Linux Memory Issues Introduction

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

‘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

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

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)

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)

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

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

Logical to Linear selector GDTR segment-registeroperand-offset virtual address-space memory segment global descriptor table base-address and segment-limit descriptor

Segment Descriptor Format Limit[ ] Base[ ] Base[ ]Base[ ] Limit [ ] 310

Linear to Physical physical address-space offsettable-index linear address CR3 dir-index page frame page directory page table

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

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

PageTable Entry Format Frame Address 0 31 Frame attributes 1112 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)

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

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

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

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

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

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

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?

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?