Operating Systems Principles. Segmentation & Shared Memory

Transcription

1 2 Operating Systems Principles Segmentation & Memory Steve Goddard 1 Memory Management Name spaces Schemes to date have considered only a single name space per process»why? (Why not have multiple spaces?) 2 n 1 P s VAS Heap Run- Time Stack Text Page 2 Page 1

2 Multiple Name s Example Protection/Fault isolation & sharing 2 n 1 Heap Run- Time Stack Text 2 n 1 1 Text 2 n n 3 1 Run- Time Stack 2 n 4 1 Heap 2 n 6 1 Libraries 2 n 5 1 User 3 Supporting Multiple Name s Segmentation n 2 Segment a memory object» A virtual address space A process now addresses objects a pair (s, addr)» s segment number» addr a virtual address within an object offset virtual address as before: (p, o) A virtual address is now a triple (s, p, o) Segment + register scheme: s n 1 p o n Single address scheme: s p o 4 Page 3 Page 4

3 Segmentation address translation One additional level of indirection The segment table CPU Memory Memory Sharing of procedures and data Why share? STBR s p o es page table + Physical es f f 11 9 o Levels of sharing» source code» object code» executable binaries s p Process P s Segment Segment S s Page 5 6 Page 5 Page 6

4 Memory Requirements for sharing Sharing in Non-Segmented/Paged Systems Trading-off protection for sharing Sharing s cannot modify their own code» Pure procedures Serially sharable code» s that are self-initializing» s that contain their own global data Concurrently sharable code» s must be reentrant Run-time stack cannot be shared Sharing can no longer be allocated contiguously» Base + Limit register schemes not applicable Loaders must determine if copies of shared objects already exist in memory» And if so where is it? Similar problems with garbage collection Process B Process A cannot (typically) contain pointers 7 8 Page 7 Page 8

5 Sharing in Paged Systems sharing code must reside in the same place in all process s VAS shared proc() begin loop x := 6 : : end while end proc (p 4,o) store x,6 p 4 p 3 p 2 p 1 jmp (p 4,o) 9 Sharing in Paged Systems sharing Thus shared pages must have the same page number in all process s VAS (p,o) jmp (p,o) p f = Page p f = Page (p,o) Physical Memory jmp (p,o) 1 Page 9 Page 1

6 Sharing in Paged Systems sharing Raw data pages can appear anywhere in a process s VAS Sharing in Paged Systems sharing data containing pointers must also reside in the same place in all process s VAS shared var foo : array[1..n] of integer Page Page Physical Memory 11 (p 2,o) shared var foo : char bar : ptr to char p 2 p 1 foo: a bar:(p 2,o) 12 Page 11 Page 12

7 Sharing in Paged Systems sharing If the data contains pointers treat pages the same as shared code foo: a bar:(p 2,o) shared var foo : char bar : ptr to char f = 1 Page f = 1 bar: (p 2,o) Physical Memory (p 2,o): a Sharing in Segmented Systems The simplest form of sharing Sharing portion of a process s segment space library seg heap stack data seg code seg Segment Libraries Heap Text Run- Time Stack Page Page 13 Page 14

8 Sharing in Segmented Systems Sharing segments Dynamic Linking & Sharing A key technology for realizing shared memory Unit of sharing a segment rather than a set of pages» Processes need only agree on 1 number rather than a sequence of numbers» If segments are paged then the page tables are automatically shared shared seg heap seg s code seg Segment shared seg heap seg s code seg Segment f = 2 f = f = 1 Segment Page (p,o) 57 jmp (p,o) Concept» Dynamically bind to shared modules at run-time Advantages» On demand linking ensures that only those modules that are actually used will be loaded» Possibly allows modules to be replaced or upgraded at run-time Disadvantages» Possible performance penalty at run-time for invoking new modules» Overhead of indirect/interpretive access» Complexity of the overall scheme Page 15 Page 16