defer inactive processing

[kmacy]

Summary

Don't immediately enqueue pages to inactive list, instead place on a list protected by the page lock then move pages to inactive when scanning

Test Plan

stress2, buildworld, netflix deployment

[mlaier]

In general, I'd like to see a little stricter fixup. Either wakeup the pagedaemon once the global deferred count crosses a threshold, or directly call fixup in the enqueue.
Other than that, this looks good to me.

sys/vm/vm_page.c

2121–2122

Since there is no lock order issue here, I'd like to see either a "forced" option to _fixup() to really collect if we are falling behind our target in the pagedaemon, or a secondary threshold that forces the consolidation (regardless of lock contention).

sys/vm/vm_pageout.c

1358–1362

Read v_inactive_deferred_count instead of looking at "merged"?

[kmacy]

mlaier's comments were addressed by my latest commit: 09fe122