Inverted Page Table in Operating System

In a computer system, an operating system (OS) manages the resources of the system, including memory, storage, and processing power. One of the critical components of an OS is the memory management system, which manages the allocation and deallocation of memory to different processes.

When a program is executed in a computer system, it requires memory to store its code and data. The memory management system of the operating system is responsible for allocating the memory required by a program and deallocating it when the program is finished executing.

One of the most common techniques used by the memory management system is the use of a page table. A page table is a data structure that maps the logical addresses used by a program to the physical addresses of the memory where the program's code and data are stored.

In this article, we will discuss one particular type of page table known as an inverted page table. We will explain how it works, its advantages, and its limitations.

What is an Inverted Page Table?

An inverted page table (IPT) is a page table that stores information about the physical pages of memory that are allocated to each process instead of storing information about the logical pages used by the process.

In a traditional page table, each entry maps a logical page number to a physical page number. In contrast, an inverted page table maps a physical page number to a process ID and a virtual page number.

Advantages of an Inverted Page Table:

1. Reduced Memory Overhead:

One of the main advantages of an inverted page table is that it reduces the memory overhead required to store the page table. In a traditional page table, the size of the page table grows with the size of the logical address space of the process.

For example, if a process has a logical address space of 4 GB and the page size is 4 KB, the size of the page table will be 1 million entries. If each entry is 4 bytes, the size of the page table will be 4 MB.

In contrast, an inverted page table's size is determined by the size of the physical memory rather than the logical address space of the process. Since the physical memory is typically much smaller than the logical address space, the size of the inverted page table is significantly smaller than that of a traditional page table.

For example, if the physical memory size is 1 GB and the page size is 4 KB, the size of the inverted page table will be 256,000 entries. If each entry is 8 bytes (4 bytes for process ID and 4 bytes for virtual page number), the size of the inverted page table will be 2 MB.

2. Faster Page Table Lookup:

Another advantage of an inverted page table is that it can perform page table lookups faster than a traditional page table. In a traditional page table, the page table entry for a logical page number must be located by traversing the page table.

In contrast, in an inverted page table, the page table entry for a physical page number can be located directly by hashing the physical page number. Since the physical page number is typically smaller than the logical page number, the hash function can be much simpler, and the lookup can be performed faster.

3. Reduced TLB Misses:

A Translation Lookaside Buffer (TLB) is a cache of recently used page table entries. When a process accesses a memory location, the TLB is checked first to see if the corresponding page table entry is present in the TLB.

If the page table entry is present in the TLB, the translation can be performed quickly without accessing the page table. If the page table entry is not present in the TLB, a TLB miss occurs, and the page table must be accessed to retrieve the page table entry.

In a traditional page table, TLB misses occur when a process accesses a different logical page number. In contrast, in an inverted page table, TLB misses occur when a process is switched out and another process is switched in, requiring a new set of page table entries to be loaded into the TLB.

Since the size of the inverted page table is much smaller than the traditional page table, the number of TLB entries required is also much smaller. This means that the TLB can be more effective in reducing TLB misses in an inverted page table.

Limitations of an Inverted Page Table:

1. Complexity:

One of the main limitations of an inverted page table is that it is more complex to implement than a traditional page table. Since an inverted page table must maintain information about which process is using which physical page, the operating system must perform additional bookkeeping to ensure that the page table is updated correctly.

In addition, the inverted page table must handle page faults differently than a traditional page table since it must locate the correct process to handle the page fault.

2. Memory Fragmentation:

Another limitation of an inverted page table is that it can lead to memory fragmentation. Memory fragmentation occurs when there are small amounts of free memory scattered throughout physical memory, making it difficult to allocate large contiguous blocks of memory to a process.

In an inverted page table, physical memory is allocated to processes based on the availability of free physical pages. This can lead to memory fragmentation if physical pages become scattered throughout physical memory.

Conclusion:

An inverted page table is a page table that stores information about the physical pages of memory that are allocated to each process instead of storing information about the logical pages used by the process. It offers several advantages over a traditional page table, including reduced memory overhead, faster page table lookup, and reduced TLB misses.

However, it also has some limitations, including increased complexity and memory fragmentation. Overall, the use of an inverted page table is a design decision that must be weighed against the specific requirements of the system and the trade-offs involved.