tom almost runs xv6 well.  xv6 runs into, what looks like, some APIC
issues.

as detected by mtrace.

mapbench now has only one real cache line of unexpected sharing:
"name": "tlbflush_req+0x0",

use virtual addresses further away from each other in mapbench,
to avoid contending for radix_node cache lines.

This code is inefficient, it doesn't guarantee to find an unmapped
area if one exists, and it only works for crange.

in terms of performance on real hardware..  need to think more.

to avoid lost tlbflush IPIs.

..allocfd might still cause bottlenecks as threads race to allocate
FD slots..