Is there an optimum number of DIMMs?

Posted on 14 Oct 2008 at 12:07

We tend to think of a computer's memory as a contiguous space, but in the physical world it's normally spread across multiple DIMMs, filling up some or all of the motherboard's slots. This raises a few practical and technological issues.

One obvious consideration is how many memory slots your motherboard has. Most come with four DIMM slots, though a few have six or even eight, while lightweight systems and laptops often have only two. If you have just two slots free, it makes little sense to fill them with two low-capacity DIMMs, as you'll be closing off options for future upgrades. Due to the dynamics of supply and demand, it may not make economic sense, either: a pair of 512MB DIMMs can cost more than twice as much as a single 1GB module.

However, two smaller DIMMs can be faster than one large one - as long as your motherboard supports dual-channel architecture. This is a similar system to RAID0 striping, but for memory rather than hard disks: two identical DIMMs run in parallel, acting as one large module with twice the bandwidth of a normal DIMM.

To quantify the benefits of dual-channel architecture, we tried our standard benchmarks with a single 2GB DIMM, then repeated them with two 1GB DIMMs in dual-channel mode.

We found the benefits varied hugely between apps: dBpoweramp and Photoshop gained no advantage, but Canopus ProCoder completed its tasks 1% more quickly with dual-channel RAM, while 3ds Max gained a 1.5% boost. Our Office tasks ran a whole 5% faster, and the multi-applications test reflected a 3% performance improvement.

If your motherboard supports dual-channel operation, the DIMM slots will be colour-coded in pairs. To take advantage of it, you simply need to install two identical modules in slots of the same colour - which is one reason why DIMMs are often sold in matched "kits". You may also have to enable dual-channel architecture in your BIOS.

Next: What does RAM speed mean?

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Author: Darien Graham-Smith