To run a SMP system with multiple CPUs you need to have CPUs that are “identical”, the question is what does “identical” mean. In this case I’m interested in Intel CPUs because SMP motherboards and server systems for Intel CPUs are readily available and affordable. There are people selling matched pairs of CPUs on ebay which tend to be more expensive than randomly buying 2 of the same CPU model, so if you can identify 2 CPUs that are “identical” which are sold separately then you can save some money. Also if you own a two CPU system with only one CPU installed then buying a second CPU to match the first is cheaper and easier than buying two more CPUs and removing a perfectly working CPU.
Intel (R) Xeon (R) E5-2640V4 SR2NZ 2.40GHZ J717B324 (e4) 7758S4100843
Above is a pic of 2 E5-2640v4 CPUs that were in a SMP system I purchased along with a plain ASCII representation of the text on one of them. The bottom code (starting with “77”) is apparently the serial number, one of the two codes above it is what determines how “identical” those CPUs are.
The code on the same line as the nominal clock speed (in this case SR2NZ) is the “spec number” which is sometimes referred to as “sspec” [1].
The line below the sspec and above the serial number has J717B324 which doesn’t have a google hit. I looked at more than 20 pics of E5-2640v4 CPUs on ebay, they all had the code SR2NZ but had different numbers on the line below. I conclude that the number on the line below probably indicates the model AND stepping while SR2NZ just means E5-2640v4 regardless of stepping. As I wasn’t able to find another CPU on ebay with the same number on the line below the sspec I believe that it will be unreasonably difficult to get a match for an existing CPU.
For the purpose of matching CPUs I believe that if the line above the serial number matches then the CPUs can be used together. I am not certain that CPUs with this number slightly mismatching won’t work but I definitely wouldn’t want to spend money on CPUs with this number being different.
smpboot: CPU0: Intel(R) Xeon(R) CPU E5-2699A v4 @ 2.40GHz (family: 0x6, model: 0x4f, stepping: 0x1)
When you boot Linux the kernel identifies the CPU in a manner like the above, the combination of family and model seem to map to one spec number. The combination of family, model, and stepping should be all that’s required to have them work together.
I think that Intel did the wrong thing in not making this clearer. It would have been very easy to print the stepping on the CPU case next to the sspec or the CPU model name. It also wouldn’t have been too hard to make the CPU provide the magic number that is apparently the required match for SMP to the OS. Having the Intel web site provide a mapping of those numbers to steppings of CPUs also shouldn’t be difficult for them.
If anyone knows more about these issues please let me know.
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