slow directory operation on huge dirs

[Wojciech Puchar]

i have UFS_DIRHASH enabled and enough large dirhash_maxmem
yet when there is heavy I/O on large directory (like dovecot indexing so 
it reads every file) it randomly works fast or slow. Most often slow.
Whole system slows down, and system cpu usage gets above 50% on quad core 
Xeon.

lowering maxvnodes from 500000 to 100000 "fixes" the problem.

what is going on here?

11.3-STABLE from maybe 2 weeks ago.