![1
Introduction to Linux,
for Embedded Engineers
Tutorial on Virtual Memory
Feb. 22, 2007
Tetsuyuki Kobayashi
Aplix Corporation
[translated by ikoma]](https://image.slidesharecdn.com/virtualmemory20070222-en-130610000534-phpapp02/85/Virtual-memory-20070222-en-1-320.jpg)
![28
Example of Memory Space
of a User Process
00101000-0011a000 r-xp 00000000 fd:00 15172739 /lib/ld-2.4.so
0011a000-0011b000 r-xp 00018000 fd:00 15172739 /lib/ld-2.4.so
0011b000-0011c000 rwxp 00019000 fd:00 15172739 /lib/ld-2.4.so
0011e000-0024a000 r-xp 00000000 fd:00 15172740 /lib/libc-2.4.so
0024a000-0024d000 r-xp 0012b000 fd:00 15172740 /lib/libc-2.4.so
0024d000-0024e000 rwxp 0012e000 fd:00 15172740 /lib/libc-2.4.so
0024e000-00251000 rwxp 0024e000 00:00 0
08048000-08049000 r-xp 00000000 fd:00 11666681 /home/koba/lab/loop/a.out
08049000-0804a000 rw-p 00000000 fd:00 11666681 /home/koba/lab/loop/a.out
b7fef000-b7ff1000 rw-p b7fef000 00:00 0
b7fff000-b8000000 rw-p b7fff000 00:00 0
bffeb000-c0000000 rw-p bffeb000 00:00 0 [stack]
cat /proc/<PROCESS_ID>/maps
file nameinode
device
major:minor
Address Range file offset
r: read
w: write
x: execute
s: shared
p: private (copy on write)](https://image.slidesharecdn.com/virtualmemory20070222-en-130610000534-phpapp02/85/Virtual-memory-20070222-en-28-320.jpg)
![37
References
• Linux kernel source code
http://www.kernel.org/
• GNU C library source code
http://www.gnu.org/software/libc/
• “Understanding the Linux Kernel (2nd Edition)”
by Daniel P. Bovet (O’Reilly) [Japanese translation; 3rd
Edition available in English]
• “Linux kernel 2.6 Kaidokushitsu”, by Hirokazu Takahashi
et. al. (SoftBank Creative) [in Japanese]
• Linux man command
• And other search results on web](https://image.slidesharecdn.com/virtualmemory20070222-en-130610000534-phpapp02/85/Virtual-memory-20070222-en-37-320.jpg)
Uploaded byTetsuyuki Kobayashi
Virtual memory 20070222-en
This document serves as a tutorial for embedded engineers on the mechanism of virtual memory in Linux, emphasizing its importance for performance optimization in embedded device projects. It covers fundamental concepts, benefits of virtualization, the architecture of virtual and physical memory, as well as practical applications like demand paging and memory mapping using system calls. The presentation also warns about the potential overhead and costs associated with virtualization, stressing the distinction between virtual and physical memory usage.
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Virtual memory 20070222-en
- 1. 1 Introduction to Linux, forEmbedded Engineers Tutorial on Virtual Memory Feb. 22, 2007 Tetsuyuki Kobayashi Aplix Corporation [translated by ikoma]
- 2. 2 Target Audience ofthis Presentation • People who have been engaged in projects on embedded devices, and who are now using Linux as operating system
- 3. 3 Goals of thisPresentation • To understand the mechanism of virtual memory in Linux, and to make use of it for the current project – Although programs work without understanding the mechanism, it is important to understand the mechanism to extract sufficient performance
- 4. 4 Basic Concepts, Firstof All • Virtual ..., Logical ... – Virtual addresses, logical devices, logical sectors, virtual machines – To handle as if it is ... • Real ..., Physical ... – Real addresses, physical devices, phisical sectors – Itself, as it is
- 5. 5 Virtualization: As if... but ... • As if it is large, but it actually small • As if it is flat, but it actually uneven • As if there are many, but there is actually one • As if exclusively usable, actually shared Virtualization is magic to hide complexity or individual depedency; as it is magic, there is a trick = Mapping between the real and the virtual Translating so that it looks as if ...
- 6. 6 Cost of Virtualization •We do virtualize as its merits are greater than its demerts, but virtualization does not always mean positive results
- 7. 7 Physical Memory andVirtual Memory • Most of ordinary embedded device projects so far have handled only physical memory • Recently, as the size of embedded systems grow, PC-oriented OSes such as Linux and Windows CE are getting widely used; these operating systems provide virtual memory systems • This presentation explains Linux
- 8. 8 Physical Memory • Singlememory space • As each device is implemented with different addresses for ROM, RAM and I/O, programmers should code accordingly ROM RAM I/O I/O I/O
- 9. 9 Virtual Memory • Merits –User programs do not depend on actual memory map (implementation address, implementation size) any more – Can use non-contiguous physical memory fragments as contiguous virtual memory – Memory protection: Can prevent irrelevant memory from being destroyed by bugs • Introducing new concepts – Address translation – Multiple memory spaces – Demand paging
- 10. 10 Conceptual Schema ofVirtual Memory Source: Wikipedia Swapping Area Physical Memory Space Virtual Address Space
- 11. 11 Address Translation ROM RAM I/O I/O I/O address x PhysicalAddr SpaceVirtual Add Space address y MMU Linux kernel controls
- 12. 12 Virtual Memory isonly in CPU ROM RAM I/O I/O I/O address x Physical Addr Space Virtual Addr Space address y MMU CPU Only physical addresses come out of CPU onto address bus. Virtual addresses can not be observed with the logic analyzer
- 13. 13 User Program Handles onlyVirtual Addresses ROM RAM I/O I/O I/O address x Physical Addr Space Virtual Addr Space address y MMU CPU Physical addresses are handled only in kernel mode, i.e. kernel itself and device drivers
- 14. 14 Address Translation withMMU Directory Entry Page Table Entry Physical Addr 011122131 22 Page Directory Page Table Page Page OffsetPage Table IndexDirectory IndexVirtual Addr Page Directory Base Register No page! ... page fault
- 15. 15 TLB • Translation LookasideBuffers • Something like hashtable getting a physical address by a using virtual address as a key • In most address translation, page is found in TLB, so there is no need to access page directory or page table ... Virtual Address Pages Physical Address Pages
- 16. 16 Multiple Memory Spaces ROM I/O I/O PhysicalAddr Space Independent virtual memory spaces per process RAM
- 17. 17 Demand Paging • Mappedper page – Page size is usually 4Kbytes • Two phase execution 1. Virtual memory is allocated(mmap); just registered in management table 2. At actual access, physical momory is allocated for the page As no physcal page is allocated unless the page is accessed Virtual memory size >= Actually required physical memory size
- 18. 18 Example of DemandPaging Behavior Virtual Addr Space Physical Addr Space read access No Corresponding Physical page! Page fault occurs; Transits into kernel mode (1)
- 19. 19 Example of DemandPaging Behavior (cont.) DMA Transfer Mapping Virtual Addr Space Physical Addr Space Kernel loads the data and maps physical address (2)
- 20. 20 Example of DemandPaging Behavior (cont.) Virtual Addr. Space Physical Addr Space Return to user mode; User program can read the data as if nothing happened (but time has elapsed actually) (3) data
- 21. 21 Page Cache Physical AddrSpace Data read from disk are kept on memory as far as space allows. Access tends to be sequential, so several pages are read at a time in advance; Thus disk access does not occur every time in (2)
- 22. 22 Page Out If nophysical memory is available in (2), a page assumed to be least used is released. If the contents of this page is not modified, it is just discarded; otherwise, the page is swapped out onto swap device. Virtual Addr Space Physicial Addr Space swap device Many of embedded Linux does not have a swap device a page to be swapped out
- 23. 23 Page Out (cont.) Arequested page is allocated using area of a page released. This “juggling” enables to larger size of virtual memory than physical memory size actually installed DMA Transfer Virtual Addr Space Physical Addr Space no page
- 24. 24 Page Sharing Virtual AddrSpace Physical Addr Space Process A Process B The same pages of a same file will be shared among more than one processes; for both read-only pages and writable pages
- 25. 25 Copy on Write ProcessA Process B write access read only Page Fault Occurs r/w private If write operation occurs on writable and private page...
- 26. 26 Copy on Write(cont.) Process A Process B write access read only Kernel copies the page and changes the page status to read/write read/write copy
- 27. 27 Memory Spaces ofProcesses 0x00000000 0xffffffff TASK_SIZE process A B C ...user space kernel space Kernel space is shared among processes; kernel space is not allowed to read/write/execute in user mode; user memory spaces are switched when processes switched About 3 GB user memory space per process TASK_SIZE is 0xc0000000 for i386; 0xbf000000 for ARM
- 28. 28 Example of MemorySpace of a User Process 00101000-0011a000 r-xp 00000000 fd:00 15172739 /lib/ld-2.4.so 0011a000-0011b000 r-xp 00018000 fd:00 15172739 /lib/ld-2.4.so 0011b000-0011c000 rwxp 00019000 fd:00 15172739 /lib/ld-2.4.so 0011e000-0024a000 r-xp 00000000 fd:00 15172740 /lib/libc-2.4.so 0024a000-0024d000 r-xp 0012b000 fd:00 15172740 /lib/libc-2.4.so 0024d000-0024e000 rwxp 0012e000 fd:00 15172740 /lib/libc-2.4.so 0024e000-00251000 rwxp 0024e000 00:00 0 08048000-08049000 r-xp 00000000 fd:00 11666681 /home/koba/lab/loop/a.out 08049000-0804a000 rw-p 00000000 fd:00 11666681 /home/koba/lab/loop/a.out b7fef000-b7ff1000 rw-p b7fef000 00:00 0 b7fff000-b8000000 rw-p b7fff000 00:00 0 bffeb000-c0000000 rw-p bffeb000 00:00 0 [stack] cat /proc/<PROCESS_ID>/maps file nameinode device major:minor Address Range file offset r: read w: write x: execute s: shared p: private (copy on write)
- 29. 29 Example of MemorySpace of a User Process (Detail) .... 0011e000-0024a000 r-xp 00000000 fd:00 15172740 /lib/libc-2.4.so Size: 1200 kB Rss: 136 kB Shared_Clean: 136 kB Shared_Dirty: 0 kB Private_Clean: 0 kB Private_Dirty: 0 kB 0024a000-0024d000 r-xp 0012b000 fd:00 15172740 /lib/libc-2.4.so Size: 12 kB Rss: 8 kB Shared_Clean: 0 kB Shared_Dirty: 0 kB Private_Clean: 0 kB Private_Dirty: 8 kB 0024d000-0024e000 rwxp 0012e000 fd:00 15172740 /lib/libc-2.4.so Size: 4 kB Rss: 4 kB Shared_Clean: 0 kB Shared_Dirty: 0 kB Private_Clean: 0 kB Private_Dirty: 4 kB .... cat /proc/<PROCESS_ID>/smaps RSS = Physical Memory Size
- 30. 30 mmap System Call •Map/Unmap files or devices onto memory • Argument prot – PROT_NONE, or OR operation of PROT_EXEC, PROT_READ, and PROT_WRITE • Argument flags – MAP_FIXED, MAP_SHARED, MAP_PRIVATE, MAP_ANONYMOUS, ... #include <sys/mman.h> void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset); int munmap(void *start, sizt_t length);
- 31. 31 mmap tips • Unlessspecified as MAP_FIXED, kernel searches available pages • If MAP_FIXED is specified and it overlaps existing pages, the pages are mumpapped internally – Thus this option is usually not used • File offset must be multiple of page size • Addresses and sizes of mmap and munmap need not be identical
- 32. 32 Usage of mmap(1) • As substitute of malloc for large size – No data copy, such as compaction, occurs – Unlike malloc/free, addr and size at munmap can be different than those at mmap • It is possible to allocate a large chunk with a single mmap, and to release piecemeal with multiple munmaps – In malloc of glibc implementation, mmap is called for a certain size or larger • DEFAULT_MMAP_THRESHOLD = (128*1024)
- 33. 33 Usage of mmap(2) • Fast file access – In system calls read and write, data is internally buffered in physical pages; from there data is copied to array specified by user – Using mmap enables to access page directly, thus number data copies can be reduced – java.nio.MappedByteBuffer in Java1.4
- 34. 34 Usage of mmap(3) • Shared memory among processes – Map the same file as readable/writable and shared from more than one processes – IPC shared memory system calls (shmget, shmat, ...) does above internally
- 35. 35 Usage of mmap(4) • Access to physical memory, I/O ports – By mapping device file /dev/mem, it becomes possible to read/write physical memory space in user mode – To access /dev/mem, root privilege is required
- 36. 36 Summary • Virtual memoryusage and physical memory usage are not same. Physical one matters in practice • Be careful when overhead of virtual memory occurs. – TLB miss – Page fault • Make use of system call mmap
- 37. 37 References • Linux kernelsource code http://www.kernel.org/ • GNU C library source code http://www.gnu.org/software/libc/ • “Understanding the Linux Kernel (2nd Edition)” by Daniel P. Bovet (O’Reilly) [Japanese translation; 3rd Edition available in English] • “Linux kernel 2.6 Kaidokushitsu”, by Hirokazu Takahashi et. al. (SoftBank Creative) [in Japanese] • Linux man command • And other search results on web
- 38. 38 One more thing:hot topics • From CELF BootTimeResources –KernelXIP –ApplicationXIP –(DataReadInPlace) • From CELF MemoryManagementResouces –Huge/large/superpages –Page cache compression