lecture27 - Memory 2 - Multi-level Page Table

Memory Management and Virtual Memory

1. Introduction to Memory Management

• Memory management creates an isolated view of memory for each process, preventing interference.

• Each process operates within its own virtual memory space, allowing for greater security and stability.

• The Memory Management Unit (MMU) is responsible for translating virtual addresses to physical addresses using a data structure called a page table. This ensures that a process can access its memory seamlessly, without knowing the physical layout of the system.

2. Virtual Memory and Address Translation

• Each running process is given a unique virtual memory space, allowing them to use the same virtual address without conflict.

• To translate a virtual address to a physical address, a page table is employed.

  • Example: In the SV39 model, a 39-bit virtual address is managed, with the address broken down into an offset (the last 12 bits) and a segment for mapping to physical memory.

3. Page Size and Offset Calculation

• The standard page size in modern systems is 4 kilobytes (4 KB = 2^12 bytes).

• The offset, represented as 12 bits, helps locate specific bytes within that page.

• With 27 bits left, the potential number of entries in a page table amounts to 2^27, which equals 134,217,728 entries.

4. Page Table Entry (PTE)

• Each Page Table Entry (PTE) typically has a minimum size of 44 bits, yielding a common size of 64 bits for ease of computation.

• Page Table Size Calculation: The total size of the page table can be calculated as follows:

  • Number of Entries: 2^27 = 134,217,728

  • Size per Entry: 8 bytes (64 bits)

  • Total Size = 134,217,728 entries x 8 bytes/entry = 1,073,741,824 bytes = 1 GB.

5. Memory Limitations and Issues

• A significant limitation in memory management is the overhead created due to large page table sizes—1 GB for 512 GB of addressable memory.

• This can lead to inefficiencies, especially when many addresses in the space remain unused or unallocated, raising questions about space optimization.

6. Multi-Level Page Table Concept

• A multi-level page table optimizes memory usage by breaking down the page table into smaller, more manageable pieces.

• Goal: Maintain the entirety of the page table within a single physical page size (4 KB).

• This can be achieved by splitting the remaining 27 bits into three sections of 9 bits each, corresponding to three tiers of page tables: top-level, middle-level, and bottom-level.

• A special register known as the Special Attribute Pointer (SAP) points to the top-level page table, streamlining access and management.

7. Physical Address and Index Explanation

• Translating physical addresses through multiple levels means that each address maps back through its respective page tables.

• Each entry in these tables either points to the next page table or ultimately to the physical page in memory.

• Additionally, a linked list structure is a possible implementation to maintain free pages in physical memory, simplifying allocation and deallocation processes.

8. Allocation Efficiency

• Allocation efficiency refers to the resource management practices that maximize memory utilization while minimizing waste.

• Example: In a best-case scenario, where memory allocation is perfectly aligned with process requirements, memory is optimally used. Conversely, the worst-case may lead to fragmentation and inefficient usage.

9. Memory Alignment Issues

• Memory alignment considers data structure sizes and their corresponding padding when stored in memory.

• Example: In a structure containing an int (typically 4 bytes) and a char (1 byte), the compiler may pad the structure to align it to an 8-byte boundary for easier access, leading to extra memory usage due to unused bytes.

10. Address Transition Example

• To illustrate the translation process, consider a virtual address of 0x0000_1234:

  • Breakdown:

    • Offset = 0x234 (last 12 bits)

    • The remaining bits are used to find the corresponding entry in the page table.

• Discussed how address alignment directly influences system performance and addressability.

11. Changes in Page Table Size

• When switching from 8-bit to 16-bit physical addresses, page table sizes require careful consideration, particularly in how the size of each entry increases.

• Illustration: If each entry grows from 4 bytes to 8 bytes, the number of required entries in a multi-level table would also change, requiring recalculation of the overall memory requirement.

12. Summary

• The lecture encompassed a detailed examination of virtual memory management, focusing on address translation processes, the multi-level page table framework, allocation efficiencies, memory alignment issues, and practical examples outlining these crucial concepts.