With chiplets being a thing these days, I guess this is really about where in the overall system is the best place to put the package boundary and socket/pins? And then another approach to that same question would be what apple is doing with in-package ddr5 (which I think I heard amd is copying with a custom line for... Microsoft / azure I think it was?).
My understanding is that one factor pushing hyperscalers towards dual-socket was the cost of the network fabric - for the longest time, having two CPU sockets per NIC / per leaf switch port was the overall system price/performance sweet spot for many workloads. More sockets required more expensive CPUs, while single-socket servers need twice as many NICs and twice as many top-of-rack switchports.<p>With newer/faster ethernet standards you still need twice as many NICs but you can often split the lanes coming out of a switch chip and use a Y cable.
EPYC brings plenty of NUMA complexity in a single socket unfortunately. If you just want to solve system performance riddles then one socket is plenty. I seem to recall that Facebook publicly announced that they switched their web server systems to 1 socket more than 8 years ago. Since that time Netflix has written several times about how they carefully keep the sides of a 2S server from interfering with each other and I always wondered why they bother, why they don't just saw the system in half and save themselves the trouble.
Not mentioned in this is the issue of memory scaling.<p>DRAM price per GB has been roughly flat for well over a decade - consumer prices hit $4/GB in 2011, and have fluctuated around there ever since - most of the drop in real cost since then has been due to declining value of that $4. Prices for large enterprise/hyper scalers are probably similar, as it’s a low-margin commodity market.<p>Two sockets gets you more memory channels and more DINNS, but as memory price causes the RAM/CPU ratio to drop, and single-channel bandwidth increases with DDR5, that becomes less important.<p>Of course that’s one of those things you can’t really say to customers, kind of like “you don’t really need 250hp in a passenger sedan”.