Virtual Memory

Abstract

Virtual memory abstracts away the majority of the complexity incorporated with physical memory to make what appears to be a large, flat main memory

Key Benefits

Process Isolation

• Impossible to modify another process’ memory if unable to address that memory directly
• Accessing invalid or uninitialized pointers may cause segmentation faults but it doesn’t impact the entire system, only the currently executed program.

Code Relocation

• Binary object files are loaded at the same virtual address, making it straightforward for linking and loading tools
• Ensures locality in the virtual address which minimizes problems with memory fragmentation

Hardware Abstraction

• The virtual address space provides a uniform and hardware-independent view of the memory, despite physical memory resources changing when more RAM is installed or a hosted VM configuration is modified

Virtual Address Space Layout

• Assuming each memory location addresses 1 byte. A 32bit processor will address $2^{32}=4,294,967,296$ bytes of memory. This is more commonly known as 4GB of Virtual Address Space
• Linux Virtual Space layouts use (user-space) 3:1 (kernel-space), or 2:2 for 4GB address spaces.
• Raspberry Pi Linux Kernel’s 32bit config specifies 2:2

Diagram:

Allocating memory

• Stack frame (descending stack) - each new call
• Within heap - malloc or new
• Grow the heap – brk/sbrk
• Map new region into memory - mmap

Memory Management Unit (MMU)

Memory Access

CPU Mediated: Non Uniform Memory Access

Non-CPU mediated:

Direct memory access (DMA)

Copies data between devices and memory buffers

Remote dynamic memory access (RDMA)

Enables pages to be transferred rapidly between nodes in LAN Copies memory from a remote buffer to a local buffer

• Used in cloud,e.g. VM migration, distributed storage services