Cas Latency

[Fred]

Recognizing that memory speed/bandwidth is king, and all other factors being even, is it fair to assume that cas latency has a fairly significant impact on results (LL testing)?

In particular, I'm looking at cas ratings of 12 vs 15 for an extra $16 :

http://www.newegg.com/Product/Produc...D=3938566&SID=

vs

http://www.newegg.com/Product/Produc...D=3938566&SID=

[Mark Rose]

I don't recall anyone doing a CAS latency benchmark comparison.

Perhaps underclock your existing RAM and see what effect it has?

[Madpoo]

Originally Posted by Fred

Recognizing that memory speed/bandwidth is king, and all other factors being even, is it fair to assume that cas latency has a fairly significant impact on results (LL testing)?

The CAS latency does play a part in other memory benchmarks so I can guess it would also impact P95 performance in the same way. But as Mark said, I don't know that anyone has done a side-by-side comparison to confirm that hypothesis.

In general though, lower latencies are going to give you better performance overall even at the same memory speed. If it were me, I'd pony up the extra $16 just to be sure.

[science_man_88]

Originally Posted by Madpoo

If it were me, I'd pony up the extra $16 just to be sure.

I'm not into hardware or software all that much but you'd pay ~47% more to have 25% more speed ? as that's what I would get thinking of rough math in my head. it's also only because they are both SDRAM that they are simply comparable as DRAM itself would have nano second timings where as SDRAM has clock cycles.

[mackerel]

Is the mobo that it will be used in not supporting faster ram? I would think that would have more of a benefit.

I have done a lot of testing in the past, but ram timings wasn't a part of it. If we take the choice as presented, I'll see if I can get one of my systems into a state to test that, but it might take a few hours as I have other things to do first.

[Mark Rose]

Originally Posted by mackerel

Is the mobo that it will be used in not supporting faster ram? I would think that would have more of a benefit.

I have done a lot of testing in the past, but ram timings wasn't a part of it. If we take the choice as presented, I'll see if I can get one of my systems into a state to test that, but it might take a few hours as I have other things to do first.

I truly appreciate all the benchmarking you do.

Perhaps we should start an ideal hardware thread that can be stickied for future reference.

[tServo]

Originally Posted by Mark Rose

I don't recall anyone doing a CAS latency benchmark comparison.

Perhaps no one here using Prime95 but if you google
cas latency anandtech or cas latency tom's hardware

you will see lots of articles about memory tests/comparisons, particularly on the
Anandtech site. They also compare DDR3 vs DDR4, raw speed vs cad latency, etc.
Anandtech's articles are quite detailed and lengthy.

Most of the posts on Tom's Hardware are from the forums of people asking questions.

[mackerel]

2400-15-15-15-35
Timings for 4096K FFT length (1 cpu, 1 worker): 15.41 ms. Throughput: 64.88 iter/sec.
Timings for 4096K FFT length (2 cpus, 2 workers): 16.28, 16.26 ms. Throughput: 122.93 iter/sec.
Timings for 4096K FFT length (3 cpus, 3 workers): 20.01, 19.98, 19.92 ms. Throughput: 150.22 iter/sec.
Timings for 4096K FFT length (4 cpus, 4 workers): 25.71, 25.76, 26.57, 26.39 ms. Throughput: 153.24 iter/sec.

2400-12-14-14-35
Timings for 4096K FFT length (1 cpu, 1 worker): 15.41 ms. Throughput: 64.88 iter/sec.
Timings for 4096K FFT length (2 cpus, 2 workers): 16.22, 16.25 ms. Throughput: 123.16 iter/sec.
Timings for 4096K FFT length (3 cpus, 3 workers): 19.75, 19.78, 19.86 ms. Throughput: 151.55 iter/sec.
Timings for 4096K FFT length (4 cpus, 4 workers): 25.85, 25.95, 25.88, 25.66 ms. Throughput: 154.82 iter/sec.

3000-15-17-17-35
Timings for 4096K FFT length (1 cpu, 1 worker): 15.29 ms. Throughput: 65.42 iter/sec.
Timings for 4096K FFT length (2 cpus, 2 workers): 15.66, 15.65 ms. Throughput: 127.75 iter/sec.
Timings for 4096K FFT length (3 cpus, 3 workers): 17.42, 17.44, 17.36 ms. Throughput: 172.35 iter/sec.
Timings for 4096K FFT length (4 cpus, 4 workers): 21.47, 21.38, 21.41, 21.44 ms. Throughput: 186.72 iter/sec.

Test system has an i5-6600k fixed at 3.5 GHz. Ram is dual channel, single rank.

Comparing the two tests at 2400, the tighter timings gives you under 1%. Turn the wick up to 3000 for a more visible boost. 25% more ram clock for 20% more performance. Think we might be a bit ram bandwidth limited here?

[Mark Rose]

So lower CAS latency helps a little for Prime95, but bandwidth is king.

[mackerel]

Up to a point. On the weekend I was using the same system to see if I could overclock the ram further. I reached 3600 at some slack timings, and it only increased throughput 3.3% over 3000. So it seems 3000 gives you most of the bandwidth you need for this system. Of course, if the CPU clock is faster, you would need faster ram again.

I think this also showed a potential error in my earlier estimates in how rank influences things. I suspected my results were slightly over-estimating the ram bandwidth required. I didn't allow for the change in throughput being non-linear depending on the balance between ram and cpu. It only approaches linear if you're really limited by one or the other, so I'll have to retest that and see what it does.

[kladner]

This is an interesting and educational thread.