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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
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