Hi Aneesh, On Fri, Jun 03, 2022 at 07:12:28PM +0530, Aneesh Kumar K.V wrote: > * The current tier initialization code always initializes > each memory-only NUMA node into a lower tier. But a memory-only > NUMA node may have a high performance memory device (e.g. a DRAM > device attached via CXL.mem or a DRAM-backed memory-only node on > a virtual machine) and should be put into a higher tier. I have to disagree with this premise. The CXL.mem bus has different latency and bandwidth characteristics. It's also conceivable that cheaper and slower DRAM is connected to the CXL bus (think recycling DDR4 DIMMS after switching to DDR5). DRAM != DRAM. Our experiments with production workloads show regressions between 15-30% in serviced requests when you don't distinguish toptier DRAM from lower tier DRAM. While it's fixable with manual tuning, your patches would bring reintroduce this regression it seems. Making tiers explicit is a good idea, but can we keep the current default that CPU-less nodes are of a lower tier than ones with CPU? I'm having a hard time imagining where this wouldn't be true... Or why it shouldn't be those esoteric cases that need the manual tuning.
