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
Kernel Memory Allocation 1
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
Kernel Memory Allocation 2
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
Kernel Memory Allocation 3
Introduction (cont)
physical
memory
Page-level
allocator
kernel
memory
allocator
network
buffers
proc
structures
Paging
system
inodes, file
descriptors
user
processes
block buffer
cache
Kernel Memory Allocation 4
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
Kernel Memory Allocation 5
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
Kernel Memory Allocation 6
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
Kernel Memory Allocation 7
Simple Power-of-Two Free List
allocated
blocks
list
headers
free
buffers
32
64
128
256
512
1024
Kernel Memory Allocation 8
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
Kernel Memory Allocation 9
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
Kernel Memory Allocation 10
Mckusick-Karels Allocator
freelistaddr[ ]
allocated blocks
free buffers
32
64
128
256
512
kmemsizes[ ]
32 512 64
F
32 128 F
32 32 256 64
F
2K
F 128
freepages
Kernel Memory Allocation 11
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
Kernel Memory Allocation 12
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
Kernel Memory Allocation 13
Buddy System
• Basic idea
– Combines free buffer coalescing with a powerof-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
Kernel Memory Allocation 14
Buddy System (e.g.)
allocate(256)  allocate(128)  allocate(64)
bitmap
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
free list headers
0
32
256
B
C
64
128
256
512
384 448 512
D D’
1023
A’
C’
B’
free
in use
A
Kernel Memory Allocation 15
Buddy System (e.g.)
 allocate(128)  release(C, 128)
bitmap
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
free list headers
0
32
256
B
C
64
128
384 448 512
D D’
256
640
512
768
F’
F
1023
E’
C’
B’
A
E
A’
Kernel Memory Allocation 16
Buddy System (e.g.)
 release(D, 64)
bitmap
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
free list headers
0
32
64
256
512
B’
B
128
256
640
F’
F
C’
512
768
1023
E’
E
A
Kernel Memory Allocation 17
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
Kernel Memory Allocation 18
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
Kernel Memory Allocation 19
Mach-OSF/1 Zone Allocator
active zones
zone of
zones
free objects
free_elements
struct
zone
struct
zone
struct
zone
struct
obj1
struct
obj1
struct
obj1
struct
objn
struct
objn
struct
objn
next_zone
first_zone
last_zone
zone of
zones
...
zone of
zones
Kernel Memory Allocation 20
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
Kernel Memory Allocation 21
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
Kernel Memory Allocation 22
Hierarchical Allocator for
Multiprocessors
Coalesce-toPage layer
per-pages
freelists
Global layer
global freelist
bucket list
target = 3
Per-CPU
cache
main freelist
aux freelist
target = 3
Kernel Memory Allocation 23
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
Kernel Memory Allocation 24
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
Kernel Memory Allocation 25
Solaris 2.4 Slab Allocator (cont)
page-level allocator
back end
vnode
cache
proc
cache
mbuf
cache
msgb
cache
active
mbufs
active
msgbs
...
front end
active
vnodes
active
procs
Kernel Memory Allocation 26
Solaris 2.4 Slab Allocator (cont)
• Implementation
slab data
unused space
coloring area
(unused)
free
active
free
active active
freelist
free
active
linked list
of slabs in
same cache
NULL
pointers
extra space for linkage
Kernel Memory Allocation 27
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 28