vm_object: use lookup_range in vm_object_page_clean_flush

[dougm]

Summary

In vm_object_page_clean_flush and vm_pageout_cluster, replace the loop that uses vm_radix_iter_next to visit consecutive pages with a call to
vm_radix_iter_lookup_range, then a loop to check the pages.

Test Plan

I tested by running several buildworlds while I counted function calls and cycles spent per call (not counting the time in vm_pageout_flush).

Original:

Calls    Cycles/call
27929    2974.9316839127787
27929    2983.8554549035052
27928    2993.4640146089946

Modified:

Calls    Cycles/call
27929    2863.995273729815
27928    2908.8322114007447
27930    2899.870032223416

[markj]

sys/vm/vm_object.c

1143–1144

From what I can see, pctrie_iter_lookup_range() doesn't update the iterator index. Is that correct? If so, how do we know what "prev" is relative to?

[dougm]

sys/vm/vm_object.c

1143–1144

That is correct.

"prev" is relative to where the cursor pointed when the function was called, which is the same place that "next" is relative to now - the pindex of the page passed into the function.

BTW, I hope to replace the "prev" parts with a backward-walking version of lookup_range, but it' s too much to put in this patch I think, and it needs more work.

[dougm]

Limit the changes to just vm_object_page_clean_flush. Provide more meaningful performance results.