PCIe is still rather slow. A single-packet transaction on PCIe costs 120ns. The lack of IP is the reason why P8/CAPI stuck to PCIe. In the original design, CAP

PCIe is still rather slow. A single-packet transaction on PCIe costs 120ns. The lack of IP is the reason why P8/CAPI stuck to PCIe. In the original design, CAPI was to simply reuse the PCI link layer, not the transaction layer. With a cache-coherent system based on PCIe, especially when the coherency layer is set at L3 like on POWER8, you are looking at ~500ns latency for a single cache line. This kind of latency is just too much for many applications.