Do Page Tables and PCBs Swap Out with a Process?

Let's understand the Process Swapping, Kernel Persistence, and Why Essential Structures Stay in RAM

Introduction

In modern operating systems, memory management is a critical component that ensures efficient utilization of physical memory. A common question arises: When a process is swapped out of main memory, are its page table and PCB (Process Control Block) also swapped out?

This article explores the mechanics of process swapping, the role of page tables, and the PCB.

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Table of Contents

1. Process Swapping vs. Paging

2. Page Tables: Structure and Persistence

3. The PCB and Its Role

4. Kernel vs. User-Space Memory

5. Code Examples and System Analysis

6. Process Swapping Overview

7. Conclusion

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1. Process Swapping vs. Paging

Swapping refers to moving an entire process from RAM to secondary storage (swap space) to free memory. Paging, however, involves moving individual pages of a process. Modern systems primarily use paging, but the terms are often used interchangeably.

Key Takeaway: Full process swapping is rare today; instead, individual pages are swapped out as needed.

2. Page Tables: Structure and Persistence

Page tables map virtual addresses to physical addresses. Each process has its hierarchical page table (e.g., in x86-64, a 4-level hierarchy).

Are Page Tables Swapped Out?

• No. Page tables are kernel data structures stored in wired memory (non-swappable). They remain in RAM even if the process is swapped out.

• Why? The kernel needs to track the process’s memory state. If page tables were swapped, the OS couldn’t restore the process efficiently.

Example: Linux Page Table Handling

typedef struct {
    unsigned long pte; // here this is very simplified for this blog
} pte_t;

// Kernel function to free process pages (simplified)
void swap_out_process(struct task_struct *process) {
    free_user_pages(process->mm); // Swaps user pages, not page tables
}

3. The PCB and Its Role

The PCB (Process Control Block) stores process metadata (e.g., state, registers, memory maps).

Is the PCB Swapped Out?

• No. The PCB resides in kernel memory (part of the task_struct in Linux). The OS needs it to manage the process, even when swapped out.

Linux PCB Example:

struct task_struct {
    volatile long state;         // Process state
    struct mm_struct *mm;        // Memory management info
    // same way for scheduler, file descriptors, etc.
};

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4. Kernel vs. User-Space Memory

• Kernel Space: Reserved for OS code/data. Never swapped.

• User Space: Process-specific data (code, heap, stack). Swappable.

5. Code Examples and System Analysis

Example 1: Monitoring Swap Usage

Use vmstat to observe swap activity:

vmstat 1

Output:

procs -----------memory---------- ---swap--
 r  b   swpd   free   buff  cache   si   so
 0  0   1024 100000   2000  30000    0    0

• swpd: Total swap used.

• si/so: Swap-ins/outs per second.

Example 2: Inspecting Process Memory

Use pmap to view a process’s memory map:

pmap -X <PID>

Output:

Address           Kbytes     RSS    Dirty Mode  Mapping
...
00007f8e3b6e8000   18064       0       0   [swp]

• [swp] indicates swapped-out pages.

6. Process Swapping Overview

Explanation:

• Page Tables and PCB stay in kernel memory during swapping.

• Only user-space pages move to swap space.

7. Conclusion

• Page Tables: Never swapped out; part of kernel memory.

• PCB: Never swapped out; critical for process management.

• User-Space Pages: Swapped as needed, but kernel structures persist.

Modern OSes prioritize efficiency by retaining essential structures in memory, ensuring swift process restoration and system stability.