What drives the timings?

[ehume]

I have 8GB of F3-16000CL9D-4GBRH running at DDR3 1600, BCLK 133 on a Gigabyte GA-P55M-UD2 MB, BIOS ver. = f5K. All voltages are set to auto in BIOS: with BCLK=133, setting VDIMM to 1.65V makes no difference.

Timings from bootup: 7-7-7-16, with tRC=23, tRFC=50, RTL=42 (RTL=41 fails POST) and 1T (this reduces latency about 10% from 2T). These are values set in the BIOS. I picked tRAS=16 because supposedly tRAS=tCL+tRCD+2.

According to SiSoft Sandra, my bandwidth runs in the mid-19 GB/s at DDR3-1600. The bandwidth varies with the mem rate, up to around 24GB/s at DDR3-2000. At DDR3-1600 my combined memory latency=66.2ns, with a power factor of 51.8. The scores vary a bit, of course, but this is the best I have done so far. And it is pretty good: working to get timings down has decreased my latency about 10%.

IMO the results are very, very good. When I get a better cooler, I will crank up the BCLK to 166 to get DDR3 2000. I have tried it out and it works well, except for the temps.

So what is my first question?

I have used CPU-Tweaker to vary the tRAS from 16 to 25 -- higher than tRC -- without making and substantial difference to the results.

In theory, I should either freeze my machine (setting tRFC=45 did that) or cause worsening of the results, but neither happened. I even set tRAS=tRC in BIOS with continued good results.

In theory, this should not be possible. Supposedly tRAS+tRP=tRC, which is why I set tRC=23 in the BIOS.

So, question 1: what is going on here?

My hypothesis is that by setting tRC=23, I am holding tRAS to 16. In essence, tRC seems to be driving tRAS when the settings conflict. But is that right?

Question 2: what is up with tRFC?

Setting it from 50 to 60 with 7-7-7-23 makes no difference to latency. Setting it to 74 or even 88 just barely nudges up the latency by about a nanosecond.

Should I look at other settings? Or should I accept these rates as about the best I am going to do at this BCLK?

Thanks.

[G.SKILL]

that's theory, but it's a little different with fact
and tRC don't have to be exactly tRAS +tRP
we just recommend you to set it that way because it would be much safe and stable to run
for this item, we guarantee for DDR3 2000 9-9-9-24-2N@1.65v, but for DDR3 1600, we are not sure how it could go
you could run memtest to check it these setting is stable to run
if it works fine, we don't think you have to change it
thanks


G.S

[ehume]

Thank you.

I think I have been lucky to get two matched pairs - four sticks - with excellent qualities. I wish I knew better the relationships between the various timings so I could predict what this memory can do.

I understand I will have to learn all over again when I move to DDR3-2000.

[G.SKILL]

for the timings, it seems too much to say
and it may differ with different module
so if you would like to know the better timing for your memory, you may try to find some info on some overclocking forum, which is based on many user's experience
thanks


G.S