory is negligible on a MacBook Pro, on Macbooks it should matter, as there's the graphics hardware using the memory, too, and consuming bandwidth. I think that's why apple sells MacBooks only with matched pairs, MBPs not.

Jojo

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Dual channel RAM was always kind of a boondoggle... I don't think I've ever seen a benchmark that shows more than a 5% increase in speed, and even then only in a couple very specific tasks.

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Ok for what I know
is that
the C2d supports 3gb at full speed 667mhz DDR2
the CD supports 2gb at full speed 1x2gb
however the zCD will support 4gb 2x$ at 533mhz
so I assume the the C2d supports @ least 4gb

Slots
IMac HAS 2 slots, so do the laptops
I've heard that the 3gb is actually a 2x2gb config, but at full speed only 3gb are seen
matched pairs do help


Note: I'm only a 14yr old Mac finatic so any of this info could be wrong

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edit:the CD supports 2gb at full speed 2x1gb
NOT as I had previously posted
and that $ is a 4

Also the CD takes the exact RAm as the C@D - none of the actually dpeneds on the processor, btw

Also in the literal sense of the word you CAN always put a larger stick in your computer, will it reconigize all of it, maybe not, but ...

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I would think that in cases where you require 3 GB of RAM for an app, if there was a speed decrease when using mis-matched modules, it would still be at least an order of magnitude faster than swapping to the HDD.

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