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“Virtual” Memory

“Virtual” Memory. How does the OS kernel provide “private” and “shared” memory areas to multiple concurrent processes?. Hardware/Firmware/Software. The PC ‘s memory management system is a cooperative venture involving hardware, software, and firmware

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“Virtual” Memory

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  1. “Virtual” Memory How does the OS kernel provide “private” and “shared” memory areas to multiple concurrent processes?

  2. Hardware/Firmware/Software • The PC ‘s memory management system is a cooperative venture involving hardware, software, and firmware • The hardware and firmware would be the same no matter which Operating System we choose to run (i.e., Linux or Windows) • An operating system utilizes the hardware (in its own unique way) to provide multiple processes with protected memory-spaces

  3. The Linux/x86 scheme • Let’s focus on the particular approach that Linux takes in utilizing the Intel x86 CPU’s memory-addressing capabilities • There are two conceptual levels (because “processor independence” is a goal Linux strives for): much of the Linux ‘mm’ code runs on almost any CPU, yet there’s some code is (unavoidably) specific to the x86

  4. Source-code organization High-level ‘mm’ code is processor-independent (It’s in the ‘/usr/src/linux/mm’ subdirectory) Low-level ‘mm’ code is processor-specific (It’s in the ‘/usr/src/linux/arch/’ subdirectories) i386/mm ppc/mm mips/mm alpha/mm . . .

  5. Our machines have 1GB of RAM 0x3FFFFFFF Main Memory (1 GB) Intel Pentium CPU 0x00000000 system bus NIC VGA FIFO (4 KB) VRAM (32 MB) some devices have RAM, too

  6. Booting up • At power-on, the CPU sees only 1-MB of the physical memory (including the ROMs) • The OS initialization code must “activate” the processor’s “protected-mode” MMU so that all the ‘Extended Memory’ (including device-memory) will become addressable • Finally the OS enables “virtual memory” to allow the illusion of a 4-GB address-space

  7. Kernel memory-mapping 0xFFFFFFFF 0x3FFFFFFF mappings 1-GB 896-MB 896-MB 0xC0000000 0x00000000 Physical memory 4-GB display memory device memory 0x00000000 Virtual Memory

  8. Application memory-mapping kernel space user space stack stack mappings text 3-GB data physical memory data text virtual memory

  9. ‘mm_struct’ • Each application-process uses a different mapping of physical memory regions into the Pentium’s “virtual” address-space, so one process can’t access memory owned by another process (i.e., private regions) • The OS switches these mapping when it suspends one process to resume another • The ‘map’ for each task is in ‘task_struct’

  10. How Linux tracks mappings mm_struct task_struct vm_area_struct vma start, end, etc mm pgd vm_area_struct start, end, etc vm_area_struct start, end, etc mapping-tables mapping-tables mapping-tables vm_area_struct start, end, etc

  11. When a program ‘forks()’ • We already know how new processes get created in the UNIX/Linux environment (i.e., via the ‘fork()’ function or system-call) • A child-process gets constructed using the same code and data as its parent-process • Yet each task’s memory-regions are kept ‘private’ (i.e., accessible to it alone) • So how exactly is this feat achieved?

  12. Similar memory-mappings virtual memory virtual memory physical memory kernel space kernel space stack user space stack user space stack data data stack text data data text text parent-process child-process

  13. Some Linux demos • We have written a Linux kernel module to display some info in a task’s ‘mm_struct’ (using a pseudo-file named ‘/proc/mm’) • We also wrote a program (‘mmfork.cpp’) that forks a child process, so each task can read and display its ‘mm_struct’ info • A user can compare the contents of the mm_struct’s info for the parent and child

  14. More demos • We wrote a Linux module (named ‘vma.c’) that displays information from a task’s list of ‘vm_area_struct’ data-structures • You could write a program, similar to our ‘mmfork.cpp’ demo, that would let a user compare the list of memory-regions which a parent-process owns and the list that its child-process has inherited

  15. In-class exercise • Write an application program that allows a user to compare BOTH the ‘mm_struct’ and the ‘vm_area_struct’ information that belong to a parent-process and to its child • You can use our ‘mm.c’ and ‘vma.c’ Linux kernel modules, and you can imitate the programming ideas we used in ‘mmfork’ • TURN IN printouts of your code and output

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