Explain the page replacement algorithms - FIFO, LRU, and Optimal

- Paging is used by most of the computer operating systems for the purpose of virtual memory management. 

- Whenever a page fault occurs, some pages are swapped in and swapped out. Who decides which pages are to be replaced and how? 

- This purpose is served by the page replacement algorithms. 

- Page replacement algorithms only decide which pages are to be page out or written to the disk when a page of the memory has to be allocated. 

- Paging takes place only upon the occurrence of a page fault.

- In such situations a free cannot suffice because either one is not available or because the number of available pages is less than threshold. 

- If a previously paged out page is reference again, then it has to be read in from the disk again. 

- But for doing this, the operating system has to wait for the completion of the input/ output operation. 

- The quality of a page replacement algorithm is denoted by the time it takes for a page in. 

- The lesser it is, the better the algorithm is. 

- The information about the access to page as provided by the hardware is studied by the page replacement algorithm and then it decides which pages should be replaced so that the number of page faults can be minimized. 

In this article we shall see about some of the page replacement algorithms.

FIFO (first – in, first – out): 

- This one being the simplest of all the page replacement algorithms has the lowest overhead and works by book – keeping in place of the OS. 

- All pages are stored in a queue in the memory by the operating system.

- The ones that have recently arrived are kept at the back while the old ones stand at the front end of the queue.

- While making replacement, the oldest page is selected and replaced. 

- Even though this replacement algorithm is cheap as well as intuitive, practically it does not perform well. 

- Therefore, it is rarely used in its original form. 

- VAX/ VMS operating systems make use of the FIFO replacement algorithm after making some modifications. 

- If a limited number of entries are skipped you get a partial second chance. 

Least recently used (LRU): 

- This replacement algorithm bears resemblance in name with the NRU. 

- However, difference is there which is that this algorithm follows that the page usage is tracked for a certain period of time. 

- It is based on the idea that the pages being used many times in current set of instructions will be used heavily in the next set of the instructions also. 

- Near optimal performance is provided by LRU algorithm in the theory however in practical it is quite expensive to be implemented. 

- For reducing the costs of the implementation of this algorithm, few implementation methods have been devised. 

- Out of these the linked list method proves to be the costliest. 

- This algorithm is so expensive because it involves moving the items about the memory which is indeed a very time consuming task. 

- There is another method that requires hardware support.

Optimal page replacement algorithm: 

- This is also known as the clairvoyant replacement algorithm. 

- The page that has to be swapped in, it is placed in place of the page which according to the operating system will be used in future after a very long time. - In practical, this algorithm is impossible to be implemented in case of the general purpose OS because approximate time when a page will be used is difficult to be predicted. 

Explain the methods for free space management? – Part 2

- Managing the free space is easy only when the space that has to be managed is divided in to units of fixed size. 

- If this is the case, the list of these fixed size units can be kept. 

- The first entry can be returned if it is requested by the client. 

- Managing free space gets difficult when the space to be managed consists of units of variable sizes. 

- This is the case with the memory allocation library at the user level. 

- This is also the case in the physical memory where the segmentation is used for the implementation of the virtual memory. 

- In such cases, external fragmentation is the main problem. 

- This leads to the splitting up of the disk space in to pieces of variable size. 

- The next coming requests might fail because of unavailability of contiguous free space. 

- For example, the request might fail even if 20 bytes are available and the request requires only 15 bytes because this 20 bytes space is non-contiguous. 

- Thus, the main problem is how the free space should be managed while satisfying the variable sized variables. 

How these strategies can manage free space while at the same time keeping the fragmentation in control.

Low level Mechanisms: 

- Most of the allocator use some common mechanisms as these: coalescing and splitting.

- Here, the free list consists of a set of elements describing about all the free spaces available in the heap. 

- Once a pointer to a space is handed over to the program, the determination of the pointers to this space becomes somewhat difficult. 

- These pointers are stored either in the registers or in the variables at some point of execution. 

- However, this is not the case of garbage collected and strongly typed languages as a measure for enabling compaction for combating fragmentation. - Suppose the program makes a request for a single byte of memory.

- In such a case the action performed by the allocator is called splitting. 

- Here a free memory chunk is searched for and split in to two. 

- The first one is returned to the calling request and the second one stays in the list itself. 

- This approach is used in the allocators where small requests are made requesting space that is of size smaller than the chunk. 

- Most of the allocators use a corollary mechanism called the coalescing of the free space. 

- Suppose a small heap is given and an application calls a function to obtain some memory. 

- This function returns the space available in the middle of the heap. 

- All these strategies are based up on simple policies. 

- An ideal allocator is the one that both minimizes fragmentation and is fast. 

- But since it is possible for the stream of free requests and allocation to be arbitrary, any strategy would go wrong if wrong inputs are given. 

- Thus, the best approach cannot be described. 

There are 4 major approaches in this regard:

1. Best fit: 

- Simplest and first searches the list for free chunks of memory that might be bigger than the size requested. 

- The smallest one in the searched ones is returned and this is known as the best fitting chunk or smallest fitting too. 

- One pass is enough for finding the appropriate block to be returned.

2. Worst fit: 

- This one is just the opposite of the best fit. 

- It looks for the largest chunk and returns the amount requested while keeping the remaining memory space.

3. First fit: 

- This one also looks for the first bigger block and out of it allocates the amount requested by the thread.

4. Next fit: 

- Here, one extra pointer is kept at the location where the last search was done. 

Explain the methods for free space management? – Part 1

- For efficient working of the programs and the entire operating system, it is important that the memory of the system should be managed. 

- When the files and programs are allocated memory space, some free space is left in the storage area. 

- It is required that these free spaces must be managed properly. 

- Since there is a limitation to the disk space, this same space has to be used again and again after deleting and creating new files. 

- A free space list is maintained by the operating system for keeping the track of the available free space. 

- All the free disk spaces are listed in free space list. 

- For the creation of a new file this free space list is searched in order to get the amount of space needed and then if the space is available, it is allocated to the file to be created. 

- In the case of deletion, after deleting the file, its space is added to the list of free spaces.

Methods for Free Space Management

There are 4 methods for the management of free space namely:

- Bit vector

- Linked list

- Grouping

- Counting

What is Bit Vector?

- Quite a many times, the free space list about which we mentioned above, is implemented as the bit vector (also known as the bitmap). 

- Here, 1 bit is used for representing each block. 

- If a particular block has been allocated to some file or program, its representative bit is set to 0 and when the block is available, the bit is set to one.

- Consider an example, suppose the following disk blocks are free and rest are allocated: 1, 2, 4, 5, 6, 7, 9, 10, 12, 13, 14, 18, 19, 21, 26, 27, 28. 

- Then for this allocation we have the following free – space bit map:

01101111011011100011010000111…

- This method of free space management is relatively simple and has good efficiency. 

- This method is known for its efficiency to locate the n consecutive free blocks or the first free block available in the storage area. 

- But this method can be inefficient if it is not kept in the main memory of the system. 

- Also, when required occasionally for the recovery needs, this map can also be written to the disk. 

- Keeping such maps in the physical memory is an easy thing if the system has a small memory but this is not always possible in the case of the systems with larger memories.

What is a Linked list?

- In this method, all the free spaces are linked together and the first block in this linked list is assigned a pointer which is stored in the cache memory. 

- Similarly, the pointer to the second block is stored in the first block.

What is Grouping?

- This method is a modified version of the free list approach and it stores the addresses of the all the free blocks in the first block that is free. 

- Here, actually the first n- 1 blocks only are free and the address of another n free blocks is stored in the last block. 

- This lets the system to find the addresses of a number of free blocks which is not possible in the case where the approach being used is the linked list approach.

What is Counting?

- This approach takes advantage of the simultaneous allocation or freeing of the contiguous blocks through clustering or by using contiguous allocation algorithm. 

- Thus, it requires only to keep the address of the first free block and the rest of the blocks follow it. 

Explain the various Disk Allocation methods? – Part 2

In this article we discuss about the non-contiguous disk allocation methods i.e., the linked allocation and the indexed allocation. 

What is a Linked Allocation?

- In linked allocation a single file might be stored all over the disk and these scattered parts are linked to each other just like a linked list. 

- Few bytes in the memory block are used for storing the address of the following linked block.

- This type of allocation has two major advantages mentioned below:

1. Simplicity and
2. Non – requirement of disk compaction

- Since the nature of the allocation method is non-contiguous, it does not lead to any external fragmentation of the disk space.

- And since all the memory blocks have been linked to each other, a memory block available anywhere in the memory can be used for satisfying the requests made by the processes. 

- Declaring the file size for the linked allocation is not required during its creation. 

- There are no issues even if the file continues to grow as long as free blocks are available and since the blocks can always be linked up. 

- As a consequence of all this, the need for disk compaction is eliminated. 

Disadvantages of Linked Allocation

But there are disadvantages of using linked allocation. They are:

Direct access to the disk blocks becomes slow: 

For finding a particular block of the file, the search has to begin at the starting point of the file and the successive pointers have to be followed till the destination block is reached.

Space required by the pointers: 

- Suppose out of 512 words of a memory block, 2 are required for storing the pointers, then we have 39 percent of the total disk being used by the pointers rather than for data. 

- This adds to the space requirement of the file blocks.

Reliability: 

- Because of all the blocks being linked via the pointers, even if one pointer gets damaged or wrong, the successive blocks can become inaccessible. 

- This problem is quite common and thus most of the operating systems avoid this problem by keeping redundant copies of these pointers in a special file. 

- The basic idea here is to keep the list of the pointers in the physical memory of the system. 

- This also allows for the faster access to the disk blocks.

What is Indexed allocation?

- The linked allocation method does not provide support for the direct access and this problem is solved by the indexed allocation method. 

- In this method, all the pointers are placed over an index. 

- Thus, these pointers together form the index block. 

- The address of this address block is then stored in the directory. 

- The pointer at the nth number in the index block points to the nth block of the associated file. 

- The purpose of the index blocks is somewhat similar to that of the page map tables; however both of these are implemented in a different way.
- First level index is used for searching the index of second level and the second one is used for searching the third one and the process may continue till the fourth level. 

- But in most of the cases the indexes of the first two levels are sufficient. 

- With this method, second level index blocks (128 x 128) can be easily addressed and files of size of up to 8mb can be supported. 

- Assuming the same thing, files of size up to 1 gb can be addressed.

Advantages and Disadvantage of Indexed Allocation

- The major advantage of this allocation technique is that it does not give rise to external fragmentation and offers a high level of efficiency in random accessing. 

- Also, using this technique mapping can be done around the disk blocks that are known to be bad. 

-Bit mapping can be used for indexing the free space. 

The biggest disadvantage of this allocation technique is the large number disk accesses required for the retrieval of the address of the destination block in the memory. 

Explain the various Disk Allocation methods? – Part 1

Management of space of the secondary data storage devices is one of the most necessary functions of the file system. This includes tracking which blocks of memory or disk are being allocated to the files and which blocks are free to be allocated. 

There are two main problems faced by the system during allocation of the space to different files. Firstly, the access to the files has to be fast and secondly, the disk space has to be effectively utilized. Both of these combine to form the big problem of disk management. These two problems are more common with the physical memory of the system. 

However, the secondary storage of the system also introduces 2 additional problems which are long disk access time and blocks of larger size to deal with. In spite of all these problems, there are considerations that are common for both the storage such as the non – contiguous and contiguous space allocation. 

Types of Disk Allocation Methods

The following are the 3 widely used allocation techniques:

Ø  Contiguous

Ø  Linked

Ø  Indexed

- The linked and the indexed allocation techniques fall under the category of the non-contiguous space allocation. 

- All the methods have their own pros and cons.

- It is in the term of blocks that all the input and output operations are carried out on the disk. 

- Software is responsible for converting from the logical record to physical storage blocks.

Contiguous Allocation: 

- This allocation technique assigns only the contiguous memory blocks to the files. 

- The size of the memory required is mentioned by the user in advance for the holding the file. 

- The file is then created only if that much amount of memory is available otherwise not. 

- It is the advantage of the contiguous memory allocation technique that all the successive file records are saved adjacent to each other physically. 

- This causes an increase on the disk access time of the files. 

- This can be concluded from the fact that if the files are scattered all about the disk, then it takes a lot more time to access them. 

- Accessing files when they have been organized in a proper order is quite easy.

- To access the files sequentially, the system uses the address of the last memory block and if required moves to the next one. 

- Both direct and sequential accesses are supported by the contiguous memory allocation technique. 

- The problem with this allocation method is that every time a new contiguous space cannot be found for the new files if a majority of the memory is already in use and if the file size is larger than the available memory.  

Non–Contiguous Memory Allocation: 

- It happens a lot of time that the files grow or shrink in size with usage and time. 

- Therefore, it becomes difficult to determine the exact space required for the files since the users do not have any advance knowledge about the growing size of their files. 

- This is why the systems using the contiguous memory allocation techniques are being replaced by the ones with the non-contiguous storage allocation which is more practical and dynamic in nature. 

- The linked allocation technique mentioned above in the article, is a disk – based implementation of the linked list. 

- Each file is considered to be a list of the disk blocks linked to each other. 

- It does not matter whether the blocks are scattered about the disk. 

- In each block, few bytes are reserved for storing the address of the next block in chain.

- The pointer to the first and the last block of the file is stored in the directory. 

What are the various Desk Scheduling methods?

About Disk Scheduling

The I/O system has got the following layers:

1. User processes: The functions of this layer including making I/O calls, formatting the I/O and spooling.
2. Device independent software: Functions are naming, blocking, protection, allocating and buffering.
3. Device drivers: Functions include setting up the device registers and checking their status.
4. Interrupt handlers: These perform the function of waking up the I/O drivers up on the completion of the I/O.
5. Hardware: Performing the I/O operations.

- Disk drives can be pictured as large 1 – D array consisting of logical blocks that are smallest unit of transfer.  

- These blocks are mapped in to the disk sectors in a sequential manner. 

- Mapping is done in the same manner. 

- The responsibility of using the hardware efficiently is the duty of the operating system for the disk drives for increasing the speed of access and bandwidth of the disk. 

Algorithms for Scheduling Disk Requests

There are several algorithms existing for the scheduling of the disk requests:

Ø  SSTF: 

- In this method the request having the minimum seek time is selected from the present head position. 

- This method is a modification of the SJF (shortest job first) scheduling and therefore contains some possibility of process starvation.

Ø  SCAN: 

- From one end of the disk, the disk arm starts and continues in the direction of the other end, serving to the requests till the opposite end. 

- At this end the head is reversed and the process continues. 

- This is sometimes called as the elevator algorithm.

Ø  C – SCAN: 

- A better algorithm then the previous one. 

- This one offers a more uniform waiting time than the previous one. 

- The movement of the head is from one end to another while it services the requests encountered along the way. 

- However, the difference is that when it comes to the other it straightaway goes to the beginning without heeding to any of the requests in the way and then again starts. 

- The cylinders are treated as the circular list wrapped around last and the first cylinder.

Ø  C – Look: 

- This is a modified version of the C – SCAN. 

- Here the arm or the head travels only up to the last request rather than going till the far end. 

- Then immediately the direction is reversed and the process continues.

- For disk scheduling it is important that the method be selected as per the requirements only. 

- The first one is the most commonly used and appeals to the needs naturally. 

- For a system where often there is a heavy load on the disk, the SCAN and C- SCAN methods can help. 

- The number as well as the kind of requests affects the performance in a number of ways.

- On the other hand, the file – allocation method influences the requests for the disk services. 

- These algorithms have to be written as an individual module of the OS so that if required it can be replaced with a different one easily. 

- As a default algorithm, the LOOK or the SSTF is the most reasonable choice. 

Ways to attach to a disk

There are two ways of attaching the disk:

Ø  Network attached: This attachment is made via a network. This is called the network attached storage. All such connected storage devices together form the storage area network.

Ø  Host attached: This attachment is made via the I/O port.

All these disk scheduling methods are for the optimization of the secondary storage access and for making the whole system efficient. 

Explain the various File Access methods?

One of the most important functions of the mainframe operating system is the access methods that make it possible for you to access the data from external devices such as the tape or disk. 

What are access methods?

- Access methods are very useful in providing an API for transferring the data from one device to another.

- Another best thing about this API was that it worked as the device driver for the operating systems on non-mainframe computers. 

- There have been a lot of reasons behind the introduction of the access methods. 

- A special program had to be written for the I/O channel and there has to be a processor entirely dedicated to controlling the access to the peripheral storage device as well as data transfer from and to the physical memory. 

- Special instructions constitute these channel programs and are known as the CCWs or the channel command words.

- To write such programs, very detailed knowledge is required regarding the characteristics of the hardware. 

Benefits of File Access Methods

There are 3 major benefits of the file access methods:

Ø  Ease of programming: The programmer does not have to deal with the procedures of the specific devices, recovery tactics and error detection. A program designed to process a particular thing will do it no matter where the data has been stored.

Ø  Ease of hardware replacement: A program cannot be altered by the programmer during the migration of data from older to newer model of the storage device provided the same access methods are supported by the new model.

Ø  Ease in sharing the data set access: The access methods can be trusted for managing the multiple accesses to the same file. At the same it ensures the security of the system and data integrity.

Some File/Storage Access Methods

Ø  Basic direct access method (BDAM)

Ø  Basic sequential access method (BSAM)

Ø  Queued sequential access method (QSAM)

Ø  Basic partitioned access method (BPAM)

Ø  Indexed sequential access method (ISAM)

Ø  Virtual storage access method (VSAM)

Ø  OAM (object access method)

- For dealing with the records of a data set both the types of access i.e., the queued and the basic are suitable. 

- The queued access methods are an improvement of the basic file access methods. 

- Read ahead scheme and internal blocking of data is well supported by these methods. 

- This allowed combining the multiple records in to one unit, thus increasing the performance. 

- In sequential methods, it is assumed that there’s only a sequential way for processing the records which is just the opposite of the direct access methods. 

- There are devices like the magnetic tape that only enforce the sequential access 

- Sequential access can be used for writing a data set and then later the direct manner can be used for processing it.

Today we have access methods that are network-oriented such as the following:

Ø  Basic telecommunications access method or BTAM

Ø  Queued tele – processing access method or QTAM

Ø  Telecommunications access method or TCAM

Ø  Virtual telecommunications access method or VTAM

- The term access method was used by the IMS or the IBM information management system for referring to the methods for manipulation of the database records. 

- The access methods used by them are:

Ø  GSAM or generalized sequential access method

Ø  HDAM or hierarchical direct access method

Ø  HIDAM or hierarchical indexed direct access method

Ø  HISAM or hierarchical indexed sequential access method

Ø  HSAM or hierarchical sequential access method

Ø  PHDAM or partitioned hierarchical direct access method

Ø  PHIDAM or partitioned hierarchical indexed direct access