

An excerpt from the Rambus ruling in April






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RE: I dont get the rambus hate to this day...
By fsardis on 7/18/2006 7:28:28 PM , Rating: 1
All memory latency is lowered by increased clock rates. This is nothing unique to RDRAM.

wrong. increasing the ram speeds inevitably means more latency. check out DDR modules for proof and even better check out the latency differences between DDR3200 and DDR4000. another good example is DDR vs DDR2.
yet another example is overclocking the ram which definitely means increasing the latencies to achieve higher speed.
so no, increasing the clocks doesnt lower the latency in any way.

By masher2 (blog) on 7/18/2006 8:05:41 PM , Rating: 2
> "wrong. increasing the ram speeds inevitably means more latency"

Err, no. It doesn't work like this at all. Latency is based on clock cycles...and the length of a cycle is the inverse of the operating frequency. Increase the clock rate on a memory module, and the latency decreases. In an exact, perfectly linear manner. Twice the clock rate = half the latency.

What you're thinking of is a different issue entirely. Quite often, you cannot increase the clock rate without relaxing the timings. And that may wind up increasing the overall latency more than what the higher clock yields. Result...more latency.

DDR2 has a higher latency than DDR. But its not due to the higher clock rate, its in despite of it. It has significantly higher timings, a prefetch buffer and additional mux/demux logic...all of which adds latency.

The OP was implying that the normal relationship between clock rate and latency was something special to RDRAMs. It is not...its a basic element of all memory technologies...though the latency is, of course, affected by many other factors besides clock rate.

By emboss on 7/18/2006 9:46:08 PM , Rating: 2
Just one minor clarification about an otherwise excellent post ... IIRC, latency (except for CAS) is actually specified in ns in the SPD data (which makes sense if you think about how RAM works). RAM controllers work at particular clock rate, which requires changing this ns time into clock cycles. Due to this quantization alone, increasing the clock speed (assuming the timings are strictly adhered to) may require an increase in latency (measured in ns).

By masher2 (blog) on 7/19/2006 12:17:13 AM , Rating: 2

Exactly excellent point which I was lax to not have mentioned.

By Acanthus on 7/19/2006 3:09:19 AM , Rating: 2
No, what i was saying was that with RDRAM, the timings were static, so the latency was significantly reduced as the clockspeed went up.

