Cas Latency vs Speed OC

[bdbeall]

Hello all,

I have a P8H61-M motherboard and an Intel G620 Proc, and am looking to upgrade my ram from a single 2GB stick to 2x 4GB sticks. I plan on upgrading my motherboard and processor in september. I want to buy 1600MHz ram for when i get my new mobo and proc, but was curious about timings and whatnot.

I know the motherboard support 1333/1066, so can i underclock 1600MHz ram to 1333 and lower the timing from 9-9-9-24 to like 7-8-8, or 8-8-8, or does memory not work this way?


If not, will i be fine running the 1600 ram on this mobo?

 

[willard]

Yes, the memory speed can be lowered to work in motherboards that do not support that speed. And generally, lowering the speed will allow you to tighten the timings, which can get a large amount of the lost speed back.

A good way to judge latency is by comparing real time latencies, not clocks.

1600 MHz memory at CAS 9 has a 9 cycle delay and has 1600 cycles per second. Dividing latency by frequency gives you the actual time, which is 0.005625 seconds, or 5.625ms.

1333 MHz memory at CAS 8 gives you a 6ms latency, so it will operate slightly slower. CAS 7 on the other hand, gives you 5.25ms latency, which is slightly better.

 

[bdbeall]

Thank you for the quick reply, this is exactly what i needed.

 

[Pinhedd]

The memory will most likely contain a valid 1333 MT/s profile with the correct timings. The 1600 MT/s profile can only be enabled on motherboards with XMP support and is not guaranteed to work with processors not tested for 1600 MT/s memory IO. You will not have to adjust the timings or do anything of the nature, it will default to 1333 on its own

 

[geofelt]

I think all 1.5v ddr3 ram will run at lower speeds by default. It takes a bios setting to get it running at 1600.

But really, latency and speed are largely irrelevant for sandy bridge.

The current Intel cpu's have an excellent integrated ram controller. It is able to keep the cpu fed with data from any speed ram.

The difference in real application performance or FPS between the fastest and slowest ram is on the order of 1-3%.

Synthetic benchmark differences will be impressive, but are largely irrelevant in the real world.

Fancy heat spreaders are mostly marketing too.

In fact tall heat spreaders are a negative because they can impact some cpu coolers.
Only if you are seeking record level overclocks should you consider faster ram or better latencies.

Read this Anandtech article on memory scaling:
http://www.anandtech.com/show/4503/sandy-bridge-memory-scaling-choosing-the-best-ddr3/1
---------------bottom line------------

DDR3 1600 is the sweet spot considering the marginal cost delta over 1333.

 

[bdbeall]

Best answer selected by bdbeall.