Add sysctl vm.objects_swap

[kib]

Summary

vm.objects_swap: disable reporting some information

For making the call faster, do not count active/inactive object queues, and do not report vnode info if any (for tmpfs).

See D29754

[kib]

Add the 'yield if hog' patch

[markj]

sys/vm/vm_object.c

2516

Why examine objects with OBJ_ANON set and OBJ_SWAP clear?

[kib]

sys/vm/vm_object.c

2516

Because these are logically swap objects. The fact that the system yet did not swapped anything from their page queue does not imply anything.

Hm, even more, if I skip such objects there, then sysinfo(8) would not be able to reconstruct full shadow chain. So it is not the question of interpretation, but in fact required to do.

[markj]

sys/vm/vm_object.c

2516

I think this deserves a comment, then, since swap_only is not quite accurate. Naively I understand that the new sysctl is used to display information about swap usage, and it is tempting to "optimize" this loop by excluding OBJT_DEFAULT objects since by definition they have never undergone pageouts.

2548

You could avoid calling pgo_getvp calls in vm_object_kvme_type() by passing swap_only ? NULL : &vp instead (with vp initialized to NULL).

[kib]

Optimize vp calculation.
Add detailed comment about new sysctl.