Inverted Page Table in Operating System

Prerequisites – Paging, Page table entries, Segmentation

Most Operating Systems implement a separate page table for each process, i.e. for ‘n’ number of processes running on a Multiprocessing/ Timesharing operating system, there is ‘n’ number of page tables stored in the memory. Sometimes when a process is very large in size and it occupies virtual memory then with the size of the process, its page table size also increases substantially.

Example: A process of size 2 GB with:
Page size = 512 Bytes
Size of page table entry = 4 Bytes, then
Number of pages in the process = 2 GB / 512 B = 222
PageTable Size = 222 * 22 = 224 bytes

Through this example, it can be concluded that for multiple processes running simultaneously in an OS, a considerable part of memory is occupied by page tables only. Operating Systems also incorporate multilevel paging schemes which further increase the space required for storing the page tables and a large amount of memory is invested in storing them. The amount of memory occupied by the page tables can turn out to be a huge overhead and is always unacceptable as main memory is always a scarce resource. Various efforts are made to utilize the memory efficiently and to maintain a good balance in the level of multiprogramming and efficient CPU utilization. 

Inverted Page Table – An alternate approach is to use the Inverted Page Table structure that consists of a one-page table entry for every frame of the main memory. So the number of page table entries in the Inverted Page Table reduces to the number of frames in physical memory and a single page table is used to represent the paging information of all the processes. Through the inverted page table, the overhead of storing an individual page table for every process gets eliminated and only a fixed portion of memory is required to store the paging information of all the processes together. This technique is called inverted paging as the indexing is done with respect to the frame number instead of the logical page number. Each entry in the page table contains the following fields.

• Page number – It specifies the page number range of the logical address.
• Process id – An inverted page table contains the address space information of all the processes in execution. Since two different processes can have a similar set of virtual addresses, it becomes necessary in the Inverted Page Table to store a process Id of each process to identify its address space uniquely. This is done by using the combination of PId and Page Number. So this Process Id acts as an address space identifier and ensures that a virtual page for a particular process is mapped correctly to the corresponding physical frame.
• Control bits – These bits are used to store extra paging-related information. These include the valid bit, dirty bit, reference bits, protection, and locking information bits.
• Chained pointer – It may be possible sometimes that two or more processes share a part of the main memory. In this case, two or more logical pages map to the same Page Table Entry then a chaining pointer is used to map the details of these logical pages to the root page table.

Working – The operation of an inverted page table is shown below. The virtual address generated by the CPU contains the fields and each page table entry contains the other relevant information required in paging related mechanism. When a memory reference takes place, this virtual address is matched by the Memory Management Unit(MMU), the Inverted Page table is searched and the corresponding frame number is obtained. If the match is found at the i^th entry then the physical address of the process is sent as the real address otherwise if no match is found then Segmentation Fault is generated. Note: Number of Entries in Inverted page table = Number of frames in Physical address Space(PAS).

Examples – The Inverted Page table and its variations are implemented in various systems like PowerPC, UltraSPARC, and the IA-64 architecture. An implementation of the Mach operating system on the RT-PC also uses this technique. 

Advantages and Disadvantages:

• Reduced memory space – Inverted Pagetables typically reduce the amount of memory required to store the page tables to a size bound of physical memory. The maximum number of entries could be the number of page frames in the physical memory.
• Longer lookup time – Inverted Page tables are sorted in order of frame number but the memory look-up takes place with respect to the virtual address, so, it usually takes a longer time to find the appropriate entry but often these page tables are implemented using hash data structures for a faster lookup.
• Difficult shared memory implementation – As the Inverted Page Table stores a single entry for each frame, it becomes difficult to implement the shared memory in the page tables. Chaining techniques are used to map more than one virtual address to the entry specified in the order of frame number.
• Optimal and less complex – it is better than simple paging process and have less complexity.

In an operating system that uses virtual memory, an Inverted Page Table (IPT) is a data structure used to map physical memory pages to virtual memory pages. Unlike a traditional Page Table, which is a per-process data structure, an IPT is a system-wide data structure that contains an entry for each physical page in memory.

Each entry in the IPT contains the virtual address(es) that map to that physical page, as well as other information such as the process ID of the process that owns the virtual address(es), the access permissions for the page, and other page attributes. This information is used by the operating system to manage the allocation and deallocation of physical memory pages, as well as to ensure that processes are accessing memory only in a safe and controlled manner.

The use of an IPT can provide several benefits over a traditional Page Table, including:

1. Reduced memory overhead: Because the IPT is a system-wide data structure, it requires less memory than a Page Table for each individual process. This can be important in systems with limited memory resources.
2. Improved cache performance: Because the IPT is smaller than a Page Table, it can be more easily stored in the CPU cache, which can improve the performance of memory access operations.
3. Simplified page swapping: When a process needs to be swapped out of memory, the IPT can be used to quickly identify all the physical pages that are associated with the process. This can simplify the process of swapping pages and reduce the overall overhead of memory management.

However, there are also some potential disadvantages to using an IPT, such as increased overhead for lookups and the potential for contention between processes for physical memory pages. As with any memory management technique, the choice of whether to use an IPT or a Page Table depends on the specific requirements and constraints of the operating system and hardware platform being used.

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