Revamp the default page clustering strategy that is used by the page fault
handler. For roughly twenty years, the page fault handler has used the
same basic strategy: Fetch a fixed number of non-resident pages both ahead
and behind the virtual page that was faulted on. Over the years,
alternative strategies have been implemented for optimizing the handling
of random and sequential access patterns, but the only change to the
default strategy has been to increase the number of pages read ahead to 7
and behind to 8.

The problem with the default page clustering strategy becomes apparent
when you look at how it behaves on the code section of an executable or
shared library. (To simplify the following explanation, I'm going to
ignore the read that is performed to obtain the header and assume that no
pages are resident at the start of execution.) Suppose that we have a
code section consisting of 32 pages. Further, suppose that we access
pages 4, 28, and 16 in that order. Under the default page clustering
strategy, we page fault three times and perform three I/O operations,
because the first and second page faults only read a truncated cluster of
12 pages. In contrast, if we access pages 8, 24, and 16 in that order, we
only fault twice and perform two I/O operations, because the first and
second page faults read a full cluster of 16 pages. In general, truncated
clusters are more common than full clusters.

To address this problem, this revision changes the default page clustering
strategy to align the start of the cluster to a page offset within the vm
object that is a multiple of the cluster size. This results in many fewer
truncated clusters. Returning to our example, if we now access pages 4,
28, and 16 in that order, the cluster that is read to satisfy the page
fault on page 28 will now include page 16. So, the access to page 16 will
no longer page fault and perform an I/O operation.

Since the revised default page clustering strategy is typically reading
more pages at a time, we are likely to read a few more pages that are
never accessed. However, for the various programs that we looked at,
including clang, emacs, firefox, and openjdk, the reduction in the number
of page faults and I/O operations far outweighed the increase in the
number of pages that are never accessed. Moreover, the extra resident
pages allowed for many more superpage mappings. For example, if we look
at the execution of clang during a buildworld, the number of (hard) page
faults on the code section drops by 26%, the number of superpage mappings
increases by about 29,000, but the number of never accessed pages only
increases from 30.38% to 33.66%. Finally, this leads to a small but
measureable reduction in execution time.

In collaboration with: Emily Pettigrew <ejp1@rice.edu>
Differential Revision: https://reviews.freebsd.org/D1500
Reviewed by: jhb, kib
MFC after: 6 weeks