Low Latency v. Higher Frequency

[David Habermehl]

Can someone please explain to me what is the impact on system performance of low latency compared to the impact of higher frequency? For example, would it be better to run DDR RAM on a PC400E-Deluxe at 2,3,2,5 and 400MHz, or at 3,3,3,6 and >400 MHz?

And while I have your attention : If my OCZ memory is warranted up to 3 volts, is there a benefit to running it at 2.65 volts instead of at 2.85 volts?

Thanks,

David

[h20-ski]

P4's like more bandwidth than lower latency. Although those 2 tend to go hand to hand.

[zapionics<alt>]

Originally Posted by David Habermehl

Can someone please explain to me what is the impact on system performance of low latency compared to the impact of higher frequency? For example, would it be better to run DDR RAM on a PC400E-Deluxe at 2,3,2,5 and 400MHz, or at 3,3,3,6 and >400 MHz?

And while I have your attention : If my OCZ memory is warranted up to 3 volts, is there a benefit to running it at 2.65 volts instead of at 2.85 volts?

Thanks,

David

Just some background info,

With all these settings the RAM manufacturers build in some "safety levels" in their products, basically the chips can handle a slight increase over their factory specified settings. This safety level ensures reliable operation even under high stress conditions. Also, and even more importantly, there is some variation between the modules coming out of the factory due to slight differences in the silicon at the atomic level between batches and slight differences in other component structures, so that there can be a bit of variation in the characteristics of modules even of the same brand and model. It is this variation that we take advantage of when overclocking the ram modules. The aim of overclocking is to get the maximum performance whilst maintaining the reliability of the modules and without causing damage to the modules.

Latency refers to how long it takes the ram to complete an instruction (either to read or write) (broadly). Typically, the lower the latency setting the better as this means instructions are processed quicker which gives better system response.

Increasing frequency on the other hand is effectively the same as increasing bandwidth which is also very desirable. Higher frequencies mean more instructions will be processed by the RAM in a given time frame.

So obviously increasing bandwidth and reducing latencies is a good thing, however there is a catch.

The problem is that the engineers are having difficulties designing RAM modules that can do both without introducing instability. Instability is what happens when the RAM chips fail to process instructions correctly and it results in errors. Memory errors usually manifest themselves as system crashes or lockups or sometimes data corruption. So there is a compromise between latency settings and frequency.

The aim is to get as high a bandwidth as you can with as low a latency as you can whilst maintaining stability. To answer your question, more bandwidth is always better so, in general, frequency is the "course" adjustment and latency is the "fine" adjustment. (IMO). Having said that, results will depend on the particular modules and applications on a case by case basis.

As far as the voltage (VDIMM) goes, when overclocking its often necessary to supply more voltage to the module to overcome problems with increasing signal losses in the circuits, however increasing voltage also increases heat which is not good as it leads to instability and component wear. It is also possible to go the other way lowering the voltage level to reduce heat, however you can only go so far before instability occurs again. Some DDR modules will sometimes overclock better at lower VDIMM due to particular characteristics of the modules.

So if your not doing any overclocking then increasing the memory voltage would be very bad as it increases heat for no purpose and decreasing it might lead to instability.

Overclocking is a compromise between all these things and needs a bit of trial and error to find the best settings.
I'm sure there are some ppl here who can give advice on the best procedure to approach overclocking and which ram is best for your system, etc.

[David Habermehl]

Thanks. I appreciate the information.