1 Memory Management
Address Binding The normal procedures is to select one of the processes in the input queue and to load that process into memory. As the process executed, it accesses instructions and data from memory, when the process terminates, and its memory space is declared available. Most systems allow a user process to reside in any part of the physical memory. Address space of the computer starts at 00000, addresses may be represented in different ways. 2
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Addresses in the source program are generally symbolic. A compiler will typically bind these symbolic addresses to re-locatable addresses. The linkage editor or loader will in turn bind these re-locatable addresses to absolute addresses. Each binding is a mapping from one address space to another. 4
The binding instruction and data to memory address can be done at any step along the way: Compile time Load time Execution time 5
Compile Time If you compile time where the process will reside in memory, the absolute code can be generated. For example, if you know a priori that a user process resides starting at location R, then the generated compile time will start at that location and extend up from there. Absolute code : - A code used when the addresses in a program are to be written in machine language exactly as they will appear when the instructions are executed by the control circuits. 6
Load time If it is not known at compile time where the process will reside in memory, then the compiler must generate re-locatable code. In this case, final binding is delayed until load time. If the starting address changes, we need only to reload the user code to incorporate this changed value. 7
Execution time If the process can be moved during its execution from one memory segment to another, then binding must be delayed until run time. 8
9 Dynamic Loading The entire program and data of a process must be in physical memory for the process to execute. To obtain better memory-space utilization, we can use dynamic loading. With dynamic loading, a routing is not loaded until it is called. All routines are kept on disk in re-locatable load format. The main program is loaded into memory and is executed. When routine needs to call another routine, the calling routine first checks to see whether the other routine has been loaded.
10 If not, the re-locatable linking loader is called to load the desired routine into memory and to update the program s address tables to reflect this change. The advantage of dynamic loading is that an unused routines is never load. Useful when large amounts of code are needed to handle infrequently occurring cases. Dynamic loading does not require support from operating system. The designer job is to design their program to take advantage of such method.
11 Logical vs. Physical Address Space The concept of a logical address space that is bound to a separate physical address space is central to proper memory management Logical address generated by the CPU; also referred to as virtual address Physical address address seen by the memory unit The compile time and load-time address binding generate identical logical and physical address. However, the execution-time address binding scheme results in differing logical and physical addresses. We usually refer to the logical address as a virtual address
Memory Management Unit (MMU) The run-time mapping from virtual to physical addresses is done by a hardware called the memory-management unit(mmu). 12
Swapping A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution. For example, a round robin scheduling algorithm. When quantum expires, the memory manager will start to swap out the process that just finished, and to swap in another process to the memory space that has been freed.( see figure on next slide). 13
14 Schematic View of Swapping
15 Roll out, roll in swapping variant used for priority-based scheduling algorithms; lowerpriority process is swapped out so higher-priority process can be loaded and executed. A process that is swapped out will swapped back into memory at same location that previously occupied. This restriction is dictated by the method of address binding. If binding is done at assembly or load time, then process can not move at different location but binding is done at execution time then a process can be swapped into a different memory space.
16 Swapping requires a backing store. Fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images.
Contiguous Memory Allocation The main memory must accommodate both the operating system and the various user processes. Main memory usually into two partitions: Resident operating system, usually held in low memory with interrupt vector User processes then held in high memory. 17
18 Concept(Base & Limit Register) To separate each program s memory space, we need the ability to determine the range of legal addresses that the program may access, and to protect the memory outside the space. We can provide this protection by using two registers. i.e. Base register and Limit register. The base register hold the smallest legal physical memory address; the limit register contains the size of range. For example, if the base register holds 300040 and limit register is 120900 then the program can legally access all addresses from 300040 through 420940 inclusive.
19 This protection is accomplished by the CPU hardware comparing every address generated in user mode with registers. Any attempt by a program executing in user node to access monitor memory or other users memory results in trap to the monitor, which treats the attempts as a fatal error. This scheme prevents the user program from modifying the code or data structure of either the operating system or other users.
20 Memory Protection Protecting the operating system from user processes, and protecting user processes from one another. The relocation register contains the value of smallest physical address; the limit register contains the range of logical addresses. With relocation and limit register, each logical address must be less than limit register. The MMU maps the logical address dynamically by adding the value in the relocation register.
21 When the CPU scheduler selects a process for execution, the dispatcher load the relocation and limit registers with correct values. Every address generated by the CPU is checked against these registers. The relocation register scheme provides an effective way to allow the OS size to change dynamically. For example, the OS contains code and buffer space for device drivers. If a device driver not in used, we do not need to keep the code and data in memory so we can able to allocate the space for another purpose. Such code is some times called transient OS code.
Limit Register Relocation Register CPU Logical Address < No Yes + Physical Address MEMORY 22
Memory Allocation One of simplest methods for memory allocation is to divide memory into several fixed-sized partitions. Each partitions may contain exactly one process. In multipartition method, when a partition is free, a process is selected from the input queue and loaded into the free partition. When process terminates, the partition will be available for another process. The operating system keeps table indicating which parts of memory are available and which are occupied. Initially, all memory is available for user processes, and is considered as one large block of available memory, a hole. 23
24 When process arrives, it search for a hole large enough, if it is found, we allocate only as much memory as is needed, keeping the rest of another processes. A set of holes, of various sizes, is scattered throughout memory at any given time. When process arrives and needs memory, system search this set of hole that is large enough for this process. If the hole is too large, it is split into two parts: one part is allocated to the arriving process; the other is returned to the set of holes. When a process terminates, it release block of memory, which is then placed back in the set of holes.
25 This procedure is a particular instance of the general dynamic storage allocation problem, which is how to satisfy a request of size n form a list of free holes. There are many solutions to this problem, which are listed below: (1) First fit (2) Best fit (3) Worst fit
26 First fit : allocate the first hole that is big enough. Searching can start either at the beginning of the set of holes or where the previous first-fit search ended. We can stop searching as we find a free hole that is large enough. Best fit : allocate the smallest hole that is big enough. We must search the entire list, unless the list is kept ordered by size. This strategy produces the smallest leftover hole. Worst fit : allocate the largest hole. Again, we must search the entire list, unless it is sorted by size. This strategy produces the largest leftover hole, which may be more useful than the smaller leftover hole from a best fit approach.
27 Simulations have shown that both first fit and best fit are better than worst fit in terms of decreasing both time and storage utilization. Neither first fit not best fit is clearly better in terms of storage utilization, but first fit is generally faster. These algorithm, however, suffer from external fragmentation. As processes are loaded and removed from memory, the free space is broken into little pieces. External fragmentation exists when enough total memory space exists to satisfy a request, but it not contiguous; storage is fragmented into a large number of small holes.
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Example Given memory partitions of 100K, 500K, 200K, 300K, and 600K (in order), how would each of the First-fit, Best-fit, and Worst-fit algorithms place processes of 212K, 417K, 112K, and 426K (in order)? Which algorithm makes the most efficient use of memory? 29
First Fit Answer 212K is put in 500K partition 417K is put in 600K partition 112K is put in 288K partition (new partition 288K = 500K - 212K) 426K must wait 30
Best fit 212K is put in 300K partition 417K is put in 500K partition 112K is put in 200K partition 426K is put in 600K partition Worst fit 212K is put in 600K partition 417K is put in 500K partition 112K is put in 388K partition 426K must wait 31
Fragmentation Memory allocation can be internal as well as external Consider a multiple partition allocation scheme with a hole of 18,464 bytes. Suppose that the next process requests 18,462 bytes. If we allocate exactly the requested block, we are left with a hole of 2 bytes. The overhead to keep track of this hole will be substantially larger than the hole itself. The general approach is to break the physical memory into fixed-sized blocks, and allocate memory in unit of block. With this approach memory allocated to a process may be slightly larger than requested memory. 32
33 Internal fragmentation : - occurs when memory is divided into fixed-sized partitions(e.g. page frames in main memory, physical block on disk). If a block of data is assigned to one or more partitions, then there may be wasted space in the last partition. This will occur if the last portion of data is smaller than the last partition. External fragmentation : - Occurs when memory is divided into variable-sized partitions corresponding to the blocks of data assigned to the memory(e.g. segments in main memory). As segments are moved into and out of the memory, gaps will occur between the occupied portions of memory.
34 There is a hole of 300K and 600K in multiple partition allocation scheme. Next process request for 700k of memory is free which satisfy the request but hole is not contiguous. So there is an external fragmentation of memory. The selection of first fit versus best fit can affect the amount of fragmentation. Depending on the total memory of memory storage and the average process size, external fragmentation may be either minor or major problem.
P50 Hole of 600k Hole of 300k OS 35
36 One technique for overcoming external fragmentation is compaction. From time to time, the operating system shifts the processes so that they are contiguous and all the free memory is together in one block.
37 Compaction implies the need for a dynamic relocation capability. If relocation is static and done at assembly or load time, compaction can not be done. Relocation is done at execution time, the compaction is possible cost is the major factor for compaction. The simplest algorithm of compaction is, the process moves towards one side of memory. It produces one large hole of available memory. But cost of this scheme can be more.
Terms Page : in virtual storage, a fixed length block that has a virtual address and that is transferred as a unit between main memory and secondary memory. Paging : - the transfer of pages between main memory and secondary memory. Physical address : - The absolute location of a unit of data in memory ( e.g. word or byte in main memory, block on secondary storage). 38
Paging Physical memory is divided into fixed-size blocks called frames and the logical memory is divided into the fixed-size blocks called pages. The size of a page is same as that of frame. The key idean of this method is to place the pages of a process into the available frames of memory, whenever, this process is to be executed. The hardware support for paging is illustrated in figure( next slide for image). The page size is same as frame size. 39
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Address Translation Scheme Address generated by CPU is divided into: Page number (p) used as an index into a page table which contains base address of each page in physical memory Page offset (d) combined with base address to define the physical memory address that is sent to the memory unit 41
Example Using a page size of 4 bytes and a physical memory of 32 bytes(8 pages). 42
We will see how the user s view of memory can be mapped into physical memory. Logical address 0 is page 0, offeset 0. Indiexing into the page table, we find that page 0 is in frame 5. Thus, logical address 0 maps to physical address 20(=5 x 4 ) + 0 ). Logical address 3 (page 0, offset 3) maps to physical address 23(=(5 x 4 + 3). Logical address 4 is page 1, offset 0; according to page table, page 1 is mapped to frame 6. thus, logical address 4 maps to physical address 24(=6 x 4) + 0). 43
Hardware Support on Paging To implement paging, the simplest method is to implement the page table as a set of registers. These registers should be built with very high-speed logic to make the paging-address translation efficient. Every access to memory must go through the paging map, so efficiency is major factor. The CPU dispatcher reloads the registers, just as it reloads the other registers. Instruction to load or modify the page-table registers are, of course, so that only OS can change the memory map. However, the size of register is limited and the size of page table is usually large Therefore, the page table is kept in main memory 44
Hardware Support on Paging If we want to access location I, we must first index into page table, this requires one memory access With this scheme, TWO memory access are needed to access a byte The standard solution is to use a special, small, fast cache, called Translation lookaside buffer (TLB) or associative memory 45
46 TLB The TLB is associative, high-speed memory. Each entry in the TLB consists of two parts: a key(or tag) and a value. When the associative memory is presented with an item, it is compared with all keys simultaneously. If the item is found, the corresponding values field is returned. The search is fast; the hardware; however is expensive. Typically the number of entries inn a TLB is small, often numbering between 64 and 1,024
47 TLB
48 TLB If the page number is not in the TLB (TLB miss) a memory reference to the page table must be made. In addition, we add the page number and frame number into TLB If the TLB already full, the OS have to must select one for replacement Some TLBs allow entries to be wire down, meaning that they cannot be removed from the TLB, for example kernel codes
49 TLB The percentage of times that a particular page number is found in the TLN is called hit ratio If it takes 20 nanosecond to search the TLB and 100 nanosecond to access memory, then a mapped memory access takes 120 nanoseconds when the page number is in the TLB. If our hit ratio is 80%, the effective memory access time equal: 0.8*(100+20) + 0.2 *(100+100)=140 nanosec If our hit ratio is 98%, the effective memory access time equal: 0.98*(100+20) + 0.02 *(100+100)=122 nanosec
50 Disadvantage of TLB is that: if two pages use the same entry of the memory, only one of them can be remembered at once, if process is referencing both pages at same time. TLB does not work very well.
Segmentation Since the user s view of memory is not the same as the actual physical memory, segmentation helps user to view memory as a collection of variable-size segment Segmentation is a memory management scheme that supports user view of memory. 51
Segmentation A program is a collection of segments. A segment is a logical unit such as: main program, procedure, function, method, object, local variables, global variables, common block, stack, symbol table, arrays 52
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54 Segmentation is memory-management scheme that supports this user view of memory. A logical address space is a collection of segments. Each segment has a name and length. The addresses specify both the segment name and the offset within the segment. The user therefor specifies each address by two quantities: a segment name and an offset.
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Segmentation The user specifies each address by two quantities: a segment name and an offset <segment-number, offset> Compare with page scheme, user specifies only a single address, which is partitioned by hardware into a page number and an offset, all invisible to the programmer 56
Segment Hardware Although the user can refer to objects in the program by a two-dimensional address, the actual physical address is still a one-dimensional sequence Thus, we need to map the segment number This mapping is effected by a segment table In order to protect the memory space, each entry in segment table has a segment base and a segment limit. 57
58 The segment base contains the starting physical address where the segment resides in memory, whereas the segment limit specifies the length of the segment.
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60 A logical address consists of two parts: a segment number, s, and an offset into that segment, d. The segment number is used as an index into the segment table. The offset d of the logical address must be between 0 and the segment limit. If it is not, we trap to the Operating system. If this offset is legal, it is added to the segment base to produce the address in physical memory of the desired byte. The segment table is thus essentially an array of base-limit register pairs.
61 Example of Segmentation
Consider the figure of previous slide. We have five segments numbered from 0 to through 4. the segment are stored in physical memory as shown. The segment table has a separate entry for each segment, giving the beginning address of the segment in physical memory(or base) and the length of that segment. 62
Difference between Segmentation and Paging Segment is logical unit, visible to user s program and is of arbitrary size. A page is physical unit, invisible to the user s program and is of fixed size. 63
64 Comparison of paging and segmentation
Virtual Memory Virtual memory is a technique that allows the execution of processes that may not completely in memory. One major advantages of this scheme is that programs can be larger than physical memory. Further, virtual memory abstract main memory into an extremely large, uniform array of storage, separating logical memory as viewed by the user from physical memory Virtual memory also allows processes to easily share files and address space, and it provides mechanism for process creation. 65
66 The ability to execute a program that is only partially in memory would consult many benefits. 1) A program would no longer be constrained by the amount of physical memory that is available. User would be able to write programs for an extremely large virtual-address space, simplifying the programming task. 2) Because each user program could take less physical memory, more program could be run at the same time, with a corresponding increase in CPU utilization and throughput, but with no increase in response time or turnaround time. 3) Less I/O would be needed to load or swap each user program into memory, so each user program would run faster.
Background 67 The instruction being executed must be in physical memory. The first approach to meeting this requirement is to place entire logical address space in physical memory. Virtual memory is the separation of user logical memory from physical memory. This separation allows an extremely large virtual memory to be provided for programmers when only a smaller physical memory is available. Virtual memory makes the task of programming much easier, because the programmer no longer needs to worry about the amount of physical memory available.
Demand Paging A demand paging is similar to a paging system with swapping. Processes reside on secondary memory (i.e. disk). When we want to execute a process, we swap into memory. Rather than swapping the entire process into memory, we use a lazy swapper. A lazy swapper never swaps a page into memory unless that page will be needed. A swapper manipulate entire processes, whereas a pager concerned with the individual pages of a process. 68
Concept When a process is to be swapped in, the pager guesses which pages will be used before the process is swapped out again. Instead of swapping in a whole process, the pager bring only those necessary pages into memory. It avoids reading into memory pages that will not be used anyway, decreasing the swap time and the amount of physical memory needed. 69
70 Transfer of a Paged Memory to Contiguous Disk Space
71 With this scheme, we need some form of hardware support to distinguish between those pages that are in memory and those pages that are one the disk. The valid-invalid bit scheme can be used for this purpose. When the bit is set to valid, this value indicates that the associated page is both legal and in memory. If the bit is set to invalid, this value indicates that the page either is not valid( that is, not in the logical address space of the process), or is valid but is currently on the disk.
72 The page-table entry for a page that is brought into memory as usual, but the page-table entry for a page that is not currently in memory is simply marked invalid, or contains the address of the page on disk. This situation is shown in next slide.
73 Page Table When Some Pages Are Not in Main Memory
74 If the process tries to access a page that was not brought into memory? Access to a page marked invalid causes a page-fault trap. The procedure for handling this page fault is straightforward. See next slide for figure.
75 Steps in Handling a Page Fault
76 1) We check an internal table for this process, to determine whether the reference was a valid or invalid memory access. 2) If the reference was invalid, we terminate the process. If it was valid, but we have not yet brought in that page, we now page it in. 3) We find a free frame. 4) We schedule a disk operation to read the desired page into the newly allocated frame. 5) When the disk read is complete, we modify the internal table kept with the process and the page table to indicate that page is now in memory. 6) We restart the instruction that was interrupted by the illegal address trap.
Page Replacement Over-allocation demonstrates itself as follows. While a user process is executing, a page fault occurs. The hardware traps to the operating system, which checks its internal tables to see that this page fault is genuine one rather than an illegal memory access. The operating system determines where the desired page is residing on the disk, but then finds that there are no free frames on the free frame list. All memory is in use( see next slide for figure) 77
78 Need For Page Replacement
79 Reference String : - we evaluate an algorithm by running it on a particular string of memory and computing the number of page faults. The string of memory reference is called a reference string.
FIFO Page replacement A FIFO replacement algorithm associates with each page the time when that page was brought into memory. When a page must be replaced, the oldest page is chosen. We replace the page at the head of the queue. When a page is brought into memory, we insert it at the tail of the queue. FIFO page replacement algorithm is easy to understand and program. However, its performance is not always good. 80
Let us consider the following string 0,1,2,3,0,1,2,3,0,1,2,3,4,5,6,7 Belady s anomaly: For some page replacement algorithms, the page fault rate may increase as the number of allocated frames increase. FIFO page replacement algorithm may space this problem. 81
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LRU Page Replacement LRU replacement associates with each page the time of that page the time of that page s last use. When a page must be replaced, LRU choose that page that has not been used for the longest period of time. This strategy is the optimal pagereplacement algorithm looking backward in time, rather than forward. 83
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Optimal Page Replacement The optimal policy selects for replacement that page for which the time to the next reference is the longest. An optimal page replacement algorithm has the lowest page fault rate of all algorithms, and will never suffer from Belady s anomaly. This algorithm is impossible to implement because it would require the operating system to have perfect knowledge of future events. The optimal page replacement algorithm says the page with the highest label should be removed. 85
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Page Reference String : - 5,4,3,2,1,4,3,5,4,3,2,1,5 Count FIFO, LRU Example 87
Second Chance Algorithm(Clock) The basic algorithm of second-chance replacement is a FIFO replacement algorithm. When a page has been replaced, however, we inspect its reference bit. If the value is 0, we proceed to replace this page. If the reference bit is set to 1, however, we give that page a second chance and move on to select the next FIFO page. When a page gets a second chance, its reference bit is cleared and its arrival time is reset to the current time. Thus, a page that is given a second chance will not be replaced until all other pages are replaced, 88
One way to implement the second chance algorithm is as a circular queue. A pointer indicates which page is to be replaced next. When a free is needed, the pointer advances until it finds a page with 0 reference bit. As it advances, it clears the reference bits. Once a victim page is found, the page is replaced, and the new page is inserted in the circular queue in that position. 89
Thrashing If a process does not have enough pages, the page-fault rate is very high. This leads to: low CPU utilization operating system thinks that it needs to increase the degree of multiprogramming another process added to the system Thrashing a process is busy swapping pages in and out 90