Articles: Memory
 

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Number of Memory Channels and Performance

As we have just seen, installing memory modules into all four channels on LGA 2011 systems may generate no performance improvement compared with the dual- or triple-channel memory configurations. The memory load should be multi-threaded, and only in this case the multiple channels of the Sandy Bridge-E memory controller will translate into performance growth. It is all pretty straight-forward in theory, but what does it look like in real applications? What types of requests are generated by typical desktop tasks? In order to answer this question we once again tested the LGA 2011 system with DDR3-1867 SDRAM working in dual-, triple- and quad-channel modes in popular general-purpose benchmarks. To get a good idea of the relative performance drop upon reduction in the number of channels we also added the results for the sale LGA 2011 platform equipped with slower quad-channel DDR3-1600 SDRAM working with the same CL9 timings.

However, I think we should start with the Stream results: this test shows really well what a given memory configuration is capable of in ideal testing conditions.

As you can see from the obtained results, the loss of one memory channel during multi-threaded load is about twice as bad as reducing its frequency by 266 MHz. the same can be concluded from the results of the theoretical bandwidth tests. Quad-channel DDR3-1867 SDRAM provides 59.7 GB/s bandwidth, while triple-channel memory – 44.8 GB/s. Quad-channel DDR3-1600 SDRAM may be able to ensure up to 51.2 GB/s bandwidth. However, it is important to remember that DDR3-1867 also has an advantage in latency, independent of the number of utilized memory channels.

Now let’s check out the results of complex benchmarks:

The most important conclusion made from the graphs above is that reducing the number of memory channels in LGA 2011 platforms doesn’t lead to any catastrophic consequences. Yes, the performance drops, but only as little as by 1-2%, which you will barely feel in real applications. Moreover, strange as it might seem, but reduction in memory frequency almost always affects the performance more seriously than the use of fewer memory channels. In other words, if you already have a high-speed dual- or triple-channel DDR3 SDRAM kit, then the only way to replace it with a quad-channel one for your LGA 2011 system is if the memory frequency doesn’t get any lower than it used to be. Otherwise, high-speed DDR3 SDRAM will deliver higher performance in most applications even though it will be using fewer channels.

In fact, the only benchmark where we saw the true advantages of the quad-channel access was the synthetic eight-thread Stream. And it indicates clearly that real desktop applications do not use enough parallel threads when working with the memory sub-system to allow Sandy Bridge-E memory controller optimized for server workloads to show its hidden potential.

 
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