On _LP64, pad struct vm_amap to 128 bytes and use the additional space to
hold the arrays for tiny amaps which are common. Carefully size the array
allocations to avoid false sharing, and for smaller amaps try to share
allocated cache lines.
- Eliminate most contention due to amap_list: maintain the list in the pool
cache constructor / destructor like we do for struct file. Cache the
mutexes we allocate here.
- Don't do PR_WAITOK mutex allocations when NOWAIT has been specified.
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