Kernel Memory Allocation 1 Chapter 12. Kernel Memory Allocation Introduction Resource Map Allocator...

Kernel Memory Allocation 1

Chapter 12. Kernel Memory Allocation

• Introduction

• Resource Map Allocator

• Simple Power-of-Two Free Lists

• Mckusick-Karels Allocator

• Buddy System

• Mach-OSF/1 Zone Allocator

• Solaris 2.4 Slab Allocator


Introduction

• Page-level allocator

– Paging system

• Supports virtual memory system

– Kernel memory allocator

• Provides odd-sized buffers of memory to various

kernel subsystems

• Kernel frequently needs chunks of memory of

various sizes usually for short periods of time


Introduction (cont)

• Users of the kernel memory allocator

– pathname translation routine

– allocb( ) allocates STREAMs buffers of

arbitrary size

– In SVR4, the kernel allocates many objects

(proc structures, vnodes, file descriptor

blocks) dynamically when needed


Introduction (cont)

Page-levelallocator

Pagingsystem

kernelmemoryallocator

networkbuffers

procstructures

inodes, filedescriptors

userprocesses

block buffercache

physicalmemory


Kernel Memory Allocator (KMA)• Functional requirements– Page-level allocator pre-allocates part of main

memory to KMA, which must use this memory pool efficiently

– KMA also have to monitor which part of its pool are allocated and which are free

• Evaluation criteria– Utilization factor

– KMA must be fast• Both average and worst-cast latency are important

– Interaction with the paging system


Resource Map Allocator• Resource map– set of <base, size> for free memory

• First fit, Best fit, Worst bit policies

• Advantages– Easy to implement– Not restricted to memory allocation– Allocates the exact numbers of bytes

requested– Client is not constrained to release the exact

region it has allocated– Allocator coalesces adjacent free regions


Resource Map Allocator (cont)

• Drawbacks

– Map may become highly fragmented

– As the fragmentation increases, so does the

size of the map

– Sorting for coalescing adjacent regions is

expensive

– Linear search to find a free region


Simple Power-of-Two Free List

32

64

128

256

512

1024

listheaders

freebuffersallocated

blocks


Simple Power-of-Two Free List (cont)

• Used frequently to implement malloc( ), and free( ) in the user-level C library

• One-word header for each buffer

• Advantages

– Avoids the lengthy linear search of the resource map method

– Eliminates the fragmentation problem


Simple Power-of-Two Free List (cont)

• Drawbacks

– Rounding of requests to the next power of two results in poor memory utilization

– No provision for coalescing adjacent free buffer to satisfy larger requests

– No provision to return surplus free buffers to the page-level allocator


Mckusick-Karels Allocator

32

64

128

256

512

freelistaddr[ ] free buffers

allocated blocks

32 512 64 F 32 128 F 32 32 256 64 F 2K F 128

freepages

kmemsizes[ ]


Mckusick-Karels Allocator (cont)

• Used in 4.4BSD, Digital UNIX

• Requires that– a set of contiguous pages

– all buffers belonging to the same page to be the same size

• States of each page– Free

• corresponding element of kmemsizes[ ] contains a pointer to the element for the next free page

– Divided into buffers of a particular size• kmemsizes[ ] element contains the size


Mckusick-Karels Allocator (cont)

– Part of a buffer that spanned multiple pages• kmemsizes[ ] element corresponding to the first

page of the buffer contains the buffer size

• Improvement over simple power-of-two

– Faster, wastes less memory, can handle large and small request efficiently

• Drawbacks

– No provision for moving memory from one list to another

– No way to return memory to the paging system


Buddy System• Basic idea– Combines free buffer coalescing with a power-

of-two allocator– When a buffer is split, each buffer is called the

buddy of the other

• Advantages– Coalescing adjacent free buffers– Easy exchange of memory between the

allocator and the paging system

• Disadvantages– Performance degradation due to coalescing– Program interface needs the size of the buffer


Buddy System (e.g.)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

32 64 128 256 512

bitmap

free list headers

B C D D’ A’

0 256 384 448 512

C’

B’

A

free in use

allocate(256) allocate(128) allocate(64)

1023


Buddy System (e.g.)

1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0

32 64 128 256 512

bitmap

free list headers

B C D D’ F

0 256 384 448 512

C’

B’

A

allocate(128) release(C, 128)

F’ E’

640 768 1023

E

A’


Buddy System (e.g.)

1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0

32 64 128 256 512

bitmap

free list headers

B B’ F

0 256 512

C’

A

release(D, 64)

F’ E’

640 768 1023

E


SVR4 Lazy Buddy System

• Basic idea

– Defer coalescing until it becomes necessary, and then to coalesce as many buffers as possible

• Lazy coalescing

– N buffers, A buffers are active, L are locally free, G are globally free

– N = A + L + G

– slack = N - 2L - G = A - L


SVR4 Lazy Buddy System (cont)

• Lazy state (slack = 0)

– buffer consumption is in a steady state and coalescing is not necessary

• Reclaiming state (slack = 1)

– consumption is borderline, coalescing is needed

• Accelerated state (slack = 2 or more)

– consumption is not in a steady state, and the allocator must coalesce faster


Mach-OSF/1 Zone Allocator

zone of zones

zone of zones

zone of zones

...

structzone

structzone

structzone

active zones

free_elements

next_zone

free objects

structobj1

structobj1

structobj1

structobjn

struct objn

struct objn

first_zone

last_zone


Mach-OSF/1 Zone Allocator (cont)• Basic idea– Each class of dynamically allocated objects is

assigned to its own size, which is simply a pool of free objects of that class

– Any single page is only used for one zone

– Free objects of each zone are maintained on a linked list, headed by a struct zone

– Allocation and release involve nothing more than removing objects from and returning objects to the free list

– If an allocation request finds the free list empty, it asks the page-level allocator for alloc( ) more bytes


Mach-OSF/1 Zone Allocator (cont)

• Garbage collection– Free pages can be returned to the paging

system and later recovered for other zones

• Analysis– Zone allocator is fast and efficient

– No provision for releasing only part of the allocated object

– Zone objects are exactly the required size

– Garbage collector provides a mechanism for memory reuse


Hierarchical Allocator for Multiprocessors

main freelist

aux freelist

target = 3

global freelist

bucket list

target = 3

per-pagesfreelistsCoalesce-to-

Page layer

Global layer

Per-CPUcache


Solaris 2.4 Slab Allocator

• Better performance and memory utilization than other implementation

• Main issues

– Object reuse• advantage of caching and reusing the same object,

rather than allocating and initializing arbitrary chunks of memory

– Hardware cache utilization • Most kernel objects have their important, frequently

accessed fields at the beginning of the object


Solaris 2.4 Slab Allocator– Allocator footprint

• Footprint of a allocator is the portion of the hardware cache and the translation lookaside buffer (TLB) that is overwritten by the allocator itself

• Large footprint causes many cache and TLB misses, reducing the performance of the allocator

• Large footprint: resource maps, buddy systems

• Smaller footprint: Mckusick-Karels, zone

• Design– slab allocator is a variant of the zone method

and is organized as a collection of object caches


Solaris 2.4 Slab Allocator (cont)

page-level allocator

vnode cache

proc cache

mbuf cache

msgb cache ...

activevnodes

activeprocs

activembufs

activemsgbs

back end

front end


Solaris 2.4 Slab Allocator (cont)• Implementation

free active free active active free active

NULL

coloring area(unused)

slab dataunused space

extra space for linkage

linked listof slabs insame cache

freelist

pointers


Solaris 2.4 Slab Allocator (cont)

• Analysis

– Space efficient

– Coalescing scheme results in better hardware cache and memory bus utilization

– Small footprint (only one slab for most requests)

Kernel Memory Allocation 1 Chapter 12. Kernel Memory Allocation Introduction Resource Map Allocator...

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