Socket A dual Channel

[paulsiu]

I notice that my socket A board support dual channel. However, I recall that socket A were strictly single channel processors. How does this work?

 

[OnEMoReTrY]

Simple. Socket A are not strictly single channel processors

 

[Peter]

This statement is technically nonsense, sorry.

On socket-A, the memory controller is in the CHIPSET not in the processor. The processor neither knows nor cares how the memory controller does its business. So you can't say "single channel processor" here. VIA and NVidia have been making chipset north bridges with dual channel RAM controllers.

The current Athlon/Sempron/Opteron/Turion platform is different. Here, the memory controller is inside the CPU, and as we all know, there are single- and dual-channel flavors.

 

[paulsiu]

OK, that makes sense then. That brings up the question of whether it's better to have a Sempron on socket A or on socket 754 since socket A can support dual channel and 754 cannot.

I did notice however that enabling dual channel on my socket A board did not improve performance by much. Some research indicate that socket A is limited by the 200 Mhz bus to 3.2Gb/s, so even if you have a dual channel DDR400 at 6.4 Gb/s, you're not going to get much improvement. Not sure what socket 754's bandwidth is.

Paul

 

[Megatomic]

It's better to have a s754 Sempron. The memory controller is single channel but it is on the CPU die and not in the chipset. Latency is MUCH MUCH reduced and performance is MUCH MUCH increased. There really is very little room for comparison when looking at s462 Sempron and s754 Sempron.

 

[Peter]

Exactly like Paul said. Socket A Sempron CPU front side bus is 166 MHz DDR, 64-bit wide. Giving the chipset dual channel (128-bit wide) 166 MHz DDR RAM is not going to improve performance at all, simply because the CPU bus forms the bottleneck, not the RAM bus on the other end.

Socket-754 Semprons in turn attach the RAM directly to the CPU, no CPU front side bus there at all. Thus, from the same RAM, you get much better performance than on socket-A. Access latencies are drastically lower, throughput is quite impressively higher as well.