Linux Memory IssuesSome Architecture History‘Backward Compatibility’Linux must accommodate legacyOther CPU ArchitecturesNodes, Zones, and PagesZones divided into PagesHow 80x86 Addresses RAMLogical to LinearSegment Descriptor FormatLinear to PhysicalPage-Size ExtensionsSlide 13PageTable Entry FormatVisualizing MemoryTwo VisualizationsVirtual Memory VisualizationPhysical Memory VisualizationWhere are process descriptors?Visualization CritiqueFuture QuestionsMore future questionsLinux Memory IssuesIntroductionSome 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’•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 existedLinux 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 APICOther 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 CPUHow 80x86 Addresses RAM•Two-stages: ‘segmentation’ plus ‘paging’•First: logical address linear address•Then: linear address physical addressLogical to LinearselectorGDTRsegment-register operand-offsetvirtual address-spacememorysegmentglobal descriptor tablebase-addressand segment-limitdescriptorSegment Descriptor FormatLimit[ 15..0 ]Base[ 15..0 ]Base[ 23..16 ]Base[ 31..24 ] Limit[19..16 ]31 0Linear to Physicalphysical address-spaceoffsettable-indexlinear addressCR3dir-indexpage frame pagedirectorypagetablePage-Size Extensions•80686 can map either 4KB or 4MB pages•With 4MB pages: middle table is omitted•Entire 4GB address-space is subdividedinto 1024 4MB-pagesLinear to Physicalphysical address-spaceoffsetlinear addressCR3dir-indexpage frame pagedirectory4-MB page-framesPageTable Entry FormatFrame Address031Frame attributes1112Some 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 columnsTwo 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 userPhysical Memory Visualization•Shows relative sizes of the three ‘zones’•Display granularity is 4KB•Data is based on ‘num_physpages’•And on the ‘Zone-Boundary’ constantslight-green: ZONE_DMAdark-green: ZONE_NORMALdark-brown: ZONE_HIGHMEMWhere are process descriptors?•White pages show ‘process descriptors’•Data is taken from the kernel’s tasklist•Searching tasklist noticibly slows drawingVisualization 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 setupFuture 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
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