Chapter 17. Memory Management

The main task of computers is to execute programs (even usually several programs running simultaneously). These programs and their data must be in the main memory of the computer during the execution.

Since the main memory is usually too small to store all these data and programs, modern computer systems have a secondary storage too for the provisional storage of the data and programs.

In this chapter the basic algorithms of memory management will be covered. In Section 17.1 static and dynamic partitioning, while in Section 17.2 the most popular paging methods will be discussed.

In Section 17.3 the most famous anomaly of the history of operating systems— the stunning features of FIFO page changing algorithm, interleaved memory and processing algorithms with lists—will be analysed.

Finally in Section 17.4 the discussion of the optimal and approximation algorithms for the optimisation problem in which there are files with given size to be stored on the least number of disks can be found.

17.1. 17.1 Partitioning

A simple way of sharing the memory between programs is to divide the whole address space into slices, and assign such a slice to every process. These slices are called partitions. The solution does not require any special hardware support, the only thing needed is that programs should be ready to be loaded to different memory addresses, i.e., they should be relocatable. This must be required since it cannot be guaranteed that a program always gets into the same partition, because the total size of the executable programs is usually much more than the size of the whole memory. Furthermore, we cannot determine which programs can run simultaneously and which not, for processes are generally independent of each other, and in many cases their owners are different users. Therefore, it is also possible that the same program is executed by different users at the same time, and different instances work with different data, which can therefore not be stored in the same part of the memory. Relocation can be easily performed if the linker does not work with absolute but with relative memory addresses, which means it does not use exact addresses in the memory but a base address and an offset. This method is called base addressing, where the initial address is stored in the so called base register. Most processors know this addressing method, therefore, the program will not be slower than in the case using absolute addresses. By using base addressing it can also be avoided that—due to an error or the intentional behaviour of a user—the program reads or modifies the data of other programs stored at lower addresses of the memory. If the solution is extended by another register, the so called limit register which stores the biggest allowed offset, i.e. the size of the partition, then it can be assured that the program cannot access other programs stored at higher memory addresses either.

Partitioning was often used in mainframe computer operating systems before. Most of the modern operating systems, however, use virtual memory management which requires special hardware support.

Partitioning as a memory sharing method is not only applicable in operating systems. When writing a program in a language close to machine code, it can happen that different data structures with variable size—which are created and cancelled dynamically—have to be placed into a continuous memory space. These data structures are similar to processes, with the exception that security problems like addressing outside their own area do not have to be dealt with. Therefore, most of the algorithms listed below with some minor modifications can be useful for application development as well.

Basically, there are two ways of dividing the address space into partitions. One of them divides the initially empty memory area into slices, the number and size of which is predetermined at the beginning, and try to place the processes and other data structures continuously into them, or remove them from the partitions if they are not needed any more. These are called fixed partitions, since both their place and size have been fixed previously, when starting the operating system or the application. The other method is to allocate slices from the free parts of the memory to the newly created processes and data structures continuously, and to deallocate the slices again when those end. This solution is called dynamic partitioning, since partitions are created and destroyed dynamically. Both methods have got advantages as well as disadvantages, and their implementations require totally different algorithms. These will be discussed in the following.

17.1.1. 17.1.1 Fixed partitions

Using fixed partitions the division of the address space is fixed at the beginning, and cannot be changed later while the system is up. In the case of operating systems the operator defines the partition table which is activated at next reboot. Before execution of the first application, the address space is already partitioned. In the case of applications partitioning has to be done before creation of the first data structure in the designated memory space. After that data structures of different sizes can be placed into these partitions.

In the following we examine only the case of operating systems, while we leave to the Reader the rewriting of the problem and the algorithms according to given applications, since these can differ significantly depending on the kind of the applications.

The partitioning of the address space must be done after examination of the sizes and number of possible processes running on the system. Obviously, there is a maximum size, and programs exceeding it cannot be executed. The size of the largest partition corresponds to this maximum size. To reach the optimal partitioning, often statistic surveys have to be carried out, and the sizes of the partitions have to be modified according to these statistics before restarting the system next time. We do not discuss the implementation of this solution now.

Since there are a constant number () of partitions, their data can be stored in one or more arrays with constant lengths. We do not deal with the particular place of the partitions on this level of abstraction either; we suppose that they are stored in a constant array as well. When placing a process in a partition, we store the index of that partition in the process header instead of its starting address. However, concrete implementation can differ from this method, of course. The sizes of the partitions are stored in array . Our processes are numbered from to . The array keeps track of the processes executed in the individual partitions, while its inverse, array stores the places where individual processes are executed. A process is either running, or waiting for a partition. This information is stored in Boolean array : if process number is waiting, then TRUE , else FALSE . The space requirements of the processes are different. Array stores the minimum sizes of partitions required to execute the individual processes.

Having partitions of different sizes and processes with different space requirements, we obviously would not like small processes to be placed into large partitions, while smaller partitions are empty, in which larger processes do not fit. Therefore, our goal is to assign each partition to a process fitting into it in a way that there is no larger process that would fit into it as well. This is ensured by the following algorithm:

Largest-Fit( )

  1  
                        FOR
                      
                      
                     
                        TO
                      
                        2    
                        DO
                      
                     
                        IF
                      
                        3       
                        THEN
                      
                     
                        Load-Largest(
                        
                        )
                      
                  

Finding the largest process the whose space requirement is not larger than a particular size is a simple conditional maximum search. If we cannot find any processes meeting the requirements, we must leave the the partition empty.

Load-Largest( )

  1     2     3  
                        FOR
                      
                      
                     
                        TO
                      
                        4    
                        DO
                      
                     
                        IF
                      
                      and  and    5       
                        THEN
                      
                        6             7  
                        IF
                      
                        8    
                        THEN
                      
                        9         10       
                     
                        FALSE
                      
                  

The basic criteria of the correctness of all the algorithms loading the processes into the partitions is that they should not load a process into a partition which does not fit. This requirement is fulfilled by the above algorithm, since it can be derived from the conditional maximum search theorem exactly with the mentioned condition.

Another essential criterion is that it should not load more than one processes into the same partition, and also should not load one single process into more partitions simultaneously. The first case can be excluded, because we call the Load-Largest algorithm only for the partitions for which and if we load a process into partition number , then we give the index of the loaded process as a value, which is a positive integer. The second case can be proved similarly: the condition of the conditional maximum search excludes the processes for which FALSE , and if the process number is loaded into one of the partitions, then the value of is set to FALSE .

However, the fact that the algorithm does not load a process into a partition where it does not fit, does not load more then one processes into the same partition, or one single process into more partitions simultaneously is insufficient. These requirements are fulfilled even by an empty algorithm. Therefore, we have to require something more: namely that it should not leave a partition empty, if there is a process that would fit into it. To ensure this, we need an invariant, which holds during the whole loop, and at the end of the loop it implies our new requirement. Let this invariant be the following: after examination of partitions, there is no positive , for which , and for which there is a positive , such as TRUE , and .

  • Initialisation: At the beginning of the algorithm we have examined partitions, so there is not any positive .

  • Maintenance: If the invariant holds for at the beginning of the loop, first we have to check whether it holds for the same at the end of the loop as well. It is obvious, since the first partitions are not modified when examining the -th one, and for the processes they contain FALSE , which does not satisfy the condition of the conditional maximum search in the Load-Largest algorithm. The invariant holds for the -th partition at the end of the loop as well, because if there is a process which fulfills the condition, the conditional maximum search certainly finds it, since the condition of our conditional maximum search corresponds to the requirement of our invariant set on each partition.

  • Termination: Since the loop traverses a fixed interval by one, it will certainly stop. Since the loop body is executed exactly as many times as the number of the partitions, after the end of the loop there is no positive , for which , and for which there is a positive , such that TRUE and , which means that we did not fail to fill any partitions that could be assigned to a process fitting into it.

The loop in rows 1–3 of the Largest-Fit algorithm is always executed in its entirety, so the loop body is executed times. The loop body performs a conditional maximum search on the empty partitions – or on partitions for which . Since the condition in row 4 of the Load-Largest algorithm has to be evaluated for each , the conditional maximum search runs in . Although the loading algorithm will not be called for partitions for which , as far as running time is concerned, in the worst case even all the partitions might be empty, therefore the time complexity of our algorithm is .

Unfortunately, the fact that the algorithm fills all the empty partitions with waiting processes fitting into them whenever possible is not always sufficient. A very usual requirement is that the execution of every process should be started within a determined time limit. The above algorithm does not ensure it, even if there is an upper limit for the execution time of the processes. The problem is that whenever the algorithm is executed, there might always be new processes that prevent the ones waiting for long from execution. This is shown in the following example.

Example 17.1 Suppose that we have two partitions with sizes of 5 kB and 10 kB. We also have two processes with space requirements of 8 kB and 9 kB. The execution time of both processes is 2 seconds. But at the end of the first second a new process appears with space requirement of 9 kB and execution time of 2 seconds again, and the same happens in every 2 seconds, i. e., in the third, fifth, etc. second. If we have a look at our algorithm, we can see that it always has to choose between two processes, and the one with space requirement of 9 kB will always be the winner. The other one with 8 kB will never get into the memory, although there is no other partition into which it would fit.

To be able to fulfill this new requirement mentioned above, we have to slightly modify our algorithm: the long waiting processes must be preferred over all the other processes, even if their space requirement is smaller than that of the others. Our new algorithm will process all the partitions, just like the previous one.

Largest-or-Long-Waiting-Fit( )

  1  
                        FOR
                      
                      
                     
                        TO
                      
                        2    
                        DO
                      
                     
                        IF
                      
                        3       
                        THEN
                      
                     
                        Load-Largest-or-Long-Waiting(
                        
                        )
                      
                  

However, this time we keep track on the waiting time of each process. Since the algorithm is only executed when one or more partitions become free, we cannot examine the concrete time, but the number of cases where the process would have fit into a partition but we have chosen another process to fill it. To implement this, the conditional maximum search algorithm has to be modified: operations have to be performed also on items that meet the requirement (they are waiting for memory and they would fit), but they are not the largest ones among those. This operation is a simple increment of the value of a counter. We assume that the value of the counter is 0 when the process starts. The condition of the search has to be modified as well: if the value of the counter of a process is too high, (i. e., higher than a certain ), and it is higher than the value of the counter of the process with the largest space requirement found so far, then we replace it with this new process. The pseudo code of the algorithm is the following:

Load-Largest-or-Long-Waiting( )

  1     2     3  
                        FOR
                      
                      
                     
                        TO
                      
                        4    
                        DO
                      
                     
                        IF
                      
                      and    5       
                        THEN
                      
                     
                        IF
                      ( and ) or                   6          
                        THEN
                      
                        7                8                9          
                        ELSE
                      
                       10  
                        IF
                      
                       11    
                        THEN
                      
                       12         13       
                     
                        FALSE
                      
                  

The fact that the algorithm does not place multiple processes into the same partition can be proved the same way as for the previous algorithm, since the outer loop and the condition of the branch has not been changed. To prove the other two criteria (namely that a process will be placed neither into more then one partitions, nor into a partition into which it does not fit), we have to see that the condition of the conditional maximum search algorithm has been modified in a way that this property stays. It is easy to see that the condition has been split into two parts, so the first part corresponds exactly to our requirement, and if it is not satisfied, the algorithm certainly does not place the process into the partition. The property that there are no partitions left empty also stays, since the condition for choosing a process has not been restricted, but extended. Therefore, if the previous algorithm found all the processes that met the requirements, the new one finds them as well. Only the order of the processes fulfilling the criteria has been altered. The time complexity of the loops has not changed either, just like the condition, according to which the inner loop has to be executed. So the time complexity of the algorithm is the same as in the original case.

We have to examine whether the algorithm satisfies the condition that a process can wait for memory only for a given time, if we suppose that there is some upper limit for the execution time of the processes (otherwise the problem is insoluble, since all the partitions might be taken by an infinite loop). Furthermore, let us suppose that the system is not overloaded, i. e., we can find a upper estimation for the number of the waiting processes in every instant of time. Knowing both limits it is easy to see that in the worst case to get assigned to a given partition a process has to wait for the processes with higher counters than its own one (at most many), and at most many processes larger than itself. Therefore, it is indeed possible to give an upper limit for the maximum waiting time for memory in the worst case: it is .

Example 17.2 In our previous example the process with space requirement of 8 kB has to wait for other processes, all of which lasts for 2 seconds, i. e., the process with space requirement of 8 kB has to wait exactly for 2k seconds to get into the partition with size of 10 kB.

In our algorithms so far the absolute space requirement of the processes served as the basis of their priorities. However this method is not fair: if there is a partition, into which two processes would fit, and neither of them fits into a smaller partition, then the difference in their size does not matter, since sooner or later also the smaller one has to be placed into the same, or into another, but not smaller partition. Therefore, instead of the absolute space requirement, the size of the smallest partition into which the given process fits should be taken into consideration when determining the priorities. Furthermore, if the partitions are increasingly ordered according to their sizes, then the index of the smallest partition in this ordered list is the priority of the process. It is called the rank of the process. The following algorithm calculates the ranks of all the processes.

Calculate-Rank( )

  1  
                     
                        Sort(
                        
                        )
                        2  
                        FOR
                      
                      
                     
                        TO
                      
                        3    
                        DO
                      
                        4          5          6       
                        WHILE
                      
                      or    7          
                        DO
                      
                     
                        IF
                      
                        8             
                        THEN
                      
                        9             
                        ELSE
                      
                       10           
                  

It is easy to see that this algorithm first orders the partitions increasingly according to their sizes, and then calculates the rank for each process. However, this has to be done only at the beginning, or when a new process comes. In the latter case the inner loop has to be executed only for the new processes. Ordering of the partitions does not have to be performed again, since the partitions do not change. The only thing that must be calculated is the smallest partition the process fits into. This can be solved by a logarithmic search, an algorithm whose correctness is proved. The time complexity of the rank calculation is easy to determine: the ordering of the partition takes steps, while the logarithmic search , which has to be executed for processes. Therefore the total number of steps is .

After calculating the ranks we have to do the same as before, but for ranks instead of space requirements.

Long-Waiting-or-Not-Fit-Smaller( )

  1  
                        FOR
                      
                      
                     
                        TO
                      
                        2    
                        DO
                      
                     
                        IF
                      
                        3       
                        THEN
                      
                     
                        Load-Long-Waiting-or-Not-Smaller(
                        
                        )
                      
                  

In the loading algorithm, the only difference is that the conditional maximum search has to be executed not on array , but on array :

Load-Long-Waiting-or-Not-Smaller( )

  1     2     3  
                        FOR
                      
                      
                     
                        TO
                      
                        4    
                        DO
                      
                     
                        IF
                      
                      and    5       
                        THEN
                      
                     
                        IF
                      ( and ) or                   6          
                        THEN
                      
                        7                8                9          
                        ELSE
                      
                       10  
                        IF
                      
                       11    
                        THEN
                      
                       12         13       
                     
                        FALSE
                      
                  

The correctness of the algorithm follows from the previous version of the algorithm and the algorithm calculating the rank. The time complexity is the same as that of the previous versions.

Example 17.3 Having a look at the previous example it can be seen that both the processes with space requirement of 8 kB and 9 kB can fit only into the partition with size of 10 kB, and cannot fit into the 5 kB one. Therefore their ranks will be the same (it will be two), so they will be loaded into the memory in the order of their arrival, which means that the 8 kB one will be among the first two.

17.1.2. 17.1.2 Dynamic partitions

Dynamic partitioning works in a totally different way from the fixed one. Using this method we do not search for the suitable processes for every empty partition, but search for suitable memory space for every waiting process, and there we create partitions dynamically. This section is restricted to the terminology of operating systems as well, but of course, the algorithms can be rewritten to solve problems connected at the application level as well.

If all the processes would finish at the same time, there would not be any problems, since the empty memory space could be filled up from the bottom to the top continuously. Unfortunately, however, the situation is more complicated in the practice, as processes can differ significantly from each other, so their execution time is not the same either. Therefore, the allocated memory area will not always be contiguous, but there might be free partitions between the busy ones. Since copying within the memory is an extremely expensive operation, in practice it is not effective to collect the reserved partitions into the bottom of the memory. Collecting the partitions often cannot even be carried out due to the complicated relative addressing methods often used. Therefore, the free area on which the new processes have to be placed is not contiguous. It is obvious, that every new process must be assigned to the beginning of a free partition, but the question is, which of the many free partitions is the most suitable.

Partitions are the simplest to store in a linked list. Naturally, many other, maybe more efficient data structures could be found, but this is sufficient for the presentation of the algorithms listed below. The address of the first element of linked list is stored in . The beginning of the partition at address is stored in , its size in , and the process assigned to it is stored in variable . If the identifier of a process is , then it is an empty one, otherwise it is a allocated. In the linked list the address of the next partition is .

To create a partition of appropriate size dynamically, first we have to divide a free partition, which is at least as big as needed into two parts. This is done by the next algorithm.

Split-Partition( )

  1     2     3     4     5   
                  

In contrast to the algorithms connected to the method of fixed partitions, where processes were chosen to partitions, here we use a reverse approach. Here we inspect the list of the processes, and try to find to each waiting process a free partition into which it fits. If we found one, we cut the required part off from the beginning of the partition, and allocate it to the process by storing its beginning address in the process header. If there is no such free partition, then the process remains in the waiting list.

Place( )

  1  
                        FOR
                      
                      
                     
                        TO
                      
                        2    
                        DO
                      
                     
                        IF
                      
                     
                     
                        TRUE
                        3       
                        THEN
                      
                     
                     
                        Fit(
                        
                        )
                     
                  

The in the pseudo code is to be replaced by one of the words First , Next , Best , Limited-Best , Worst or Limited-Worst .

There are several possibilities for choosing the suitable free partition. The more simple idea is to go through the list of the partitions from the beginning until we find the first free partition into which it fits. This can easily be solved using linear searching.

First-Fit( )

  1     2  
                        WHILE
                      
                     
                     
                        TRUE
                      and 
                     
                        NIL
                        3    
                        DO
                      
                     
                        IF
                      
                      and    4       
                        THEN
                      
                     
                        Split-Partition(
                        
                        )
                        5             6             7          
                     
                        FALSE
                        8     
                  

To prove the correctness of the algorithm several facts have to be examined. First, we should not load a process into a partition into which it does not fit. This is guaranteed by the linear search theorem, since this criteria is part of the property predicate.

Similarly to the fixed partitioning, the most essential criteria of correctness is that one single process should not be placed into multiple partitions simultaneously, and at most one processes may be placed into one partition. The proof of this criteria is word by word the same as the one stated at fixed partitions. The only difference is that instead of the conditional maximum search the linear search must be used.

Of course, these conditions are not sufficient in this case either, since they are fulfilled by even the empty algorithm. We also need prove that the algorithm finds a place for every process that fits into any of the partitions. For this we need an invariant again: after examining processes, there is no positive , for which , and for which there is a partition, such that , and .

  • Initialisation: At the beginning of the algorithm we have examined many partitions, so there is no positive .

  • Maintenance: If the invariant holds for at the beginning of the loop, first we have to check whether it holds for the same at the end of the loop as well. It is obvious, since the first processes are not modified when examining the -th one, and for the partitions containing them , which does not satisfy the predicate of the linear search in the First-Fit algorithm. The invariant statement holds for the -th process at the end of the loop as well, since if there is a free memory slice which fulfills the condition, the linear search certainly finds it, because the condition of our linear search corresponds to the requirement of our invariant set on each partition.

  • Termination: Since the loop traverses a fixed interval by one, it certainly stops. Since the loop body is executed exactly as many times as the number of the processes, after the loop has finished, it holds that there is no positive , for which , and for which there is a partition, such that , and , which means that we did not keep any processes fitting into any of the partitions waiting.

Again, the time complexity of the algorithm can be calculated easily. We examine all the processes in any case. If, for instance, all the processes are waiting, and the partitions are all reserved, the algorithm runs in .

However, when calculating the time complexity, we failed to take some important points of view into consideration. One of them is that is not constant, but executing the algorithm again and again it probably increases, since the processes are independent of each other, start and end in different instances of time, and their sizes can differ considerably. Therefore, we split a partition into two more often than we merge two neighbouring ones. This phenomenon is called fragmentation the memory. Hence, the number of steps in the worst case is growing continuously when running the algorithm several times. Furthermore, linear search divides always the first partition with appropriate size into two, so after a while there will be a lot of small partitions at the beginning of the memory area, unusable for most processes. Therefore the average execution time will grow as well. A solution for the latter problem is to not always start searching at the beginning of the list of the partitions, but from the second half of the partition split last time. When reaching the end of the list, we can continue at the beginning until finding the first suitable partition, or reaching the starting partition again. This means we traverse the list of the partitions cyclically.

Next-Fit( )

  1  
                        IF
                      
                     
                     
                        NIL
                        2    
                        THEN
                      
                        3    
                        ELSE
                      
                        4  
                        WHILE
                      
                      and    5    
                        DO
                      
                     
                        IF
                      
                     
                     
                        NIL
                        6       
                        THEN
                      
                        7    
                        IF
                      
                      and    8       
                        THEN
                      
                     
                        Split-Partition(
                        
                        )
                        9            10            11          
                     
                        FALSE
                       12            13     
                  

The proof of the correctness of the algorithm is basically the same as that of the First-Fit , as well as its time complexity. Practically, there is a linear search in the inner loop again, only the interval is always rotated in the end. However, this algorithm traverses the list of the free areas evenly, so does not fragment the beginning of the list. As a consequence, the average execution time is expected to be smaller than that of the First-Fit .

If the only thing to be examined about each partition is whether a process fits into it, then it can easily happen that we cut off large partitions for small processes, so that there would not be partitions with appropriate sizes for the later arriving larger processes. Splitting unnecessarily large partitions can be avoided by assigning each process to the smallest possible partition into which it fits.

Best-Fit( )

  1     2  
                     
                        NIL
                        3     4  
                        WHILE
                      
                     
                     
                        NIL
                        5    
                        DO
                      
                     
                        IF
                      
                      and  and    6       
                        THEN
                      
                        7             8       9  
                        IF
                      
                     
                     
                        NIL
                       10    
                        THEN
                      
                     
                        Split-Partition(
                        
                        )
                       11         12         13       
                     
                        FALSE
                      
                  

All the criteria of the correctness of the algorithm can be proved in the same way as previously. The only difference from the First-Fit is that conditional minimum search is applied instead of linear search. It is also obvious that this algorithm will not split a partition larger than minimally required.

However, it is not always efficient to place each process into the smallest space into which it fits. It is because the remaining part of the partition is often too small, unsuitable for most of the processes. It is disadvantageous for two reasons. On the one hand, these partitions are still on the list of free partitions, so they are examined again and again whenever searching for a place for a process. On the other hand, many small partitions together compose a large area that is useless, since it is not contiguous. Therefore, we have to somehow avoid the creation of too small free partitions. The meaning of too small can be determined by either a constant or a function of the space requirement of the process to be placed. (For example, the free area should be twice as large as the space required for the process.) Since this limit is based on the whole partition and not only its remaining part, we will always consider it as a function depending on the process. Of course, if there is no partition to fulfill this extra condition, then we should place the process into the largest partition. So we get the following algorithm.

Limited-Best-Fit( )

  1     2  
                     
                        NIL
                        3     4  
                        WHILE
                      
                     
                     
                        NIL
                        5    
                        DO
                      
                     
                        IF
                      
                      and  and              (( and 
                     
                        Limit(
                        
                        )
                     )             or 
                     
                        NIL
                      or (
                     
                        Limit(
                        
                        )
                      and ))  6       
                        THEN
                      
                        7                8          9  
                        IF
                      
                     
                     
                        NIL
                       10    
                        THEN
                      
                     
                        Split-Partition(
                        
                        )
                       11         12         13       
                     
                        FALSE
                      
                  

This algorithm is more complicated than the previous ones. To prove its correctness we have to see that the inner loop is a conditional minimum searching. The first part of the condition, i. e. that , and means that we try to find a free partition suitable for the process. The second part is a disjunction: we replace the item found so far with the newly examined one in three cases. The first case is when , and Limit( ) , which means that the size of the examined partition is at least as large as the described minimum, but it is smaller than the the smallest one found so far. If there were no more conditions, this would be a conditional minimum search for the conditions of which we added that the size of the partition should be above a certain limit. But there are two other cases, when we replace the previously found item to the new one. One of the cases is that NIL , i. e., the newly examined partition is the first one which is free, and into which the process fits. This is needed because we stick to the requirement that if there is a free partition suitable for the process, then the algorithm should place the process into such a partition. Finally, according to the third condition, we replace the previously found most suitable item to the current one, if Limit( ) and , which means that the minimum found so far did not reach the described limit, and the current item is bigger than this minimum. This condition is important for two reasons. First, if the items examined so far do not fulfill the most recent condition, but the current one does, then we replace it, since in this case Limit( ) , i. e., the size of the current partition is obviously larger. Second, if neither the size of partition found so far, nor that of the current one reaches the described limit, but the currently examined one approaches it better from below, then Limit( ) holds, therefore, also in this case we replace the item found so far by the current one. Hence, if there are partitions at least as large as the described limit, then the algorithm places each process into the smallest one among them, and if there is no such partition, then in the largest suitable one.

There are certain problems, where the only requirement is that the remaining free spaces should be the largest possible. It can be guaranteed if each process is placed into the largest free partition:

Worst-Fit( )

  1     2  
                     
                        NIL
                        3     4  
                        WHILE
                      
                     
                     
                        NIL
                        5    
                        DO
                      
                     
                        IF
                      
                      and  and    6       
                        THEN
                      
                        7             8       9  
                        IF
                      
                     
                     
                        NIL
                       10    
                        THEN
                      
                     
                        Split-Partition(
                        
                        )
                       11         12         13       
                     
                        FALSE
                      
                  

We can prove the correctness of the algorithm similarly to the Best-Fit algorithm; the only difference is that maximum search has to be used instead of conditional maximum search. As a consequence, it is also obvious that the sizes of the remaining free areas are maximal.

The Worst-Fit algorithm maximises the smallest free partition, i. e. there will be only few partitions which are too small for most of the processes. It follows from the fact that it always splits the largest partitions. However, it also often prevents large processes from getting into the memory, so they have to wait on an auxiliary storage. To avoid this we may extend our conditions with an extra an one, similarly to the Best-Fit algorithm. In this case, however, we give an upper limit instead of a lower one. The algorithm only tries to split partitions smaller than a certain limit. This limit also depends on the space requirement of the process. (For example the double of the space requirement.) If the algorithm can find such partitions, then it chooses the largest one to avoid creating too small partitions. If it finds only partitions exceeding this limit, then it splits the smallest one to save bigger ones for large processes.

Limited-Worst-Fit( )

  1     2  
                     
                        NIL
                        3     4  
                        WHILE
                      
                     
                     
                        NIL
                        5    
                        DO
                      
                     
                        IF
                      
                      and  and              (( and 
                     
                        Limit(
                        
                        )
                     ) or 
                     
                        NIL
                      or             (
                     
                        Limit(
                        
                        )
                      and ))  6       
                        THEN
                      
                        7             8       9  
                        IF
                      
                     
                     
                        NIL
                       10    
                        THEN
                      
                     
                        Split-Partition(
                        
                        )
                       11         12         13       
                     
                        FALSE
                      
                  

It is easy to see that this algorithm is very similar to the Limited-Best-Fit , only the relation signs are reversed. The difference is not significant indeed. In both algorithms the same two conditions are to be fulfilled: there should not be too small partitions, and large free partitions should not be wasted for small processes. The only difference is which condition is taken account in the first place and which in the second. The actual problem decides which one to use.

Exercises

17.1-1 We have a system containing two fixed partitions with sizes of 100 kB, one of 200 kB and one of 400 kB. All of them are empty at the beginning. One second later five processes arrive almost simultaneously, directly after each other without significant delay. Their sizes are 80 kB, 70 kB, 50 kB, 120 kB and 180 kB respectively. The process with size of 180 kB ends in the fifth second after its arrival, but by that time another process arrives with space requirement of 280 kB. Which processes are in which partitions in the sixth second after the first arrivals, if we suppose that other processes do not end until that time, and the Largest-Fit algorithm is used? What is the case if the Largest-or-Long-Waiting-Fit or the Long-Waiting-or-Not-Fit-Smaller algorithm is used with threshold value of 4?

17.1-2 In a system using dynamic partitions the list of free partition consists of the following items: one with size of 20 kB, followed by one of 100 kB, one of 210 kB, one of 180 kB, one of 50 kB, one of 10 kB, one of 70 kB, one of 130 kB and one of 90 kB respectively. The last process was placed into the partition preceding the one of 180 kB. A new process with space requirement of 40 kB arrives into the system. Into which partition is it to be placed using the First-Fit , Next-Fit , Best-Fit , Limited-Best-Fit , Worst-Fit or the Limited-Worst-Fit algorithms?

17.1-3 An effective implementation of the Worst-Fit algorithm is when the partitions are stored in a binary heap instead of a linear linked list. What is the time complexity of the Place algorithm perform in this case?