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OS Fall02 Virtual Memory: Page Replacement Operating Systems Fall 2002.

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Presentation on theme: "OS Fall02 Virtual Memory: Page Replacement Operating Systems Fall 2002."— Presentation transcript:

1 OS Fall02 Virtual Memory: Page Replacement Operating Systems Fall 2002

2 OS Fall02 Realizing Virtual Memory Hardware support Memory Management Unit (MMU): address translation, bits, interrupts Operating system support Page replacement policy Resident set management Load control degree of multiprogramming

3 OS Fall02 Page Replacement Policy Resident set maintenance Fixed or variable allocation Per-process or global replacement Page replacement problem A fixed number of frames, M, is used to map the process virtual memory pages Which page should be replaced when a page fault occurs and all M frames are occupied?

4 OS Fall02 Requirements and Metrics Workload: a sequence of virtual memory references (page numbers) Page fault rate = #page faults/#memory references Minimize the page fault rate for workloads obeying the principle of locality Keep hardware/software overhead as small as possible

5 OS Fall02 Algorithms Optimal (OPT) Least Recently Used (LRU) First-In-First-Out (FIFO) Clock

6 OS Fall02 Optimal Policy (OPT) Replace the page which will be referenced again in the most remote future Impossible to implement Why? Serves as a baseline for other algorithms

7 OS Fall02 Least Recently Used (LRU) Replace the page that has not been referenced for the longest time The best approximation of OPT for the locality constrained workloads Possible to implement Infeasible as the overhead is high Why?

8 OS Fall02 First-In-First-Out (FIFO) Page frames are organized in a circular buffer with a roving pointer Pages are replaced in round-robin style When page fault occur, replace the page to which the pointer points to Simple to implement, low overhead High page fault rate, prone to anomalous behavior

9 OS Fall02 Clock (second chance) Similar to FIFO but takes page usage into account Circular buffer + page use bit When a page is referenced: set use_bit=1 When a page fault occur: For each page: if use_bit==1: give page a second chance: use_bit=0; continue scan; if use_bit==0: replace the page

10 OS Fall02 Example: Page 727 is needed n... Page 9 use = 1 Page 19 use = 1 Page 1 use = 0 Page 45 use = 1 Page 191 use = 1 Page 556 use = 0 Page 13 use = 0 Page 67 use = 1 Page 33 use = 1 Page 222 use = 0 next frame pointer

11 OS Fall02 After replacement n... Page 9 use = 1 Page 19 use = 1 Page 1 use = 0 Page 45 use = 0 Page 191 use = 0 Page 727 use = 0 Page 13 use = 0 Page 67 use = 1 Page 33 use = 1 Page 222 use = 0 next frame pointer

12 OS Fall02 Example of all algorithms

13 OS Fall02 LRU and non-local workloads Workload: … Typical for array based applications What is the page fault rate for M=1, …,5? A possible alternative is to use a Most Recently Use (MRU) replacement policy

14 OS Fall02 Belady s Anomaly It is reasonable to expect that regardless of a workload, the number of page faults should not increase if we add more frames: not true for the FIFO policy: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4,

15 OS Fall02 Algorithm comparison

16 OS Fall02 Clock algorithm with 2 bits Use modified bit to evict unmodified (clean) pages in preference over modified (dirty) pages Four classes: u=0; m=0: not recently used, clean u=0; m=1: not recently used, dirty u=1; m=0: recently used, clean u=1; m=1: recently used, dirty

17 OS Fall02 First scan: look for (0,0) frame, do not change the use bit If (0,0) frame is found, replace it Second scan: look for (0,1) frame, set use bit to 0 in each frame bypassed If (0,1) frame is found, replace it If all failed, repeat the above procedure this time we will certainly find something Clock algorithm with 2 bits

18 OS Fall02 Page buffering Evicted pages are kept on two lists: free and modified page lists Pages are read into the frames on the free page list Pages are written to disk in large chunks from the modified page list If an evicted page is referenced, and it is still on one of the lists, it is made valid at a very low cost

19 OS Fall02 Resident set management With multiprogramming, a fixed number of memory frames are shared among multiple processes How should the frames be partitioned among the active processes? Resident set is the set of process pages currently allocated to the memory frames

20 OS Fall02 The working set model [Denning 68] Working set is the set of pages in the most recent page references Working set is an approximation of the program locality

21 OS Fall02 The working set strategy Monitor the working set for each currently active process Adjust the number of pages assigned to each process according to its working set size Monitoring working set is impractical The optimal value of is unknown and would vary

22 OS Fall02 Approximating the WS Global page replacement All memory frames are candidates for page eviction a faulting process may evict a page of other process Processes with larger WS are expanding whereas those with smaller WS are shrinking Problem: may unjustly reduce the WS of some processes Combine with page buffering

23 OS Fall02 Approximating the WS Local page replacement Only the memory frames of a faulting process are candidates for replacement Dynamically adjust the process allocation Page-Fault Frequency (PFF) algorithm

24 OS Fall02 Page-Fault Frequency (PFF) Approximate the page-fault frequency: Count all memory references for each active process When a page fault occurs, compare the current counter value with the previous page fault counter value for the faulting process If < F, expand the WS; Otherwise, shrink the WS by discarding pages with use_bit==0

25 OS Fall02 Multiprogramming level Too many processes in memory Thrashing, inability to run new processes The solution is swapping: save all the resident set of a process to the disk (swapping out) load the pages of another process instead (swapping in) Long-term and medium term scheduling decides which processes to swap in/out

26 OS Fall02 Long (medium) term scheduling Decision of which processes to swap out/in is based on The CPU usage Creating a balanced job mix with respect to I/O vs. CPU bound processes Two new process states: Ready swapped Blocked swapped

27 OS Fall02 UNIX process states running user running kernel ready user ready kernel blocked zombie sys. call interrupt schedule created return terminated wait for event event done schedule preempt interrupt ready swapped blocked swapped Swap out event done Swap out Swap in

28 OS Fall02 Segmentation with paging Segmentation simplifies protection and sharing, enforce modularity, but prone to external fragmentation Paging transparent, eliminates ext. fragmentation, allows for sophisticated memory management Segmentation and paging can be combined

29 OS Fall02 Address translation Main Memory Page Frame Offset Paging Page Table P# + Frame #Offset Seg Table Ptr + S # SegmentationProgram Segment Table Seg #Page #Offset

30 OS Fall02 Page size considerations Small page size better approximates locality large page tables inefficient disk transfer Large page size internal fragmentation Most modern architectures support a number of different page sizes a configurable system parameter

31 OS Fall02 Next: File system, disks, etc


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