Gestion de la mémoire
Allocation d'espace pour l'accroissement de la pile et d'un segment de données. Modèle de mémoire fusionné (mélange de pagination et segmentation).
Systèmes dExploitation - Gestion de la mémoire
contigüe. Monoprogrammation. Multiprogrammation. Pagination. Segmentation. Systèmes d'Exploitation. Gestion de la mémoire. Didier Verna didier@lrde.epita.fr.
Module 7 Gestion de la mémoire
Partitions dynamiques: fragmentation externe qui conduit au besoin de compression. ?. Segmentation sans pagination: pas de fragmentation interne mais
Smart card introduction
La segmentation est visible au programmeur mais la pagination ne l'est pas. • Le segment est une unité logique de protection et partage tandis que la page ne l
Chapitre 4
Programmation du Noyau Linux. Olivier Dalle. Mémoire. Translation d'adresses. Unité de Segmentation. Unité de Pagination. Programme. Adresse Logique.
La gestion de la mémoire
segmentation de l'espace d'adressage des programmes pour les raisons suivantes La pagination consiste à diviser la mémoire et les processus en blocs de ...
Gestion de la mémoire
En combinant segmentation et pagination. MULTICS combine les avantages des deux techniques en s'affranchissant des principaux défauts qu'ils ont : fragmentation
La gestion de la mémoire
? la segmentation : les programmes sont découpés en parcelles ayant des longueurs variables appelées. «segments». ? la pagination : elle consiste à diviser la
1. Segmentation 2. Pagination
Segmentation. On considère la table des segments suivante pour un processus P1 : Index Base Limite Calculez les adresses physiques correspondant aux
Page Segmentation using Visual Adjacency Analysis - arXivorg
Page segmentation is a web page analysis process that divides a page into cohesive segments such as sidebars headers and footers Current page segmentation approaches use either the DOM textual content or rendering style information of the page However these approaches have a number of drawbacks such as a large number of
Page Segmentation Using Convolutional Neural Network and - Spr
Paging Segmentation Page size Page size is de?ned by the hardware Often of the form 2n between 512 bytes and 16 MB typically 4-8 KB page number page offset p d m ?n n Must be carefully chosen: too large means more internal fragmentation too small means too much overhead in paging management and processing
W4118: segmentation and paging - Department of Computer
Implementation of page table Page table is stored in memory Page table base register (PTBR) points to the base of page table • x86: cr3 OS stores base in process control block (PCB) OS switches PTBR on each context switch Problem: each data/instruction access requires two memory accesses Extra memory access for page table 21
What is page segmentation?
It is aimed at segmenting the entire document images into small regions with homogeneous contents and high level semantics such as paragraphs, titles, lists, tables and figures. Page segmentation is a prerequisite for many following applications such as text line transcription, table structure analysis and figure classification, etc.
How effective is page segmentation using convolutional neural network and graphical model?
In this paper, we propose an effective method for page segmentation using convolutional neural network (CNN) and graphical model, where the CNN is powerful for extracting visual features and the graphical model explores the relationship (spatial context) between visual primitives and regions.
What are the advantages of paging a segment?
Given to segmentation unit Linear address given to paging unit Check: permissions Segment sharing Easier to relocate segment than entire program Avoids allocating unused memory Flexible protection Efficient translation Segments have variable lengths how to fit? Eliminate fragmentation due to large segments
What are the principles of segmentation?
Segmentation A. Divide an object into independent parts. B. Make an object easy to disassemble. C. Increase the degree of fragmentation or segmentation. Principle 2. Taking out A. Separate an interfering part or property from an object, or single out the only necessary part. Principle 3.
Paging & Segmentation
Frédéric Haziza
Department of Computer Systems
Uppsala University
Spring 2007
PagingSegmentation
Outline
1Paging
Implementation
Protection
Sharing
2Segmentation
SetupImplementation
PagingSegmentation
Definition
Paging
Memory-management technique that permits the physical address space of a process to be non-contiguousAvoids external fragmentation
PagingSegmentation
Basics
New setup:
Physical memoryis divided intoframes
Logical memoryis divided intopages
A frame
has the same size as a page is a place where a (logical) page can be (physically) placedPagingSegmentation
Logical to Physical
Translation
Logical address space→page table per process→physical address spacePage 0
Page 1
Page 2
Page 3
Logical memory
01 14 2337
Page table
↓Frame number 01Page 0
23Page 2
4Page 1
5 67Page 3
Physical memory
Note: Similar torelocation register per page
PagingSegmentation
How does it work?
See black board...
...take your notes...PagingSegmentation
Page size
Page size is defined by the hardware.
Often of the form 2n, between 512 bytes and 16 MB, typically 4-8 KB page number page offset pd m-nn Must be carefully chosen: too large means more internal fragmentation, too small means too much overhead in paging management and processing.If page table entry = 4 byte (= 32 bits), 2
32page frames.
If page frame = 4 KB?can address 244bytes (=16 TB) of physycial memory.PagingSegmentation
Characteristics
The current page table (address) is saved and restored when doing a context switch.The OS also has a
frame tablecontaining information about all frames, e.g. whether they are free or which process(es) is/are using itPagingSegmentation
Characteristics
No external fragmentation
all frames (physical memory) can be used by processesPossible internal fragmentation
on average 1/2 page per process (the last page) The physical memoryused by a processis no longer contiguous The logical memory of a process is still contiguousThe logical and physical addresses are separated
the process does not see the translation or the difference tohaving physical memoryPagingSegmentation
Implementation
Implementation must be done in hardware for efficiency. All process address references go through address translation: memory not allocated to the process can not be touched by the processes.PagingSegmentation
Small page tables
Example (PDP-11)
16-bit addresses with 8 KB page size
?8 pages kept in dedicated registers, updated (like all registers) at context switch.PagingSegmentation
Larger page tables
Modern systems can have millions of pages.
?Move the page table to main memory, addressed by PageTable Base Register
(PTBR) which is updated at context switch. Problem: Each logical memory reference generates two physical references (one for page table, one for real access).Solution: Can we remember one maybe?
Translation Look-aside Buffer (TLB): fast cache with associative memory.(Keeps only recently used Page Table entries) Associative memory looks up the key(page number)in parallel with cache - very fast, very expensive.PagingSegmentation
Very large page tables
two-level page tablesSplit the page number part in two:
outer and inner page number/tables.Each page table will now be 1K(easier to handle)
Other solutions:
64-bit architectures require more complexity: multi-level
page tables or hashed page tables.Inverted page tables
(See section 8.5 page 297)PagingSegmentation
Protection
Simply add a protection bits to the page table entry.Typically: Valid/Invalid and Read/Write/Execute
Lookup in parallel: Neglectable overhead
Note that we created a problem due to internal fragmentationPagingSegmentation
Shared pages
Example
40 users, each of whom executes a text editor
150 KB of code
50 KB of data
?We need 8 000 MBIf one copy of the text editor: Need 2 150 KB
If code is
reentrant, it can be shared.Non self-modifying code
code state (variables, temp results) should be local to each process ?shared code pages should not be writable.PagingSegmentation
Shared pages
Example
Compiler, Window System, run-time libraries, database systems,...Used for shared data (Cf Process Communication)
PagingSegmentation
Segmentation
User preference
View memory as a collection of variable-sized segments, rather than a linear array of bytes Separate segments for different types of memory content: main program program libraries constants stack objects symbol table and each segment can have its own protection, grow independently, etc...PagingSegmentation
Segmentation
User preference
Elements are identified within a segment by theiroffsetfrom the beginning of the segmentExample
the 1ststatement of the program the 7thentry of the stack the 5thinstruction of function foo()PagingSegmentation
Segmentation
Segmentation
Memory-management scheme that supports this user view of memory Logical address space is a collection of segments. name lengthLogical address
Pair:
Note: paging needs one address, implicitly split by hardware PagingSegmentation
Address translation
User can now refer to objects in the program by a
two-dimenseional address But the actual physical memory is still a one-dimensional sequence of bytesMust find a mapping from logical to physical
Address translation is done similar to the old memory partitions, but with baseandlimitper segment, stored in a segment table.PagingSegmentation
Characteristics
Segmentation
"paging with variable page size"Advantages:
memory protectionadded to segment tablelike paging sharingof memorysimilar to paging (but per area rather than per page)Drawbacks:
allocation algorithms as for memory partitions external fragmentation, back to compaction problem...Solution: combine segmentation and paging!
Read section 8.7 on Linux & Intel Pentium systems
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