Memory management in Linux
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A brief overview of the memory management subsystem in the Linux kernel.
![What is memory? ,[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Memory management in Linux
- 1. Memory management in Linux Raghu Udiyar Technical Support Engineer Red Hat
- 3. Can a disk be considered memory? Yes, in a way
- 5. Latency
- 6. Volatile
- 8. CPU cache (L1, L2..) - 3 – 14 cycles
- 9. RAM (main memory) - ~250 cycles
- 10. Disk - ~40 million cycles
- 12. These are then taken and divided into page frames of 4Kb
- 13. E.g. 4Gb = ~1 million pages
- 15. Every process has a virtual view of the memory
- 16. A process will think it is the only task running on the system
- 17. This virtual memory is then mapped to physical memory using page tables
- 18. Virtual Memory vi bash 2GB Physical Memory Virtual memory
- 19. Virtual Memory vi bash 2GB Physical Memory Virtual memory Page Table
- 21. User and Kernel address space Kernel User Kernel User 32bit 64bit 3Gb 1GB 256TB 256TB
- 23. malloc(1G) = Ok
- 24. memset(1G) = Ok
- 25. malloc(4GB) = Ok
- 27. Kernel almost always grants Virtual memory
- 28. Page fault occurs only when that memory is used
- 30. In such cases the “OOM killer” is invoked
- 31. It will try to free memory by killing a task based on heuristics
- 34. All i/o in Linux goes through the page cache Write-back (delayed) Clean/Dirty pages Page cache Process Disk
- 36. # dd if=/dev/zero of=512M-file count=512000 bs=1k
- 37. # time cat 512M-file &> /dev/null
- 38. # echo 1 > /proc/sys/vm/drop_caches
- 39. # time cat 512M-file &> /dev/null
- 41. Program text - read only
- 42. File backed - clean or dirty pages
- 43. Anonymous pages – Not file-backed, e.g stack and heap
- 44. Only anonymous memory is paged out to swap area
- 46. Considerably slow - ~40 million cycles
- 47. Swapping is a misnomer, pages paged out/in is more appropriate
- 48. Only anonymous pages are paged out
- 50. Tools - vmstat
- 51. Tools - ps
- 53. (10mins default)
- 54. Check “man sar” for details on all switches
- 56. #sar -f sa05 -tq
- 57. #sar -f sa05 -tr
- 58. #sar -f sa05 -tW
- 62. Check : https://access.redhat.com/kb/docs/DOC-52712
- 65. Swap usage doesn't necessarily indicate a problem
- 66. Instead, check #sar -tW for problems
- 68. In most case, this is not advised, and arises from misunderstanding of how the page cache works
- 69. Controlling page cache size : https://access.redhat.com/kb/docs/DOC-41749
- 71. Some confuse this with swap in/out
- 72. These are just the number of sectors read in or written to disk.
- 73. See : https://access.redhat.com/kb/docs/DOC-61288
- 75. Check : https://access.redhat.com/kb/docs/DOC-52474
- 76. Thank you Questions?