Multi-Channel Memory Architecture

Welcome to the ITFreeTraining video on multi-channel memory architecture. Multiple channel is a technology that increases the amount of data that can be transferred between memory and the memory controller by grouping memory modules together. By the end of this video, you will have a better understanding of this technology, how to implement it and what performance effects you can expect to receive.

Download the PDF handout: http://itfreetraining.com/handouts/ap...

Memory Wall/Bandwidth Wall
0:24 Before I look at how multi-channels work, I will first look at the memory wall (also referred to as the bandwidth wall). This will give you a better understanding of why multi-channel memory was developed.

In the early days of computing, CPU speed increased very quickly. Over some years, CPU speed doubled each year. The problem was memory speed was not increasing at the same rate as CPU speed. This created a gap between CPU performance and memory performance.

This gap in performance is referred to as the memory wall or bandwidth wall. This was more of a problem in the old days of computing as CPU speed kept increasing much faster than memory speed. However, nowadays CPU speed does not increase anywhere near as fast as in the old days, but performance can be improved by adding features like additional cores to a CPU. The slow increase of CPU speed has allowed memory speed to increase and catch up with CPU speed.

No Multi-Channel (Single)
1:24 To understand how multi-channel works, I will first look at what occurs when it is not used. Consider that you have a memory controller, either inside the CPU or on its own chip. Inside the computer, there are two memory modules.

The memory controller needs to access the memory modules. With two memory modules, dual- channel would be used. Without it enabled, which is the case in this example, the memory controller shares the access path from the memory controller with the two memory modules.

This essentially means that it can only access one memory module at a time. While the first memory module is being accessed, the second memory module cannot be accessed. So what is the difference between this and dual-channel access?

Dual-Channel
2:11 When dual-channel is enabled, the memory controller is able to access both memory modules at the same time. By being able to access two memory modules at the same time, this increases the amount of data that can either be read or written to the memory modules at once.

I will look at triple and quad-channel shortly, but first I will look at the requirements needed to run multi-channel.

Requirements
2:32 In order to use multi-channel, first the memory modules must have the same DIMM configuration. This essentially means that both need to be of the same size and have the same number of chips on them. Traditionally, you won’t be able to mix and match, for example a 4GB memory module with an 8GB memory module. If the memory modules have a different number of chips, most likely they will operate differently. For example, how they access and transfer data will differ - so they will not work together.

Having said all this, some of the newer motherboards and CPUs will allow differently sized memory to be configured using multi-channel. They achieve this by mapping the matching memory amount to the multi-channel. Any extra memory would be mapped to its own channel. This essentially means you would get a speed improvement for the lower memory amount that is mapped directly to another memory module and no speed increase for the memory that cannot be mapped, since there is not enough memory in the other memory module to do this.

When you have been shopping for RAM, you may have noticed memory modules sold as a kit. For example, this one shown. Manufacturers will often sell two memory modules bundled together to ensure that there are no problems working in multi-channel mode.

Description too long for YouTube. For the rest of the description please see http://itfreetraining.com/ap/3a25

References
“Random-access memory”
“Multi-channel memory architecture” https://en.wikipedia.org/wiki/Multi-c...
“Clock rate” https://en.wikipedia.org/wiki/Clock_rate
“RAM Performance Benchmark: Single-Channel vs. Dual-Channel - Does It Matter?” https://www.gamersnexus.net/guides/13...

Credits
Trainer: Austin Mason http://ITFreeTraining.com
Voice Talent: HP Lewis http://hplewis.com
Quality Assurance: Brett Batson http://www.pbb-proofreading.uk