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Help: Why does faster memory have higher latency values?

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Last response: in Memory
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December 19, 2005 2:35:35 PM

I'm trying understand this latency thing. From what I've read, the lower the values the better. Why, then, do faster memories have higher latency numbers than the lower ones? i.e. the following from newegg.com

Am I looking at this wrong? The lower latency is cheaper, also.
I'm confused!

OCZ Gold Series 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 667 (PC2 5400) Unbuffered Dual Channel Kit System Memory Model OCZ26672048ELDCGE-K - Retail
Cas Latency: 4
Features: Gold Mirrored Copper Heatspreader
Heat Spreader: Yes
Manufacturer Warranty: Lifetime
Timing: 4-4-4-8
Voltage: 1.8V
Model #: OCZ26672048ELDCGE-K
Item #: N82E16820227047


OCZ Gold Series 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 533 (PC2 4200) Unbuffered Dual Channel Kit System Memory Model OCZ25332048ELDCGE-K - Retail
Cas Latency: 3
Features: Gold Mirrored Copper Heatspreader
Heat Spreader: Yes
Manufacturer Warranty: Lifetime
Timing: 3-3-3-8
Voltage: 1.8V
Model #: OCZ25332048ELDCGE-K
Item #: N82E16820227045
December 19, 2005 2:47:39 PM

It gets very technical very quickly, but in very basic terms...

Latency is the amount of time spent waiting so the memory can be ready to perform the next step of accessing memory, writing memory, etc.

Latency values are in clock ticks. But memory latency is in actual fractions of a second, and doesn't go down much any more (DRAM memory technology is pretty mature) as technology improves. As the clock rate goes up, the latency in ticks has to go up to give the chips enough physical time to be ready. So at DDR400 CAS 2 is good. But at DDR800, the memory needs as much physical time to prepare (stabilize actually) as DDR400, so the CAS has to be at least 4 because the clock rate is twice as fast.

Did that make sense?

Mike.
December 19, 2005 3:25:34 PM

I think this makes sense.

So the number is based on different clock speeds. So you can't actually compare the numbers between to different speeds of memory? Is the memory actually waiting on the processor? The faster memory goes through four ticks which might be equivalent to two ticks on the slower memory.

Is is it fair to say that the latencey between the to types of memory is fairly equal?

Am I going to notice a diffrence between the two while I'm trying to run a game at max res and detail?


Thanks for your help!
December 19, 2005 5:10:04 PM

Quote:
So you can't actually compare the numbers between to different speeds of memory?

Not directly.
Quote:
Is the memory actually waiting on the processor?

No. The memory is waiting on itself to be stable enough so it can go to the next step.

Quote:
Is is it fair to say that the latencey between the to types of memory is fairly equal?

In real time, Yes.

Quote:
Am I going to notice a diffrence between the two while I'm trying to run a game at max res and detail?

Probably not. Res & details are more video card related than CPU & RAM related. The faster memory will perform faster, but I don't know if its a noticeable or significant amount or not.

Mike.
!