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8/3/2019 Win Lin Memory

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Memory Management inMemory Management inWindows and LinuxWindows and Linux

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Windows Memory ManagementWindows Memory Management

Virtual memory manager (VMM)

Executive component responsible for managing memory

Lazy allocation

Avoid allocating memory until necessary

Prefetching

Move pages from disk to main memory before they are needed

Pagefile

Stores pages that do not fit in main memory Windows XP supports up to 16 pagefiles


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Memory OrganizationMemory Organization

32-bit virtual address space

Windows 64-Bit Edition has 64-bit address space.

4GB virtual address space per process

User space vs. System space

Process can access only user space

VMM stores page tables and other data in system space

2GB user space, 2GB system space

4KB pages


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Two-level hierarchical memory map

Page directory table

Page directory entries (PDEs) point to page table

One page directory table per process

Location in page directory register

Page table

Page table entries (PTEs) point to page frames

Page frame

Contains page of data TLB (translation lookaside buffer) accelerates address

translation


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Virtual address translationVirtual address translation


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Memory AllocationMemory Allocation


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Page frame statesPage frame states


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Page Replacement ProcessPage Replacement Process


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Linux Memory ManagementLinux Memory Management

Memory manager supports 32- and 64-bit addresses

Also supports NUMA (nonuniform memory access

architectures)

To allow Linux to scale from desktops andworkstations, to servers and supercomputers


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Page Table OrganizationPage Table Organization


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Linux uses paging exclusively Often implemented using a single page size

On 32-bit systems, kernel can address 4GB of data

On 64-bit systems, the kernel supports up to 2 petabytes of data

Three levels of page tables

Page global directory

Page middle directory

Page tables

On systems that support only two levels of page tables, pagemiddle directory contains exactly one entry

Virtual address space organized into virtual memory areas togroup information with same permissions (similar tosegments)


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Linux on the IALinux on the IA--32 architecture32 architecture

Kernel attempts to reduce overhead due to TLB flushing on

context switch

Divides each 4GB address space into a 3GB region for process

data and instructions and a 1GB address space for kernel dataand instructions

Most of the kernels address space is directly mapped to main

memory so that it can access information belonging to any

process


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Kernel virtual address space mappingKernel virtual address space mapping


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Memory zones DMA memory: first 16MB of main memory

Kernel attempts to make memory available in this region for legacyhardware

Normal memory: between 16MB and 896MB on the IA-32architecture

Stores user data and most kernel data

High memory: > 896MB on the IA-32 architecture

Contains memory that the kernel does not permanently map to itsaddress space

Bounce buffer Allocates low memory temporarily for I/O

Data is bounced to high memory after I/O completes


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Physical memory zones on the IAPhysical memory zones on the IA--3232


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Page ReplacementPage Replacement

Page replacement is performed independently for each pagezone Algorithm is a variant of the clock page-replacement algorithm

Two linked lists per zone

Active list contains pages that have been referenced recently

Inactive list contains pages that have been used less recently

Page enters system at the head of the inactive list, referenced bit set

If the page is active or inactive and its referenced bit is off, the bit is turnedon

Ensures that recently referenced pages are not selected forreplacement

If page is inactive and is being referenced for the second time (referenced

bit is on), page is moved to head of the active list, referenced bit is cleared Allows the kernel to distinguish between referenced pages that havebeen accessed once and those that have been accessed more than oncerecently

The latter are placed in the active list so they are not selected forreplacement


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PagePage--replacement system overviewreplacement system overview


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SwappingSwapping

kswapd(the kernel swap daemon)

Periodically frees page frames by flushing dirty pages to disk

Swaps pages from the tail of the inactive list

First determines if the page has a valid entry in the swap cache

Enables clean pages to be freed immediately

Cannot free a page frame if

Page is shared

kswapdmust unmap multiple references to the page

Reverse mapping improves efficiency

Page is dirty

kswapdmust flush it to disk

Performed asynchronously bypdflush

Page is locked (e.g., currently under I/O)

kswapdmust wait until page is unlocked

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