Operating System Structures CH.9
Lazy swapper
Rather than swapping the entire process into memory, though, we use a lazy swapper. A lazy swapper never swaps a page into memory unless that page will be needed.
Page replacement is basic to _______ paging
demand
Process of page replacement
1. Find the location of the desired page on the disk. 2. Find a free frame: a. If there is a free frame, use it. b. If there is no free frame, use a page-replacement algorithm to select a victim frame. c. Write the victim frame to the disk; change the page and frame tables accordingly. 3. Read the desired page into the newly freed frame; change the page and frame tables. 4. Continue the user process from where the page fault occurred.
In Linux, a slab may be in one of three possible states:
1. Full. All objects in the slab are marked as used. 2. Empty. All objects in the slab are marked as free. 3. Partial. The slab consists of both used and free objects.
The slab allocator provides two main benefits:
1. No memory is wasted due to fragmentation. Fragmentation is not an issue because each unique kernel data structure has an associated cache, and each cache is made up of one or more slabs that are divided into chunks the size of the objects being represented. Thus, when the kernel requests memory for an object, the slab allocator returns the exact amount of memory required to represent the object. 2. Memory requests can be satisfied quickly. The slab allocation scheme is thus particularly effective for managing memory when objects are frequently allocated and deallocated, as is often the case with requests from the kernel. The act of allocating—and releasing—memory can be a time-consuming process. However, objects are created in advance and thus can be quickly allocated from the cache. Furthermore, when the kernel has finished with an object and releases it, it is marked as free and returned to its cache, thus making it immediately available for subsequent requests from the kernel.
In any case, we are faced with three major components of the page-fault service time:
1. Service the page-fault interrupt. 2. Read in the page. 3. Restart the process.
Kernel memory is often allocated from a free-memory pool different from the list used to satisfy ordinary user-mode processes. There are two primary reasons for this:
1. The kernel requests memory for data structures of varying sizes, some of which are less than a page in size. As a result, the kernel must use memory conservatively and attempt to minimize waste due to fragmentation. This is especially important because many operating systems do not subject kernel code or data to the paging system. 2. Pages allocated to user-mode processes do not necessarily have to be in contiguous physical memory. However, certain hardware devices interact directly with physical memory—without the benefit of a virtual memory interface—and consequently may require memory residing in physically contiguous pages.
A page fault causes the following sequence to occur:
1. Trap to the operating system. 2. Save the user registers and process state. 3. Determine that the interrupt was a page fault. 4. Check that the page reference was legal and determine the location of the page on the disk. 5. Issue a read from the disk to a free frame: a. Wait in a queue for this device until the read request is serviced. b. Wait for the device seek and/or latency time. c. Begin the transfer of the page to a free frame. 6. While waiting, allocate the CPU to some other user (CPU scheduling, optional). 7. Receive an interrupt from the disk I/O subsystem (I/O completed). 8. Save the registers and process state for the other user (if step 6 is executed). 9. Determine that the interrupt was from the disk. 10. Correct the page table and other tables to show that the desired page is now in memory. 11. Wait for the CPU to be allocated to this process again. 12. Restore the user registers, process state, and new page table, and then resume the interrupted instruction
Handling a page fault in steps
1. We check an internal table (usually kept with the process control block) for this process to determine whether the reference was a valid or an 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 (by taking one fromthe free-frame list, for example). 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 the page is now in memory. 6. We restart the instruction that was interrupted by the trap. The process can now access the page as though it had always been in memory
slab allocation slab cache object
A second strategy for allocating kernel memory is known as slab allocation. A slab is made up of one or more physically contiguous pages.Acache consists of one or more slabs. There is a single cache for each unique kernel data structure —for example, a separate cache for the data structure representing process descriptors, a separate cache for file objects, a separate cache for semaphores, and so forth. Each cache is populated with objects that are instantiations of the kernel data structure the cache represents.
Pager
A swapper manipulates entire processes, whereas a pager is concerned with the individual pages of a process.We thus use "pager," rather than "swapper," in connection with demand paging.
Page fault
Access to a page marked invalid causes a page fault. The paging hardware, in translating the address through the page table, will notice that the invalid bit is set, causing a trap to the operating system.
pure demand paging
After this page is brought into memory, the process continues to execute, faulting as necessary until every page that it needs is in memory. At that point, it can execute with no more faults. This scheme is _________________: never bring a page into memory until it is required.
coalescing
An advantage of the buddy system is how quickly adjacent buddies can be combined to form larger segments using a technique known as coalescing. In Figure 9.26, for example, when the kernel releases the CL unit it was allocated, the system can coalesce CL and CR into a 64-KB segment. This segment, BL, can in turn be coalesced with its buddy BR to form a 128-KB segment. Ultimately, we can end up with the original 256-KB segment.
Demand Paging
An alternative strategy is to load pages only as they are needed. This technique is known as demand paging and is commonly used in virtual memory systems. With demand-paged virtual memory, pages are loaded only when they are demanded during program execution.
working-set model working-set window working set
As mentioned, the working-set model is based on the assumption of locality. This model uses a parameter, to define the working-set window. The idea is to examine the most recent page references. The set of pages in the most recent page references is the working set (Figure 9.20). If a page is in active use, it will be in the working set. If it is no longer being used, it will drop from the working set time units after its last reference. Thus, the working set is an approximation of the program's locality.
Buddies
Assume the size of a memory segment is initially 256 KB and the kernel requests 21 KB of memory. The segment is initially divided into two buddies—which we will call AL and AR—each 128 KB in size. One of these buddies is further divided into two 64-KB buddies— BL and BR.
raw disk
Because of such problems, some operating systems give special programs the ability to use a disk partition as a large sequential array of logical blocks, without any file-system data structures. This array is sometimes called the raw disk, and I/O to this array is termed raw I/O. Raw I/O bypasses all the filesystem services, such as file I/O demand paging, file locking, prefetching, space allocation, file names, and directories
Memory mapping
Consider a sequential read of a file on disk using the standard system calls open(), read(), and write(). Each file access requires a system call and disk access. Alternatively, we can use the virtual memory techniques discussed so far to treat file I/O as routine memory accesses. This approach, known as memory mapping a file, allows a part of the virtual address space to be logically associated with the file. As we shall see, this can lead to significant performance increases.
Ways of implementing least recently used (LRU) algorithm
Counters. In the simplest case, we associate with each page-table entry a time-of-use field and add to the CPU a logical clock or counter. The clock is incremented for every memory reference. Whenever a reference to a page is made, the contents of the clock register are copied to the time-of-use field in the page-table entry for that page.We replace the page with the smallest time value. This scheme requires a search of the page table to find the LRU page and a write to memory (to the time-of-use field in the page table) for each memory access. The times must also be maintained when page tables are changed (due to CPU scheduling). Overflow of the clock must be considered. Stack. Another approach to implementing LRU replacement is to keep a stack of page numbers. Whenever a page is referenced, it is removed from the stack and put on the top. In this way, the most recently used page is always at the top of the stack and the least recently used page is always at the bottom (Figure 9.16) Removing a page and putting it on the top of the stack then requires changing six pointers at worst. Each update is a little more expensive, but there is no search for a replacement; the tail pointer points to the bottom of the stack, which is the LRU page.
frame-allocation algorithm
Designing appropriate algorithms to solve these problems is an important task, because disk I/O is so expensive. Even slight improvements in demand-paging methods yield large gains in system performance.
Simplest page replacement algorithm
FIFO
locality of reference
Fortunately, analysis of running processes shows that this behavior is exceedingly unlikely. Programs tend to have________________, described in Section 9.6.1, which results in reasonable performance from demand paging.
global replacement
Global replacement allows a process to select a replacement frame from the set of all frames, even if that frame is currently allocated to some other process; that is, one process can take a frame from another. Local replacement requires that each process select from only its own set of allocated frames. One problem with a global replacement algorithm is that a process cannot control its own page-fault rate. The set of pages in memory for a process depends not only on the paging behavior of that process but also on the paging behavior of other processes
Memory resident
Hence, if we guess right and page in all pages that are actually needed and only those pages, the process will run exactly as though we had brought in all pages. While the process executes and accesses pages that are memory resident, execution proceeds normally.
Anonymous memory
However, swap space must still be used for pages not associated with a file (known as anonymous memory); these pages include the stack and heap for a process. This method appears to be a good compromise and is used in several systems, including Solaris and BSD UNIX.
Page replacement
If no frame is free, we find one that is not currently being used and free it.We can free a frame by writing its contents to swap space and changing the page table (and all other tables) to indicate that the page is no longer in memory (Figure 9.10). We can now use the freed frame to hold the page for which the process faulted
Interrupt driven
If the CPU does not poll the control bit, but instead receives an interrupt when the device is ready for the next byte, the data transfer is said to be interrupt driven.
Programmed I/O
If the CPU uses polling to watch the control bit, constantly looping to see whether the device is ready, this method of operation is called programmed I/O (PIO).
pageout solaris
If the number of free pages falls below lotsfree, a process known as a pageout starts up. The pageout process works as follows: The front hand of the clock scans all pages in memory, setting the reference bit to 0. Later, the back hand of the clock examines the reference bit for the pages inmemory, appending each page whose reference bit is still set to 0 to the free list and writing to disk its contents if modified. Solaris maintains a cache list of pages that have been "freed" but have not yet been overwritten.
least recently used (LRU) algorithm
If we use the recent past as an approximation of the near future, then we can replace the page that has not been used for the longest period of time. LRU replacement associates with each page the time of that page's last use. When a page must be replaced, LRU chooses the page that has not been used for the longest period of time. We can think of this strategy as the optimal page-replacement algorithm looking backward in time, rather than forward.
How does Solaris deal with pages
In Solaris, when a thread incurs a page fault, the kernel assigns a page to the faulting thread from the list of free pages it maintains.
Data warehouses and LRU and MRU systems
In another example, data warehouses frequently perform massive sequential disk reads, followed by computations and writes. The LRU algorithm would be removing old pages and preserving new ones, while the application would more likely be reading older pages than newer ones (as it starts its sequential reads again). Here, MFU would actually be more efficient than LRU.
Thrashing
In fact, look at any process that does not have "enough" frames. If the process does not have the number of frames it needs to support pages in active use, it will quickly page-fault. At this point, it must replace some page. However, since all its pages are in active use, it must replace a page that will be needed again right away. Consequently, it quickly faults again, and again, and again, replacing pages that it must bring back in immediately. This high paging activity is called thrashing. A process is thrashing if it is spending more time paging than executing.
There are many other algorithms that can be used for page replacement. For example, we can keep a counter of the number of references that have been made to each page and develop the following two schemes
LFU least freq used MFU most freq used
Solaris memory mapping
Let's take Solaris as an example. If a file is specified as memory-mapped (using the mmap() system call), Solaris maps the file into the address space of the process. If a file is opened and accessed using ordinary system calls, such as open(), read(), and write(), Solaris still memory-maps the file; however, the file is mapped to the kernel address space.
stack algorithm
Like optimal replacement, LRU replacement does not suffer from Belady's anomaly. Both belong to a class of page-replacement algorithms, called stack algorithms, that can never exhibit Belady's anomaly. A stack algorithm is an algorithm for which it can be shown that the set of pages in memory for n frames is always a subset of the set of pages that would be in memory with n + 1 frames.
local replacement
Local replacement requires that each process select from only its own set of allocated frames. Local replacement might hinder a process, however, by not making available to it other, less used pages of memory.
Manipulating memory using functions
Manipulating files through memory rather than incurring the overhead of using the read() and write() system calls simplifies and speeds up file access and usage.
power-of-2 allocator
Memory is allocated from this segment using a power-of-2 allocator, which satisfies requests in units sized as a power of 2 (4 KB, 8 KB, 16 KB, and so forth). A request in units not appropriately sized is rounded up to the next highest power of 2.
optimal page-replacement algorithm
One result of the discovery of Belady's anomaly was the search for an optimal page-replacement algorithm—the algorithm that has the lowest page-fault rate of all algorithms and will never suffer from Belady's anomaly. Such an algorithm does exist and has been called OPT or MIN. It is simply this: Replace the page that will not be used for the longest period of time. Use of this page-replacement algorithm guarantees the lowest possible page fault rate for a fixed number of frames.
clock
One way to implement the second-chance algorithm (sometimes referred to as the clock algorithm) is as a circular queue. A pointer (that is, a hand on the clock) indicates which page is to be replaced next.
zero-fill-on-demand
Operating systems typically allocate these pages using a technique known as zero-fill-on-demand. Zero-fill-on-demand pages have been zeroed-out before being allocated, thus erasing the previous contents.
how does over allocation happen?
Over-allocation of memory manifests itself as follows. While a user process is executing, a page fault occurs. 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
The hardware to support demand paging is the same as the hardware for paging and swapping:
Page table. This table has the ability to mark an entry invalid through a valid-invalid bit or a special value of protection bits. Secondary memory. This memory holds those pages that are not present in main memory. The secondary memory is usually a high-speed disk. It is known as the swap device, and the section of disk used for this purpose is known as swap space. Swap-space allocation is discussed in Chapter 10.
Prepaging
Prepaging is an attempt to prevent this high level of initial paging. The strategy is to bring into memory at one time all the pages that will be needed. Some operating systems—notably Solaris—prepage the page frames for small files.
hit ratio
Recall that the hit ratio for the TLB refers to the percentage of virtual address translations that are resolved in the TLB rather than the page table. Clearly, the hit ratio is related to the number of entries in the TLB, and the way to increase the hit ratio is by increasing the number of entries in the TLB.
Enhancements in solaris
Recent releases of the Solaris kernel have provided enhancements of the paging algorithm. One such enhancement involves recognizing pages from shared libraries. Pages belonging to libraries that are being shared by several processes—even if they are eligible to be claimed by the scanner— are skipped during the page-scanning process. Another enhancement concerns distinguishing pages that have been allocated to processes frompages allocated to regular files. This isknownas priority paging and is covered in Section 12.6.2.
vfork()
Several versions of UNIX (including Solaris and Linux) provide a variation of the fork() system call—________ (for virtual memory fork)—that operates differently from fork() with copy-on-write.With ________ , the parent process is suspended, and the child process uses the address space of the parent. Because________ does not use copy-on-write, if the child process changes any pages of the parent's address space, the altered pages will be visible to the parent once it resumes. Therefore, ________ must be used with caution to ensure that the child process does not modify the address space of the parent. ________ is intended to be used when the child process calls exec() immediately after creation.
In addition to separating logical memory from physical memory, virtual memory allows files and memory to be shared by two or more processes through page sharing (Section 8.5.4). This leads to the following benefits:
System libraries can be shared by several processes through mapping of the shared object into a virtual address space.Although each process considers the libraries to be part of its virtual address space, the actual pages where the libraries reside in physical memory are shared by all the processes (Figure 9.3). Typically, a library is mapped read-only into the space of each process that is linked with it. • Similarly, processes can share memory. Recall from Chapter 3 that two or more processes can communicate through the use of shared memory. Virtual memory allows one process to create a region of memory that it can share with another process. Processes sharing this region consider it part of their virtual address space, yet the actual physical pages of memory are shared, much as is illustrated in Figure 9.3. • Pages can be shared during process creation with the fork() system call, thus speeding up process creation.
NUMA
Systems in which memory access times vary significantly are known collectively as non-uniform memory access (NUMA) systems, and without exception, they are slower than systems in which memory and CPUs are located on the same motherboard If we treat memory as uniform in such a system, CPUs may wait significantly longer for memory access than if we modify memory allocation algorithms to take NUMA into account. Similar changes must be made to the scheduling system. The goal of these changes is to have memory frames allocated "as close as possible" to the CPU on which the process is running. The definition of "close" is "with minimum latency," which typically means on the same system board as the CPU.
Port
The CPU transfers data through these kinds of devices by reading and writing a few device registers, called an I/O port. To send out a long string of bytes through a memory-mapped serial port, the CPU writes one data byte to the data register and sets a bit in the control register to signal that the byte is available
Recent distributions of Linux now include two other kernel memory allocators— the _______ and _______ allocators. (Linux refers to its slab implementation as SLAB.)
The SLOB allocator is designed for systems with a limited amount of memory, such as embedded systems. SLOB (which stands for Simple List of Blocks) works by maintaining three lists of objects: small (for objects less than 256 bytes), medium (for objects less than 1,024 bytes), and large (for objects less than 1,024 bytes). Memory requests are allocated from an object on an appropriately sized list using a first-fit policy.
TLB reach
The TLB reach refers to the amount of memory accessible from the TLB and is simply the number of entries multiplied by the page size. Ideally, the working set for a process is stored in the TLB. If it is not, the process will spend a considerable amount of time resolving memory references in the page table rather than the TLB.
second chance algorithm
The basic algorithm of second-chance replacement is a FIFO replacement algorithm. When a page has been selected, however, we inspect its reference bit. If the value is 0, we proceed to replace this page; but if the reference bit is set to 1, we give the 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 have been replaced (or given second chances). In addition, if a page is used often enough to keep its reference bit set, it will never be replaced.
Windows named shared memory object
The call to CreateFileMapping() creates a named shared-memory object called SharedObject. The consumer process will communicate using this shared-memory segment by creating a mapping to the same named object. The producer then creates a view of the memory-mapped file in its virtual address space. By passing the last three parameters the value 0, it indicates that the mapped view is the entire file. It could instead have passed values specifying an offset and size, thus creating a view containing only a subsection of the file. (It is important to note that the entire mapping may not be loaded into memory when the mapping is established. Rather, the mapped file may be demand-paged, thus bringing pages into memory only as they are accessed.) The MapViewOfFile() function returns a pointer to the shared-memory object; any accesses to this memory location are thus accesses to the memory-mapped file
Equal allocation
The easiest way to split m frames among n processes is to give everyone an equal share, m/n frames (ignoring frames needed by the operating system for the moment). For instance, if there are 93 frames and five processes, each process will get 18 frames. The three leftover frames can be used as a free-frame buffer pool. This scheme is called equal allocation.
Shared memory in windows API
The general outline for creating a region of shared memory using memory mapped files in the Windows API involves first creating a file mapping for the file to be mapped and then establishing a view of the mapped file in a process's virtual address space. A second process can then open and create a view of the mapped file in its virtual address space. The mapped file represents the shared-memory object that will enable communication to take place between the processes.
Counting-Based Page Replacement LFU
The least frequently used (LFU) page-replacement algorithm requires that the page with the smallest count be replaced. The reason for this selection is that an actively used page should have a large reference count. A problem arises, however, when a page is used heavily during the initial phase of a process but then is never used again. Since it was used heavily, it has a large count and remains in memory even though it is no longer needed. One solution is to shift the counts right by 1 bit at regular intervals, forming an exponentially decaying average usage count.
locality model
The locality model states that, as a process executes, it moves from locality to locality. A locality is a set of pages that are actively used together (Figure 9.19). A program is generally composed of several different localities, which may overlap.
Counting-Based Page Replacement MFU
The most frequently used (MFU) page-replacement algorithm is based on the argument that the page with the smallest count was probably just brought in and has yet to be used.
second-chance page-replacement algorithm
The number of bits of history included in the shift register can be varied, of course, and is selected (depending on the hardware available) to make the updating as fast as possible. In the extreme case, the number can be reduced to zero, leaving only the reference bit itself.
cause of thrashing
The operating system monitors CPU utilization. If CPU utilization is too low, we increase the degree of multiprogramming by introducing a new process to the system. A global page-replacement algorithm is used; it replaces pages without regard to the process to which they belong.Nowsuppose that a process enters a new phase in its execution and needs more frames. It starts faulting and taking frames away from other processes. These processes need those pages, however, and so they also fault, taking frames from other processes. These faulting processes must use the paging device to swap pages in and out. As they queue up for the paging device, the ready queue empties. As processes wait for the paging device, CPU utilization decreases. The CPU scheduler sees the decreasing CPU utilization and increases the degree of multiprogramming as a result. The new process tries to get started by taking frames from running processes, causing more page faults and a longer queue for the paging device. As a result, CPU utilization drops even further, and the CPU scheduler tries to increase the degree of multiprogramming even more. Thrashing has occurred, and system throughput plunges. The pagefault rate increases tremendously. As a result, the effective memory-access time increases. No work is getting done, because the processes are spending all their time paging. This phenomenon is illustrated in Figure 9.18, in which CPU utilization is plotted against the degree of multiprogramming. As the degree of multiprogramming increases, CPU utilization also increases, although more slowly, until a maximum is reached. If the degree of multiprogramming is increased even further, thrashing sets in, and CPU utilization drops sharply. At this point, to increase CPU utilization
pageout algorithm
The pageout algorithm uses several parameters to control the rate at which pages are scanned (known as the scanrate). The scanrate is expressed in pages per second and ranges from slowscan to fastscan. When freememory falls below lotsfree, scanning occurs at slowscan pages per second and progresses to fastscan, depending on the amount of free memory available.
reference bit
The reference bit for a page is set by the hardware whenever that page is referenced (either a read or a write to any byte in the page). Reference bits are associated with each entry in the page table.
page-fault frequency (PFF)
The specific problem is how to prevent thrashing. Thrashing has a high page-fault rate. Thus, we want to control the page-fault rate. When it is too high,we know that the process needs more frames. Conversely, if the page-fault rate is too low, then the process may have too many frames. We can establish upper and lower bounds on the desired page-fault rate (Figure 9.21). If the actual page-fault rate exceeds the upper limit, we allocate the process another frame. If the page-fault rate falls below the lower limit, we remove a frame from the process. Thus, we can directly measure and control the page-fault rate to prevent thrashing.
working-set minimum
The working-set minimum is the minimum number of pages the process is guaranteed to have in memory. If sufficient memory is available, a process may be assigned as many pages as its working-set maximum. windows
page-replacement algorithm
There are many different page-replacement algorithms. Every operating system probably has its own replacement scheme. How do we select a particular replacement algorithm? In general, we want the one with the lowest page-fault rate.
Belady's Anonmaly
This most unexpected result is known as Belady's anomaly: for some page-replacement algorithms, the page-fault rate may increase as the number of allocated frames increases. We would expect that giving more memory to a process would improve its performance. In some early research, investigators noticed that this assumption was not always true. Belady's anomaly was discovered as a result
working set strategy
This working-set strategy prevents thrashing while keeping the degree of multiprogramming as high as possible. Thus, it optimizes CPU utilization. The difficulty with the working-set model is keeping track of the working set. The working-set window is a moving window. At each memory reference, a new reference appears at one end, and the oldest reference drops off the other end. A page is in the working set if it is referenced anywhere in the working-set window.
Memory mapped I/O
To allow more convenient access to I/O devices, many computer architectures provide memory-mapped I/O. In this case, ranges of memory addresses are set aside and are mapped to the device registers. Reads and writes to these memory addresses cause the data to be transferred to and from the device registers. This method is appropriate for devices that have fast response times, such as video controllers. IBM/PC
Shared memory in windows API code and function calls
To establish a memory-mapped file, a process first opens the file to be mapped with the CreateFile() function, which returns a HANDLE to the opened file. The process then creates a mapping of this file HANDLE using the CreateFileMapping() function. Once the file mapping is established, the process then establishes a view of the mapped file in its virtual address space with the MapViewOfFile() function. The view of the mapped file represents the portion of the file being mapped in the virtual address space of the process —the entire file or only a portion of it may be mapped.
proportional allocation,
To solve this problem, we can use proportional allocation, in which we allocate available memory to each process according to its size.
Some versions of the _______ system use this method in conjunction with the second-chance algorithm
UNIX
Which operating systems use page buffering algorithms with the FIFO replacement algorithm
VAX/VMS
Sparse address space
Virtual address spaces that include holes are known as sparse address spaces. Using a sparse address space is beneficial because the holes can be filled as the stack or heap segments grow or if we wish to dynamically link libraries (or possibly other shared objects) during program execution
Enhanced second chance algorithm
We can enhance the second-chance algorithm by considering the reference bit and the modify bit (described in Section 9.4.1) as an ordered pair. With these two bits, we have the following four possible classes: 1. (0, 0) neither recently used nor modified—best page to replace 2. (0, 1) not recently used but modified—not quite as good, because the page will need to be written out before replacement 3. (1, 0) recently used but clean—probably will be used again soon 4. (1, 1) recently used and modified—probably will be used again soon, and the page will be need to be written out to disk before it can be replaced
local replacement algorithm (or priority replacement algorithm)
We can limit the effects of thrashing by using a local replacement algorithm (or priority replacement algorithm). With local replacement, if one process starts thrashing, it cannot steal frames from another process and cause the latter to thrash as well.
Modify Bit (dirty bit)
We can reduce this overhead by using a modify bit (or dirty bit). When this scheme is used, each page or frame has a modify bit associated with it in the hardware. The modify bit for a page is set by the hardware whenever any byte in the page is written into, indicating that the page has been modified. When we select a page for replacement, we examine its modify bit. If the bit is set, we know that the page has been modified since it was read in from the disk. In this case, we must write the page to the disk. If the modify bit is not set, however, the page has not been modified since it was read into memory. In this case, we need not write the memory page to the disk: it is already there.
automatic working-set trimming
When the amount of free memory falls below the threshold, the virtual memory manager uses a tactic known as automatic working-set trimming to restore the value above the threshold. Automatic working-set trimming works by evaluating the number of pages allocated to processes. If a process has been allocated more pages than its working-set minimum, the virtual memory manager removes pages until the process reaches its working-set minimum. windows
categories for process of page placement
With multiple processes competing for frames, we can classify page-replacement algorithms into two broad categories: global replacement and local replacement.
lgroups
a process with many running threads may end up with those threads scheduled on many different system boards. How is the memory to be allocated in this case? Solaris solves the problem by creating lgroups (for "latency groups") in the kernel. Each lgroup gathers together close CPUs and memory. In fact, there is a hierarchy of lgroups based on the amount of latency between the groups. Solaris tries to schedule all threads of a process and allocate all memory of a process within an lgroup. If that is not possible, it picks nearby lgroups for the rest of the resources needed.
Windows implements virtual memory using demand paging with
clustering
What does fork() do?
creates a child process that is a duplicate of its parent. Traditionally, fork() worked by creating a copy of the parent's address space for the child, duplicating the pages belonging to the parent. creates a process
What does exec() do?
executes a process but may be unnecessary because of copy-on write
We must solve two major problems to implement demand paging: we must develop a
frame-allocation algorithm and a page-replacement algorithm That is, if we have multiple processes in memory, we must decide how many frames to allocate to each process; and when page replacement is required, we must select the frames that are to be replaced.
Because they keep information about which virtual memory page is stored in each physical frame, inverted page tables reduce the amount of physical memory needed to store this information. However, the __________ no longer contains complete information about the logical address space of a process, and that information is required if a referenced page is not currently in memory.
inverted page table
Virtual memory
involves the separation of logical memory as perceived by users from physical memory. This separation allows an extremely large virtual memory to be provided for programmers when only a smaller physical memory is available
When demand paging is used, we sometimes need to allow some of the pages to be________ in memory
locked
Associated with this list of free pages is a parameter—_______—that represents a threshold to begin paging. solaris
lotsfree
If we increase our degree of multiprogramming, we are ________ memory. If we run six processes, each of which is ten pages in size but actually uses only five pages, we have higher CPU utilization and throughput, with ten frames to spare.
over-allocating
A database process may want to manage a chunk of memory, for example, moving blocks between disk and memory itself because it has the best knowledge of how it is going to use its data. Such _________ of pages in memory is fairly common, and most operating systems have a system call allowing an application to request that a region of its logical address space be pinned. Note that this feature could be abused and could cause stress on the memory-management algorithms.
pinning
Many operating systems provide a ______ of free pages for such requests. These free pages are typically allocated when the stack or heap for a process must expand or when there are copy-on-write pages to be managed
pool
We evaluate an algorithm by running it on a particular string of memory references and computing the number of page faults. The string of memory references is called a ___________. We can generate____________ artificially (by using a random-number generator, for example), or we can trace a given system and record the address of each memory reference.
reference strings
virtual address space of a process
refers to the logical (or virtual) view of how a process is stored in memory. Typically, this view is that a process begins at a certain logical address—say, address 0—and exists in contiguous memory
With a smaller page size, then, we have better ________, allowing us to isolate only the memory that is actually needed. With a larger page size, we must allocate and transfer not only what is needed but also anything else that happens to be in the page, whether it is needed or not. Thus, a smaller page size should result in less I/O and less total allocated memory.
resolution
Memory is better utilized with _______ pages, however. If a process is allocated memory starting at location 00000 and continuing until it has as much as it needs, it probably will not end exactly on a page boundary. Thus, a part of the final page must be allocated (because pages are the units of allocation) but will be unused (creating internal fragmentation).
smaller
Let's consider a scenario in which the kernel requests memory from the slab allocator for an object representing a process descriptor. In Linux systems, a process descriptor is of the type _____________, which requires approximately 1.7 KB of memory. When the Linux kernel creates a new task, it requests the necessary memory for the _______________ object from its cache. The cache will fulfill the request using a ___________object that has already been allocated in a slab and is marked as free.
struct task_struct
Important for allocating frames
we always want the minimum number of frames to be availble
copy-on-write
which works by allowing the parent and child processes initially to share the same pages. These shared pages are marked as copy-on-write pages, meaning that if either process writes to a shared page, a copy of the shared page is created.
