Dual Channel 667 = Single Channel 1333 (or 1066) ?

[Gabriel Torres]

Hi there

I've just wrote down to update our tutorial on timings to add all these math...

So if i run the RAM i bought (Kingston HyperX 1200 Mhz CL5) at 1066 Mhz (533 Mhz: real clock) then the clock cyle is ~1.8 ns (1.876...) which makes the total latency with CL5, 9 ns (9.38...). Just a little bit higher.

Perfect math.

If i run it at 1200 Mhz (600 Mhz: real clock) then the clock cycle is 1.6 ns (1.666...) which makes the total latency with CL5, 8.3 ns!!! (8.333...).

Perfect math one more time.

That means it is better than a 667 Mhz CL3 RAM.

Yes.

And one question Bart did:

Is latency more likely to determine performance than the amount of bandwidth?

Latency vs. bandwidth is a very old war. It will depend on the circumstances.

Higher bandwidth is better if data is stored in subadjacent addresses. This happens because SDRAM-based memories (DDR, DDR2, etc) can deliver from the second data on with zero latency. For instance, if you would read four data that are stored side-by-side on a memory with CL5 the first data would be delivered after 5 real clock cycles but the other three data would be delivered immediately, and since DDR/DDR2/DDR3 memories deliver two data per clock cycle, each data would be delivered in half clock cycle. In other words, first data 5 clock cycles, the other three data half clock cycle each, in sequence.

But in the example above if data aren't located in sequential addresses, then the CPU will have to wait for the latency: 5 clock cycles for the first data, 5 clock cycles for the second data and so on.

So for random memory accesses lower latency is better.

I attached a chart from a DDR2 datasheet from Micron that shows the time table for a DDR2 memory, after the above explanations you will understand it.

Cheers,
Gabriel.

[ComputerWormTR]

Hi Gabriel!

I wonder something. When the data is written in sequential addresses, so it can be read without latency? Is that up to the software? And why not all of the data (when it's possible) is written sequentially? or is it? Thx in advance.