pv list lock is the main bottleneck during poudriere -j 104 and
pmap_remove_pages is the most impactful consumer. It frees chunks with the lock
held even though it plays no role in correctness. Moreover chunks are often
freed in groups, sample counts during buildkernel (0-sized frees removed):
value ------------- Distribution ------------- count
0 | 0
1 | 8
2 |@@@@@@@ 19329
4 |@@@@@@@@@@@@@@@@@@@@@@ 58517
8 | 1085
16 | 71
32 |@@@@@@@@@@ 24919
64 | 899
128 | 7
256 | 2
512 | 0
Thus:
1. batch freeing
2. move it past unlocking pv list
Reviewed by: alc (previous version), markj (previous version), kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21832
4093e719b4