Cas Latency vs Frequency

I would like to know which dim would be better for my build. the One has a lower Cas latency and the other runs at a higher frequency. Also what is the relation of cas latency and frequency?

1) 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600
Cas Latency 7... Timing 7-9-8-24
or
2)8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 2133 (PC3 17000)
Cas Latency 9 .... Timing 9-11-10-28

*For an ASRock z77 Extreme 4 running an i7 3770k

Both use a voltage of 1.65V

Or would a 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)
Cas Latency 9......Timing 9-9-9-24

be enough?

Did you buy the ram already or are you overclocking your current ram?

I think he's trying to decide which to buy.

It's simple, you have to balance the two for optimal ram efficiency.
You won't see much difference between the two unless you are a bencher and want the best possible score at superpi32m or something.

What's the rest of your pc or projected build? What is it for? Budget? Location?

1) DDR3 1600 7-9-8-24
or
2) DDR3 2133 9-11-10-28

This might help. CAS latency (and the other timings as well) are in units of clock cycles. So CAS 7 means 7 clock cycles. Clock cycle time is dependent on frequency, so let's figure out how long the interval is for a clock cycle on each.

1) 1600MHz = 1,600M cycles/second = 0.625ns / cycle
2) 2133MHz = 2,333M cycles/second = 0.427ns / cycle

So the timings for each, now expressed in ns, are:

1) 7 - 9 - 8 = 4.38ns - 5.63ns - 5.00ns
2) 9 - 11 - 10 = 3.84ns - 4.70ns - 4.27ns

Shorter intervals are better for timing (less time to wait means it's faster, which is why lower timings are better), so that means that the 2133MHz RAM, despite having larger values for timings, actually waits less. 9 cycles takes less time at 2133MHz than 7 cycles does at 1600MHz.

That makes it easy - the latencies on the 2133MHz RAM are actually faster, *and* the bandwidth is higher as well due to the higher clock speeds. Win/win for the 2133MHz.

What does that s value stand for?
cycles / s(?) = ns

EDIT: Found this: (CAS / DATA RATE) * 2000 = X ns

Doesn't add up...I'm confused now. :/

cycles / s = cycles per second
ns = nanoseconds

And how do you find the value for cycles/s?

How do you factor in the command rate (1T-2T)?

ivanlabrie said:

And how do you find the value for cycles/s?

Click to expand...

Cycles/s is based on the clock speed. 2133MHz means 2133 million cycles / second. So one cycle is 1/2133000000 of a second, which comes out to 0.427ns. A nanosecond is one billionth of a second.

How do you factor in the command rate (1T-2T)?

Click to expand...

That's a good question. I don't know, actually. Some people say it's a cycle across the board, but I don't know if that's true. The stuff I wrote above is assuming equal CAS latency.

Apparently command rate is the time between activation and issuing a command for a chip:

http://www.hardwaresecrets.com/article/Understanding-RAM-Timings/26/6
http://www.techpowerup.com/printarticle.php?id=131

lnandez said:

I would like to know which dim would be better for my build. the One has a lower Cas latency and the other runs at a higher frequency. Also what is the relation of cas latency and frequency?

1) 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600
Cas Latency 7... Timing 7-9-8-24
or
2)8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 2133 (PC3 17000)
Cas Latency 9 .... Timing 9-11-10-28

*For an ASRock z77 Extreme 4 running an i7 3770k

Click to expand...

2133 kit will be faster and you can always set it to lower clock and tighter timings so will probably make something near that 1600 7-9-8 or 1866 8-9-8.

Command Rate is not affecting performance so much on Ivy Bridge or generally any new platform. There is little difference in some benchmarks but in daily work it's nothing.
Most memory kits are rated for CR 2T but mainly because of compatibility with most available platforms. On IB most kits are running fine at CR 1T.

Getting ram at 2133 would be the better buy then. if so is there any model you recommend. Id have set the speed and timings manually through the extreme 4's bios or is it set auto?