Correlation between CPU/FSB/RAM frequencies and voltage

T

TheKitsuneWithATie

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Hi,

I just want to learn more about how the different frequencies are set and how they correlate between one another. I'd also like to know how voltages are set.

Right now thats what I understand about how modern systems work:
  • The CPU sets the BCLK (Base CLocK) that is used by the CPU itself, the RAM and FSB as the "reference";
  • The CPU's frequency is set by multiplying by the BCLK by a coefficient set by its manufacturer (that coefficient can be either locked or unlocked for overclocking);
  • Modifying the CPU's frequency multiplier won't affect FSB's nor RAM's frequencies;
  • Modifying the BCLK will affect the CPU's, RAM's and FSB's frequencies.
If I got something wrong please tell me.

So, with all of these informations in mind, I have a few questions:
  • How are the RAM's and FSB's frequencies calculated? Do they have they're own frequency multiplier or do they work in a different way?
  • When overclocking, will raising the BCLK give a significant performance improvement? As it should in theory affect the CPU, FSB and RAM at the same time. I read that the headroom is lower than when using the CPU multiplier;
  • How does RAM overclocking works, over than augmenting the BCLK?
  • Is there a way to overclock the FSB? I read that the FSB frequency doesn't have to be modified in modern systems when overclocking;
  • I read everywhere about "raising the voltage" to get a stable overclock, but it seems pretty dangerous. What correlation is there between frequency and voltage when overclocking the CPU (Vcore) and RAM? How do I make sure that the set voltage isn't to low nor dangerous?
I know that's quite a lot of questions, but I don't want to go head first overclocking systems without understanding exactly what I'm doing.

Thank you in advance and have a nice day.
 
This is something that you will want to spend a while researching and watching videos on before you dive into it. Keep in mind though that you'll likely never know exactly what you're doing but at some point you'll know enough to accomplish something and have some fun along the way.

One thing to note is that FSB was done away with around 2008 and replaced with quickpath interconnect (QPI). This is more a point-to-point link between the cpu and memory, like athlon did awhile ago with the a64 hypertransport. QPI frequencies are processor specific and change based off of ram overclocking, so you won't be modifying them directly save for the voltage at times.

  • Ram frequency is limited by the multipliers your motherboard is capable of providing unless you are changing bclk which is rarely necessary on current processors. The quoted speed is the I/O bus frequency times two (dual data rate). So ddr4-3200 runs at a bus speed of 1600mhz bidirectionally. The memory modules themselves run at 1/4 the bus speed (in this case 400mhz) but this isn't really relevant to overclocking.
  • Raising the BCLK will give a relatively proportional performance increase, but there's generally little headroom to make on the newest generation proccesors and raising the clock multiplier while leaving bclk at 100mhz is currently the method of choice. You might have read an older article that focused on different cpus.
  • You can directly set ram speeds from the bios on modern boards. My z270 board goes up to 4133mhz without modifying bclk. You also overclock ram by changing the timings of in the individual actions taking place in your ram. There are dozens of them (CAS being the most important) and you can spend weeks or months dialing them in for that very slight performance increase.
  • Provided you have sufficient cooling, raising the voltage on current i7s will usually have a positive impact on the max stable frequency for at least the first .2-.3 volts. The point of diminishing returns for me using a few 7700ks is usually around 80C - meaning when I hit 80C under a full synthetic load it's unlikely that more voltage will lead to anything but higher temperatures. This might be pretty variable and current i7s are getting increasingly harder to cool due to the shrinking die size (less surface area for heat transfer and denser silicon). Ram is variable in its voltage tolerance based on the specific type. Some ddr4 doesn't respond much to more voltage but others (like Samsung b-die) will tolerate upwards of 1.8v despite a rating around ~1.35v. The highest performing memory through the years has historically used high voltage and required active cooling.
Hopefully this gives you an idea of the basics. Let me know if you have any more questions.
 
Thank you a lot for your detailed answer, this helps a lot.

But I have a few more questions:
  • If I understood correctly, RAM overclocking capacity is on the motherboard side? If the BIOS doesn't offer any RAM overclocking options, are there other tools that can do so? Is the overclocking option only provided by the BIOS or does it have to be an "active" motherboard feature (I don't how I can phrase this better)?
  • What defines the BCLK? Is it the CPU with its QuickPath Interconnect/HyperTransport or is it the motherboard? From what I understood, it should be on the CPU side but I'd prefer for someone to confirm that I got it correctly;
  • I asked similar questions on another forum and someone told me that the different components (CPU/FSB/QPI/HT/RAM) can each have their own BCLK on modern motherboards, is this true? And if so, how could this work if every components has a different BCLK?
  • When should I increase voltages when overclocking? Should I overclock the component (RAM/CPU), and if the overclock is unstable, try to increase the voltage and see if it makes it stable? Moreover, how do I know that the voltage is enough, how do I make sure that it isn't too high?
Thanks again for your help.
 
  • The memory controller is integrated on the chip of current processors, so ram overclocking capacity is affected by both the strength of the chip and the motherboard. There's some decent variability of these two at least at the highest level, so serious overclockers will take many cpus and motherboards and put them through a binning process to determine the very best performing units. Overclocking has to be done through the bios. There are some programs that can change some values while in windows, but to my knowledge they're still doing so through the bios. The bios itself is a program that can be modified so more options could become available by flashing a new one.
  • BCLK is base clock and is set by a clock generator circuit on the motherboard. The motherboard then multiplies this base frequency by different values for the different component interconnects. Motherboards can implement a lock on the base clock between some components as well, but this is variable based on the specific chipset and often unnecessary on the most current setups.
  • While BCLK generally refers to the cpu bus speed, it's possible for other components to have different clock rates. This is seen less on modern rigs as there is little reason to modify BCLK beyond a few MHZ of fine tuning. Clock multipliers are instead used to achieve the desired frequencies of different interconnects or buses.
  • A decent amount of trial and error goes into overclocking. Experience streamlines the process, but you generally want to overclock in reasonably small steps, raising voltage as necessary to increase stability while remaining within your thermal limits. You test these limits and the overall stability by running a program like prime95 to put more load on the cpu than it should ever see in normal usage and monitor the temperatures while doing this. If the cpu is unstable the stress-testing program will see the errors it's generating. People come up with their own definitions of "stable" to suit their needs.

Why don't you post the details of your setup and we can talk about your specific options for overclocking.
 
Thank you for taking time to answer my questions.

I currently have two systems I'd like to overclock.

The first one has the follow configuration:
  • CPU: I5-3570K
  • Motherboard: Asus P8Z77-V PRO/THUNDERBORD;
  • RAM: 2x 4Gb HyperX @ 1600MHz.
But sadly this system is starting to have display glitches, I think the motherboard is faulty (I tried multiple GPUs and the same happens every time), so I could use it as a training computer.

But I'm planning on building a new computer, that is marginally different than the previous one, as I'm using Opterons:
  • CPUs: 2x Opteron 6128 HE 8 cores @ 2GHz;
  • Motherboard: Asus KGPE-D16;
  • RAM: 2x 16Gb Samsung M393B2K70BM1-CF8Q1 ECC @ 1066MHz.
I don't even know if I'm going to be able to overclock this one, but I'd like to as the 2GHz CPUs could benefit from a slight overclocking.

This article shows a quad CPU Opteron setup that got overclocked to 4GHz thanks to the software Turion Power Control. But he uses engineering samples so I don't even know if the same can be done on my two 6128 HE.
 
I would stick with the first system for tinkering, provided you can sort out/live with the display issues. The Asus KGPE-D16 motherboard doesn't appear to support overclocking and Opteron 6128s are locked so I'm not sure how much TPC could do for this setup.
 
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