The Linux kernel is monolithic: it means that every hardware driver runs in kernel memory space. Every time you add a driver, you add stuff to the kernel. Moreover, for performance reasons, several user-space elements make their way into the kernel and increase its bloat even further.

Moreover, Linux has historically been developed for x86 platforms, and is tightly linked to the architecture, going as far as using hardware interrupts in code (essentially, mixing x86 assembly in the much more generic and portable C code).
….
According to kernel documentation, SLUB has been written to replace SLAB on multicore systems: SLAB is faster than SLUB, no question about it, but on multicore system it is also heavier on the RAM. In very extreme cases, SLAB can end up using more than a gigabyte of RAM, which negates its performance advantages by causing paging and data bus contention. SLUB solves this problem by being heavier on the CPU but using much less RAM – in these extreme cases, SLUB performs better ‘live’ than SLAB. Since it also isn’t THAT costlier CPU-wise and machines get more cores all the time (an Intel quad-core with Hyper-threading will count as an 8-core, and waste around 17 Mb of RAM – say goodbye to your CPU cache), it’s become the default.

Full Story