Generally his skepticism was driven by 2 things around the time the microkernel debate happened: - Microkernels need more development effort for equal function

Generally his skepticism was driven by 2 things around the time the microkernel debate happened: - Microkernels need more development effort for equal functionality, something that was a constraint when he was doing Linux on his own. - The context switching penalties on 386s were ginormous - it's much more reasonable now, I'd even say it's not much more penalizing than thread switching. - We live in a multi-master world with IOMMU - tons of devices have virtual address spaces on their own, with most of them doing DMA transfers.