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==Hardware==
* PAE, page tables, PTEs, TLB -- explain ''FIXME''
==Huge Pages==
==Huge Pages==
Making pages larger means fewer TLB misses for a given TLB size (due to more pages being supportable in the same amount of memory, due to narrower page identifiers), large mapping/releasing operations will be faster (due to fewer page table entries needing to be handled), and less memory is devoted to page table entries for a given amount of memory being indexed. The downside is possible wastage of main memory (due to pages not being used as completely). A 2002 paper from Navarro et al at Rice proposed transparent operating system support: "[http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.3.2392 Transparent Operating System Support for Superpages]". FreeBSD and Solaris support large pages transparently, while applications must explicitly take advantage of them on Linux (through at least 2.6.30).
Making pages larger means fewer TLB misses for a given TLB size (due to more pages being supportable in the same amount of memory, due to narrower page identifiers), large mapping/releasing operations will be faster (due to fewer page table entries needing to be handled), and less memory is devoted to page table entries for a given amount of memory being indexed. The downside is possible wastage of main memory (due to pages not being used as completely). A 2002 paper from Navarro et al at Rice proposed transparent operating system support: "[http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.3.2392 Transparent Operating System Support for Superpages]". FreeBSD and Solaris support large pages transparently, while applications must explicitly take advantage of them on Linux (through at least 2.6.30).

Revision as of 05:43, 19 June 2009

Hardware

  • PAE, page tables, PTEs, TLB -- explain FIXME

Huge Pages

Making pages larger means fewer TLB misses for a given TLB size (due to more pages being supportable in the same amount of memory, due to narrower page identifiers), large mapping/releasing operations will be faster (due to fewer page table entries needing to be handled), and less memory is devoted to page table entries for a given amount of memory being indexed. The downside is possible wastage of main memory (due to pages not being used as completely). A 2002 paper from Navarro et al at Rice proposed transparent operating system support: "Transparent Operating System Support for Superpages". FreeBSD and Solaris support large pages transparently, while applications must explicitly take advantage of them on Linux (through at least 2.6.30).

Linux

  • They were a 2003 Kernel Summit topic, after seeing first introduction in Linux 2.5.36 (LinuxGazette primer article)
  • Rohit Seth provided the first explicit large page support to applications as covered in this LWN article
    • alloc_hugepages, free_hugepages, get_large_pages(2) and shared_large_pages(2) were present in kernels 2.5.36-2.5.54
  • hugetlbfs and assorted infrastructure replaced these. Mel Gorman's Linux MM wiki has a good page on hugetlbfs.
  • Val Henson wrote a good 2006 KHB article in LWN on transparent largepage support

Applications

  • MySQL can use hugetlbfs via the large-pages option
  • kvm can use hugetlbfs with the --mem-path option since kvm-62, released in late 2008

Page Clustering

Page clustering (implemented by William Lee Irwin for Linux in 2003, and not to be confused with page-granularity swap-out clustering). There's good coverage in this KernelTrap article. This is essentially huge pages without hardware support, and therefore with some overhead and no improvements in TLB-relative performance. It was written up in Irwin's 2003 OLS paper, "A 2.5 Page Clustering Implementation".