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I understand under certain circumstances a CPU and memory may need to be matched on speed to ensure there is not something like a 'wasted cycle' in waiting for a component to respond. I have heard of situations where memory or the CPU might waste every second cycle because the other component can't quite keep up with the requests. It seems like under-clocking one to match the other would be the logical step, but I also understand DDR memory runs on an internal refresh cycle to continuously rewrite the memory contents. This cycle is likely unchangeable, thus potentially a mechanism for this 'wasted cycle'.

I have acquired an Intel XEON CPU that is rated to run with at best 2166 MHz RAM, yet when searching, this exact speed of RAM is more expensive than its faster equivalent part.

Might I actually expect a performance hit when using this faster memory?

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    CPU and memory have been decoupled from each other for at least a decade: They run on different speeds, and the CPU uses caches to make up for that, so the CPU will always wait (or do something else) when it has to access memory locations not in the cache. So this has nothing to do with the other question, which is "will particular DDR RAMs work with some other speed than the one they are made for". I don't know, and I guess it depends on the particular RAM. And if it works at a different speed, it might need different voltages. – dirkt Jun 23 at 14:28
  • If CPUs and memory have been decoupled, what is the meaning of the Memory Types: DDR4 1600/1866/2133 in the Intel listing for their products? (In this case the E5 2698 v3) – J Collins Jun 23 at 14:36
  • It's the clock speed of the memory interface (which is different from the clock speed of the CPU). – dirkt Jun 23 at 16:17
  • So are we talking now about three speeds? CPU clock speed, memory interface speed and memory speed itself, each of which can be different? I would imagine the memory speed and memory interface speed are equal. – J Collins Jun 23 at 18:42
  • No, we are talking about two speeds: CPU clock speed and memory speed. The memory interface produces the memory clock, and the memory runs at that speed. BTW, I looked at the serial EEPROM data of my memory chips, and it says "Maximum module speed" in addition to a time base, so there is a good chance that modules with faster rating will also run at slower speeds. – dirkt Jun 23 at 19:18

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