1. Background. 2. Demand Paging
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1 COSC Operating Systems Design, Fall 2001, Byunggu Yu Chapter 10 Virtual Memory 1. Background PROBLEM: The entire process must be loaded into the memory to execute limits the size of a process (it must be smaller than the size of physical memory) limits the degree of multiprogramming SOLUTION: Virtual Memory allows the execution of processes that may not be completely in memory increases the degree of multiprogramming 2. Demand Paging 2.1 Basic Idea Demand paging: Whenever a process is swapped in, pager guesses the pages that will be used before the process is swapped out again. Then, instead of swapping in a whole process, the pager brings only those necessary pages into memory. 2.2 Implementation Page table is modified: each entry has valid-invalid bit Valid: The page is both legal and in memory Invalid: The page is not legal or not in memory Note that a page is legal if it is in the process' address space When a process is loaded into the memory to start its execution (i.e., startup): -- Frame allocation: Option #1: No physical frame is allocated (i.e., Pure Demand Paging) Option #2: Allocate some frames at process startup -- Swap space allocation: Option #1: copy the entire process image to the swap space Option #2: don't copy the process image to the swap space at process startup. Instead, write the pages to swap space as they are replaced
2 During the execution of a process (Page hit/page fault) -- CPU generates a logical memory address whose page number is i -- Read the i th entry of the page table. If the valid-invalid bit value is 1, the rest of the procedure is the same as that of Paging (i.e., page hit). -- Else (page fault) 1.Trap to the OS 2.Check the values of PTBR and PTLR of the current process to see if the page is legal 3.If the page number is illegal then terminate the current process 4.Else 4.1. Find a free frame from the free-frame list (or frame pool) 4.2.Schedule a disk operation to read the content of the i th page into the free frame. (e.g., the PCB can have the location of the ith page for non-contiguous swap space allocation or the base/limit of the process image in the swap space for contiguous swap space allocation. Various possible options) 4.3.When the read operation is completed, modify the page table (and possibly PCB) of the process 4.4.Resume the process and restart the interrupted instruction from the first step (i.e., fetch instruction). But what happens if there is no free frame in the free-frame list (frame pool)? --> Page replacement (Selecting a victim page) 3. Page Replacement 3.1 Basic Ideas In many cases, the current instruction and the related operands are required to be in the main memory to execute the instruction. Considering a multi-level pointer, the pointer values are not necessarily packed into a single page. What happens if there is no free frame when a page fault occurs? 1.Solution #1: Terminate the process 2.Solution #2: Swap out the process and free all its frames. 3.Solution #3: Find a victim page and, if necessary, write the page on the corresponding swap space disk block. Then copy the required page to the frame.
3 In this section, we discuss the solution #3. To implement the solution, we should modify Step 4.1 of the page-fault handling procedure in Section 2.2 of this lecture note. Modified Step 4.1: If there is a free frame in the frame list (or frame pool), then use it. Else Use a page-replacement algorithm to select a victim page Copy the victim page to the corresponding swap space if necessary Change the page table and the frame list (frame pool) accordingly and use the frame. Note that Step is not always necessary. We need not write back the victim page if it is a part of the code section or if it has not been modified since it was read in from the swap space. However, to accommodate this, we should modify the page table to include modify bit. Each page table entry has a modify bit. The modify bit is set whenever any word or byte in the corresponding page is changed, indicating that the page has been modified. Some systems (e.g., Linux and the variants) maintain a separate frame table (frequently called page frame pool). This frame table includes m entries, where m is the total number of physical frames constituting the physical memory. Each entry of the table represents the status of the corresponding frame. For example, we can store modify bit, lock bit and free bit that can tell us whether the frame content need to be written back to the swap space, whether the page in the frame can be selected as a victim frame for a page replacement, and whether the frame is free or not. 3.2 Page-Replacement Algorithms * FIFO (First-In First-Out) Algorithm The victim frame (the frame that contains the victim page) is the oldest one. Implementation: Additional FIFO queue of allocated frames is needed. -- Victim frame is the head of the FIFO queue -- New frame is the tail of the FIFO queue Global allocation: Need a global FIFO queue. Possibly, we can implement a FIFO queue in the frame table. Local allocation: Need one FIFO queue for each process. Possibly, we can implement the page table as FIFO queue. * Optimal Algorithm for comparison study Victim frame is the one that will not be used for the longest period of time. Implementation: Almost impossible to implement, since it requires the future knowledge of the memory references. However, we can use it for comparative
4 performance evaluation: When you propose a page-replacement algorithm, you can compare it with this optimal algorithm. * LRU (Least Recently Used) Algorithm Victim frame is the one that has not been accessed for the longest period of time. Implementation: Use Counter or Stack Counter: Modify the page table (local counter) or frame table (global counter) each entry has a time-of-use field. Whenever a reference to the frame is made, the content of the clock (or counter) register is copied to the timeof-use field. In this way, we have the time of the last reference to the frame. The one who has the smallest time-of-use value is the victim. Problem #1: H/W counter or clock is required Problem #2: Must search the page table to find the LRU frame (note, if a list (LRU list) is used, the structure must be updated whenever there is a memory access). Problem #3: time-of-use must be updated for every page reference (overhead) Stack: Additional stack of frame (or page) numbers. The bottom of the stack is the LRU frame (or page). Problem: Stack must be updated for every page reference (overhead) Without the H/W support (e.g., counter or stack registers), the LRU algorithm is very slow, since the time-of-use or the stack must be updated very frequently (additional memory accesses). Many computer systems provide a reference bit for each page or frame. The bit associated each page reference is set to one by the H/W. The following three variants of the LRU algorithm use this minimum H/W support. * LRU Approximation Algorithm #1: Additional-Reference-Byte Algorithm Victim: the frame indicated by the page (or frame) entry whose 8-bit byte (reference-byte) value is the smallest one. Each page table (or frame table) entry has reference-byte (8-bit byte). At regular interval (e.g., 100 milliseconds), the timer interrupt routine writes the most recent reference bit into the high-order bit (MSB) of the 8-bit byte, shifting the other bits right 1 bit, discarding the low-order bit (LSB). --> the history of page accesses during the last eight time intervals. e.g., A page (or frame) with a reference-byte value of has been used more recently than the page that is associated with * LRU Approximation Algorithm #2: Second-Chance Algorithm Victim: The first page (or frame) in the FIFO queue whose reference-bit is zero. The FIFO queue is a circular queue. That is, the tail of the queue points to the head of the queue. To find the victim, OS scans the queue from the last stop point in a circular fashion until it finds a page whose current reference bit is zero
5 (victim page). As the pointer advances, it sets the reference bits to zero (see Figure in p. 342). * LRU Approximation Algorithm #3: Enhanced Second-Chance Algorithm Victim: Scan the entire circular queue possibly several times and find the first page (or frame) that is in the lowest class (class is <reference bit, modify bit>. see the four classes of nonempty frames in P.343) --> by considering the modify-bit, we can reduce the number of write operations (disk I/Os) on the swap space. * We skip Counting Algorithm and Page Buffering Algorithm 4. Allocation of Frames 4.1 Minimum/Maximum Number of Frames The maximum number of frames per process is defined by the amount of available physical memory. The minimum number of frames per process is defined by the underlying computer architecture. For example, if the architecture allows up to 15 levels of indirect memory addressing (you can imagine the maximum number of * s in the front of a pointer type variable in C), the minimum number of frames should be Equal Allocation and Proportional Allocation There are n processes and m physical frames. Equal Allocation: Each process has m/n frames Proportional Allocation: Each process has si/s*m frames, where si is the size of the process and S is the total amount of address space of the n processes. That is, a larger process gets a larger number of frames. 4.3 Global allocation vs. Local allocation Global allocation: select the victim frame from the frames of the physical memory. Local allocation: select the victim frame from the current process' frames.
6 5. Thrashing Trashing: Very high paging activity Too many processes in memory --> # of allocated frames < its locality --> processes spend more time on paging than executing instructions. --> The process is in a thrashing State. 5.1 Solution #1: Local Replacement Algorithm: A process cannot steal a frame from another process. --> no global thrashing (no propagation) --> Increase I/O requests from the thrashing process --> decrease the system performance. 5.2 Solution #2: Locality Model with Working-Set Model D=WSSi, where WSSi is the number of different pages in the most recent memory accesses of Pi. The value represents the average number of memory accesses by a process in a single locality. If D > m, select a victim process, suspend it, and swap it out. 5.3 Solution #3: PFF (Page-Fault Frequency) strategy Predetermined upper bound and lower bound of page-fault rate. When there is a free frame: If the current process' page-fault rate exceeds the upper bound then allocate a frame to the current process. If the current process' page-fault rate falls below the lower bound then remove a frame from the current process. When there is no free frame: If the current process' page-fault rate exceeds the upper bound then select a victim process and suspend it. If the current process' page-fault rate falls below the lower bound then remove a frame from the current process. 6. Other Considerations 6.1 Prepaging When we start or restart a process, pager brings enough number of pages, which constitute a single locality, into the main memory at once. 6.2 Page Size (usually 1K, 2K, 4K, or 8K-byte) A smaller page size --> increases the size of page table, --> decreases the internal fragments, --> increases the number of disk I/Os, --> improves the locality representation, and
7 --> increases the page-fault rate. A larger page size --> decreases the size of page tables, --> increases the internal fragments, --> decreases the number of disk I/Os, --> aggravates the locality representation, and --> decreases the page-fault rate. 6.3 I/O Interlock Process 1 requests a disk I/O to read one data page from the disk into Frame 5. CPU switches to Process 2 Process 2 generates a page-fault Frame 5 is selected as a victim frame by a global allocation strategy Process 2 uses Frame 5 CPU switches back to Process 1 Process 1 accesses Frame 5 What happens? --> Solution: Each entry of page table or frame table has lock-bit. Process 1 can lock Frame 5. If the frame is locked, it cannot be selected for replacement. 6.6 Real-Time Processing Virtual memory is not a good technique for real-time systems, since the page-fault service and disk I/O operations are expensive. Solaris2: allow both real-time and time-sharing computing within a system --> allows a process to tell the system which pages are important to that process. A real-time process requests enough number of important frames and lock the frames. --> But other nearthrashing processes can reduce the I/O response time --> Priority based scheduling -->...
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