What does this equation mean?

That capacity is not free. Sun and colleagues built the first comprehensive evaluation of real, shipping CXL memory devices — as opposed to the emulated-via-remote-NUMA-node substitute most earlier research used — across three commercial devices on a current-generation Intel Xeon platform, and their results are a useful correction to intuition built on emulation: true CXL memory can show up to 26 percent lower latency and 3 to 66 percent higher bandwidth efficiency than emulated CXL memory did in prior studies, meaning some of the pessimism in the earlier literature was measuring the wrong thing [ 6 ] . Their paper is also explicit about where the physical trade comes from: a CXL memory device connects over PCIe rather than the DDR bus, at roughly three times fewer pins than a DDR5 interface, in exchange for a much longer link latency — on the order of 40 nanoseconds for a PCIe 4.0 hop versus under 1 nanosecond for a native DDR4 link [ 6 ] . That is the pin-versus-latency trade the earlier equation predicts: CXL keeps W narrow and serial, and pays in access time instead.