K 10
svn:author
V 7
glebius
K 8
svn:date
V 27
2018-06-08T00:15:08.406633Z
K 7
svn:log
V 1194
UMA memory debugging enabled with INVARIANTS consists of two things:
trashing freed memory and checking that allocated memory is properly
trashed, and also of keeping a bitset of freed items. Trashing/checking
creates a lot of CPU cache poisoning, while keeping debugging bitsets
consistent creates a lot of contention on UMA zone lock(s). The performance
difference between INVARIANTS kernel and normal one is mostly attributed
to UMA debugging, rather than to all KASSERT checks in the kernel.

Add loader tunable vm.debug.divisor that allows either to turn off UMA
debugging completely, or turn it on only for a fraction of allocations,
while still running all KASSERTs in kernel. That allows to run INVARIANTS
kernels in production environments without reducing load by orders of
magnitude, but still doing useful extra checks.

Default value is 1, meaning debug every allocation. Value of 0 would
disable UMA debugging completely. Values above 1 enable debugging only
for every N-th item. It isn't possible to strictly follow the number,
but still amount of debugging is reduced roughly by (N-1)/N percent.

Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D15199

END