Silberschatz and Galvin  Operating System Concepts Workshop 3: Agenda Homework review Study group presentations Lecture – memory concepts Group activity – memory concepts (45 m) Lecture – virtual memory Group activity – memory management algorithms Midterm review Summary and next week preview

Silberschatz and Galvin  Operating System Concepts Workshop 3: Homework Chapter 8 –8.1 –8.2 Chapter 9 –9.1. –9.7

Silberschatz and Galvin  Operating System Concepts Workshop 3: Study Group Project

Silberschatz and Galvin  Operating System Concepts Module 8: Memory Management Background Logical versus Physical Address Space Swapping Contiguous Allocation Paging (Segmentation) (Segmentation with Paging)

Silberschatz and Galvin  Operating System Concepts Background Program must be brought into memory and placed within a process for it to be executed. Input queue – collection of processes on the disk that are waiting to be brought into memory for execution. User programs go through several steps before being executed.

Silberschatz and Galvin  Operating System Concepts Instructions and Data Memory Addresses Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes. Load time: Must generate relocatable code if memory location is not known at compile time. Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps (e.g., base and limit registers). See page 241 in textbook Can be set up at three different points.

Silberschatz and Galvin  Operating System Concepts Address Binding Dynamic loading Dynamic linking (ex: DLL) Overlays (rarely used today)

Silberschatz and Galvin  Operating System Concepts Logical vs. Physical Address Space Logical address – generated by the CPU; also referred to as virtual address. Physical address – address seen by the memory unit. Logical and physical addresses are the same in compile-time and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme.

Silberschatz and Galvin  Operating System Concepts Memory-Management Unit ( MMU ) Hardware device that maps virtual to physical address. In MMU scheme, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory. The user program deals with logical addresses; it never sees the real physical addresses.

Silberschatz and Galvin  Operating System Concepts Swapping A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution. Backing store – fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images. Roll out, roll in – used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed. Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped. Modified versions of swapping are found on many systems, i.e., UNIX and Microsoft Windows.

Silberschatz and Galvin  Operating System Concepts Schematic View of Swapping

Silberschatz and Galvin  Operating System Concepts Contiguous Memory Allocation Multiple-partition allocation –Hole – block of available memory; holes of various size are scattered throughout memory. –When a process arrives, it is allocated memory from a hole large enough to accommodate it. –Operating system maintains information about: a) allocated partitions b) free partitions (hole) OS process 5 process 8 process 2 OS process 5 process 2 OS process 5 process 2 OS process 5 process 9 process 2 process 9 process 10

Silberschatz and Galvin  Operating System Concepts Dynamic Storage-Allocation Problem First-fit: Allocate the first hole that is big enough. Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size. Produces the smallest leftover hole. Worst-fit: Allocate the largest hole; must also search entire list. Produces the largest leftover hole. How to satisfy a request of size n from a list of free holes. First-fit and best-fit better than worst-fit in terms of speed and storage utilization.

Silberschatz and Galvin  Operating System Concepts Fragmentation External fragmentation – total memory space exists to satisfy a request, but it is not contiguous. Internal fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used. Reduce external fragmentation by compaction –Shuffle memory contents to place all free memory together in one large block.

Silberschatz and Galvin  Operating System Concepts Paging Logical address space of a process can be noncontiguous; process is allocated physical memory whenever the latter is available. Divide physical memory into fixed-sized blocks called frames (size is power of 2, between 512 bytes and 8192 bytes). Divide logical memory into blocks of same size called pages. Keep track of all free frames. To run a program of size n pages, need to find n free frames and load program. Set up a page table to translate logical to physical addresses. Internal fragmentation.

Silberschatz and Galvin  Operating System Concepts 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.

Silberschatz and Galvin  Operating System Concepts Address Translation Architecture

Silberschatz and Galvin  Operating System Concepts Paging Example

Silberschatz and Galvin  Operating System Concepts Implementation of Page Table Page table is kept in main memory. Page-table base register (PTBR) points to the page table. Page-table length register (PRLR) indicates size of the page table. In this scheme every data/instruction access requires two memory accesses. One for the page table and one for the data/instruction. The two memory access problem can be solved by the use of a special fast-lookup hardware cache called associative registers or translation look-aside buffers (TLBs) Effective access time depends on hit ratio

Silberschatz and Galvin  Operating System Concepts Memory Protection Memory protection implemented by associating protection bit with each frame. Valid-invalid bit attached to each entry in the page table: –“valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page. –“invalid” indicates that the page is not in the process’ logical address space.

Silberschatz and Galvin  Operating System Concepts Shared Pages Shared code –One copy of read-only (reentrant) code shared among processes (i.e., text editors, compilers, window systems). –Shared code must appear in same location in the logical address space of all processes. Private code and data –Each process keeps a separate copy of the code and data. –The pages for the private code and data can appear anywhere in the logical address space.

Silberschatz and Galvin  Operating System Concepts Segmentation user spacephysical memory space

Silberschatz and Galvin  Operating System Concepts Segmentation Architecture Logical address consists of a two part: segment-number, offset, Segment table – maps two-dimensional physical addresses; each table entry has: –base – contains the starting physical address where the segments reside in memory. –limit – specifies the length of the segment. Segment-table base register (STBR) points to the segment table’s location in memory. Segment-table length register (STLR) indicates number of segments used by a program;

Silberschatz and Galvin  Operating System Concepts Segmentation Architecture (Cont.) Relocation. –dynamic –by segment table Sharing. –shared segments –same segment number Allocation. –first fit/best fit –external fragmentation

Silberschatz and Galvin  Operating System Concepts Segmentation Architecture (Cont.) Protection. With each entry in segment table associate: –validation bit = 0  illegal segment –read/write/execute privileges Protection bits associated with segments; code sharing occurs at segment level. Segments can vary in length A segmentation example is shown in the following diagram

Silberschatz and Galvin  Operating System Concepts Sharing of segments

Silberschatz and Galvin  Operating System Concepts Segmentation with Paging – Intel 386 As shown in the following diagram, the Intel 386 uses segmentation with paging for memory management with a two- level paging scheme.

Silberschatz and Galvin  Operating System Concepts Intel address translation

Silberschatz and Galvin  Operating System Concepts Comparing Memory-Management Strategies Hardware support Performance Fragmentation Relocation Swapping Sharing Protection

Silberschatz and Galvin  Operating System Concepts Memory Concepts: Group Activity

Silberschatz and Galvin  Operating System Concepts Module 9: Virtual Memory Background Demand Paging Performance of Demand Paging Page Replacement Page-Replacement Algorithms Allocation of Frames Thrashing Other Considerations Demand Segmenation

Silberschatz and Galvin  Operating System Concepts Background Virtual memory – separation of user logical memory from physical memory. –Only part of the program needs to be in memory for execution. –Logical address space can therefore be much larger than physical address space. –Need to allow pages to be swapped in and out. Virtual memory can be implemented via: –Demand paging –Demand segmentation

Silberschatz and Galvin  Operating System Concepts Demand Paging Bring a page into memory only when it is needed. –Less I/O needed –Less memory needed –Faster response –More users Page is needed  reference to it –invalid reference  abort –not-in-memory  bring to memory

Silberschatz and Galvin  Operating System Concepts Valid-Invalid Bit With each page table entry a valid–invalid bit is associated (1  in-memory, 0  not-in-memory) Initially valid–invalid but is set to 0 on all entries. Example of a page table snapshot. During address translation, if valid–invalid bit in page table entry is 0  page fault  Frame #valid-invalid bit page table

Silberschatz and Galvin  Operating System Concepts Page Fault If there is ever a reference to a page, first reference will trap to OS  page fault OS looks at another table to decide: –Invalid reference  abort. –Just not in memory. Get empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction

Silberschatz and Galvin  Operating System Concepts What happens if there is no free frame? Page replacement – find some page in memory, but not really in use, swap it out. –algorithm –performance – want an method which will result in minimum number of page faults.

Silberschatz and Galvin  Operating System Concepts Page Replacement Prevent over-allocation of memory by modifying page-fault service routine to include page replacement. Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk. Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory.

Silberschatz and Galvin  Operating System Concepts Page-Replacement Algorithms Want lowest page-fault rate. Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string. In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5.

Silberschatz and Galvin  Operating System Concepts First-In-First-Out (FIFO) Algorithm Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 3 frames (3 pages can be in memory at a time per process) Use queue or timestamp 4 frames page faults page faults 4 43

Silberschatz and Galvin  Operating System Concepts Optimal Algorithm Replace page that will not be used for longest period of time. 4 frames example 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 How do you know this? Used for measuring how well your algorithm performs page faults 4 5

Silberschatz and Galvin  Operating System Concepts Least Recently Used (LRU) Algorithm Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Queue, timestamp Counter implementation –Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter. –When a page needs to be changed, look at the counters to determine which are to change

Silberschatz and Galvin  Operating System Concepts LRU Algorithm (Cont.) Stack implementation – keep a stack of page numbers in a double link form: –Page referenced:  move it to the top  requires 6 pointers to be changed –No search for replacement

Silberschatz and Galvin  Operating System Concepts Counting Algorithms Keep a counter of the number of references that have been made to each page. LFU Algorithm: replaces page with smallest count. MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used.

Silberschatz and Galvin  Operating System Concepts Allocation of Frames Each process needs minimum number of pages. Two major allocation schemes. –fixed allocation  Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages.  Proportional allocation – Allocate according to the size of process. –priority allocation: If process P i generates a page fault:  select for replacement a frame from a process with lower priority number.  select for replacement one of its frames.

Silberschatz and Galvin  Operating System Concepts Global vs. Local Allocation Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another. Local replacement – each process selects from only its own set of allocated frames.

Silberschatz and Galvin  Operating System Concepts 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.

Silberschatz and Galvin  Operating System Concepts Thrashing Diagram Why does paging work? Locality model –Process migrates from one locality to another. –Localities may overlap. Why does thrashing occur?  size of locality > total memory size

Silberschatz and Galvin  Operating System Concepts Working Set Page Faulting Establish “acceptable” page-fault rate. –If actual rate too low, process loses frame. –If actual rate too high, process gains frame.

Silberschatz and Galvin  Operating System Concepts Demand Segmentation Used when insufficient hardware to implement demand paging. OS/2 allocates memory in segments, which it keeps track of through segment descriptors Segment descriptor contains a valid bit to indicate whether the segment is currently in memory. –If segment is in main memory, access continues, –If not in memory, segment fault.

Silberschatz and Galvin  Operating System Concepts Virtual Memory: Group Activity (30m)

Silberschatz and Galvin  Operating System Concepts Next Week: WS4 Preview Homework review Collect study group papers Logical and physical file concepts Group activity on above File management Group activity on file system performance Summary and preview Mid term exam! (1 hr) – see Website for sample questions

Silberschatz and Galvin  Operating System Concepts Study group milestone: WS4