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Linux Memory Issues

Linux Memory Issues. Introduction. Some Architecture History. 8080 (late-1970s) 16-bit address (64-KB) 8086 (early-1980s) 20-bit address (1-MB) 80286 (mid-’80s) 24-bit address (16-MB) 80386 (late-’80s) 32-bit address (4-GB) 80686 (late-’90s) 36-bit address (64-GB). ‘Backward Compatibility’.

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Linux Memory Issues

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  1. Linux Memory Issues Introduction

  2. Some Architecture History • 8080 (late-1970s) 16-bit address (64-KB) • 8086 (early-1980s) 20-bit address (1-MB) • 80286 (mid-’80s) 24-bit address (16-MB) • 80386 (late-’80s) 32-bit address (4-GB) • 80686 (late-’90s) 36-bit address (64-GB)

  3. ‘Backward Compatibility’ • Most buyers resist ‘early obsolescence’ • New processors need to run old programs • Early design-decisions leave their legacy • 8086 could run recompiled 8080 programs • 80x86 can still run most 8086 applications • Win95/98 could run most MS-DOS apps • But a few areas of incompatibility existed

  4. Linux must accommodate legacy • Legacy elements: hardware and firmware • CPU: reset-address and interrupt vectors • ROM-BIOS: data area and boot location • Display Controllers: VRAM & video BIOS • Support chipsets: 15MB ‘Memory Window’ • SMP: Local and I/O APIC

  5. Other CPU Architectures • Besides IA-32, Linux runs on other CPUs (e.g., PowerPC, MC68000, IBM360, Sparc) • So must accommodate their differences • Memory-Mapped I/O • 64-bit address-buses • Non-Uniform Memory Access (NUMA)

  6. Nodes, Zones, and Pages • Nodes: to accommodate NUMA systems • 80x86 doesn’t support NUMA • So on 80x86 Linux uses just one ‘node’ • Zones: to accommodate distinct regions • Three zones on 80x86: • ZONE_DMA (memory below 16-MB) • ZONE_NORMAL (from 16-MB to 896-MB) • ZONE_HIGHMEM (memory above 896-MB)

  7. Zones divided into Pages • 80x86 supports 4-KB page-frames • Linux uses an array of ‘page descriptors’ • Array of page descriptors: ‘mem_map’ • physical memory is ‘mapped’ by CPU

  8. How 80x86 Addresses RAM • Two-stages: ‘segmentation’ plus ‘paging’ • First: logical address  linear address • Then: linear address  physical address

  9. Logical to Linear virtual address-space segment-register operand-offset selector global descriptor table memory segment base-address descriptor and segment-limit GDTR

  10. Segment Descriptor Format 31 0 Limit [19..16 ] Base[ 31..24 ] Base[ 23..16 ] Base[ 15..0 ] Limit[ 15..0 ]

  11. Linear to Physical linear address physical address-space dir-index table-index offset page table page frame page directory CR3

  12. Page-Size Extensions • 80686 can map either 4KB or 4MB pages • With 4MB pages: middle table is omitted • Entire 4GB address-space is subdivided into 1024 4MB-pages

  13. Linear to Physical linear address physical address-space dir-index offset page directory page frame CR3 4-MB page-frames

  14. PageTable Entry Format 31 12 11 0 Frame Address Frame attributes Some Frame Attributes: P : (present=1, not-present=0) R/W : (writable=1, readonly=0) U/S : (user=1, supervisor=0) D : (dirty=1, clean=0) A : (accessed=1, not-accessed=0) S : (size 4MB = 1, size 4KB = 0)

  15. Visualizing Memory • Virtual address-space (4-GB) • subdivided into 4MB pages (1024 pels) • Text characters: 16 rows by 64 columns • Physical address-space (1-GB) • subdivided into 4KB pages (262,144 pels) • Graphics pixels: 512 rows by 512 columns

  16. Two Visualizations • ‘pgdir.c’ • ‘zones.c’

  17. Virtual Memory Visualization • Shows which addresses are ‘mapped’ • Display granularity is 4MB • Data is gotten from task’s page-directory • Page-Directory location is in register CR3 • Legend: ‘-’ = frame not mapped ‘3’ = r/w by supervisor ‘7’ = r/w by user

  18. Physical Memory Visualization • Shows relative sizes of the three ‘zones’ • Display granularity is 4KB • Data is based on ‘num_physpages’ • And on the ‘Zone-Boundary’ constants light-green: ZONE_DMA dark-green: ZONE_NORMAL dark-brown: ZONE_HIGHMEM

  19. Where are process descriptors? • White pages show ‘process descriptors’ • Data is taken from the kernel’s tasklist • Searching tasklist noticibly slows drawing

  20. Visualization Critique • ‘Double-drawing’ is an annoyance • No title to say what we’re seeing • No legend to explain the color usage • No indication of granularity or orientation • Display is tightly tied to hardware setup

  21. Future Questions • Where are the ‘page descriptors’? • How much memory used by ‘mem_map’? • Where are user-space memory-regions? • Where are the device-driver modules? • How much memory is really allocated? • Has the ‘free’ memory gotten fragmented?

  22. More future questions • Where are the ‘accessed’ pages? • Where are the ‘dirty’ pages? • How big is the video frame-buffer? • And where is it mapped? • How big are device-memory regions? • And where are they mapped?

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