EPYC CCX configuration question

[ssan]

Hello,
Maybe someone of you can help me on this:

I saw that on an H13SSL Mainboard allows switching off cores at BIOS level, e.g. you can run a 64 core CPU consisting of 8(CCX) x 8(cores per CCX) = 64 cores with reduced cores per CCX, e.g. as 8x5 or 8x3.

Now let's consider this example:
There is an EPYC 9275F with 8x3 configuration. Which difference would it make (performance-wise) when I compare this one to an EPYC 9555 (8x8 cores) with 5 cores per CCX switched off, so it effectively also behaves as 8x3?

I'm aware that the "crippled" 9555 wouldn't (necessarily) meet the TDP/boost-frequency/... specs of the 9275F, but how much would the results differ in reality? Would switching off cores at BIOS level also reduce L3 cache maybe or imply other drawbacks that I am missing?

This is quite hypothetical, but it might help me decide which CPU best suits my needs.
I kindly appreciate your thoughts!

 

[RolloZ170]

when I compare this one to an EPYC 9555 (8x8 cores) with 5 cores per CCX switched off, so it effectively also behaves as 8x3?

if the PPT/TDP and fused clocks are same yes.
most 8x8 have lower clocks, and if they don't go higher even if TDP space left you will have less performance.

 

[TrevorH]

If you run linux and have libvirt installed for virtualization support then you can run `virsh capabilities` and the info it shows is pretty good at working out which processor number is in which CCX.

 

[ssan]

Thanks for our replies! Given the current CPU prices, that don't increase that much with the number of cores, I'd rather opt for a CPU with high core count and large cache and would reduce the number of working cores if that benefits me.