Transparent Hugepages in RHEL 6
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Introduction to linux memory management and advantages of using huge pages for certain applications, and final wrap-up with benefits of Transparent hugepages available in RHEL 6.
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Transparent Hugepages in RHEL 6
- 1. Transparent Hugepages in RHEL 6 Raghu Udiyar 22 July 2011
- 3. Merged into upstream kernel 2.6.38
- 4. Available in RHEL 6.0 on GA
- 6. Memory is divided into pages of equal size Transparent Huge Pages Pages Memory
- 8. For other arch's : ia64 = 8KiB, ppc64 = 4,8 and 64KiB 4KiB Page x86
- 10. Programs never work directly with physical memory
- 12. Programs always allocate virtual memory
- 13. A virtual page is called simply a Page
- 15. Page : Memory Management Process 2 Process 1
- 17. These page tables are stored in memory
- 18. Looking up these page tables is a processes called a
- 19. Page Walk
- 22. Hardware assisted but it is still expensive
- 23. Typically, takes 10-100 cpu clock cycles
- 24. The obvious optimization is to cache the results
- 26. If TLB hit, return address
- 28. Look up takes 0.5 – 1 clock cycl e
- 30. But they are very fast
- 31. Typically, a TLB can only hold 8 to 1024 entries
- 32. Plus, TLB cache has to be flushed on every context switch
- 33. Therefore, TLB is a scarce resource and every attempt has to be made to maximize its usage
- 35. 4KiB page size produces a large overhead to manage these pages
- 36. Example :
- 41. Ans : The address of the root table is stored in a special CR3 register.
- 44. Therefore, Increases TLB hit ratio
- 45. Decrease in number of pages to manage
- 46. Previous Oracle example :
- 48. Page Table size? ~33MiB (1000 processes)
- 51. To verify :
- 52. # cat /proc/meminfo | grep Huge
- 56. Hugepagesize: 2048 kB
- 57. A reboot may be required if not enough contiguous physical pages are available.
- 58. Refer to : https://access.redhat.com/kb/docs/DOC-56553
- 60. Not available for normal applications
- 61. Explicit application support is required
- 62. Cannot be swapped to disk – locked in memory
- 63. Only really used by large database applications, such as Oracle and Sybase
- 68. THP is enabled by default for all applications
- 69. Transparent to the application – No configuration required
- 70. All applications benefit from using Huge pages
- 71. Up to 10% performance improvement
- 72. Therefore, we successfully have a mixed or hybrid page size scenario Transparent Huge Pages
- 74. Example : 2MiB memory requests
- 75. THP are swappable, by breaking a hugepage to 4KiB pages
- 76. A kernel thread Khugepaged will scan running processes and attempt to switch normal pages with Hugepages
- 77. THP tries to maximizes Huge page use
- 78. Kernel itself maps it's memory using hugepages
- 80. Page cache and shared memory still in progress
- 81. To check THP usage on RHEL 6 or F14 and above :
- 82. # grep AnonHugePages /proc/meminfo
- 83. AnonHugePages: 632832 kB
- 86. Thank You Raghu Udiyar 22 July 2011