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17
Free-Space Management
In this chapter, we take a small detour from our discussion of virtual-
izing memory to discuss a fundamental aspect of any memory manage-
ment system, whether it be a malloc library (managing pages of a pro-
cess’s heap) or the OS itself (managing portions of the address space of a
process). Specifically, we will discuss the issues surrounding free-space
management.
Let us make the problem more specific. Managing free space can cer-
tainly be easy, as we will see when we discuss the concept of paging. It is
easy when the space you are managing is divided into fixed-sized units;
in such a case, you just keep a list of these fixed-sized units; when a client
requests one of them, return the first entry.
Where free-space management becomes more difficult (and interest-
ing) is when the free space you are managing consists of variable-sized
units; this arises in a user-level memory-allocation library (as in malloc()
and free()) and in an OS managing physical memory when using seg-
mentation to implement virtual memory. In either case, the problem that
exists is known as external fragmentation: the free space gets chopped
into little pieces of different sizes and is thus fragmented; subsequent re-
quests may fail because there is no single contiguous space that can sat-
isfy the request, even though the total amount of free space exceeds the
size of the request.
free used free
0 10 20 30
The figure shows an example of this problem. In this case, the total
free space available is 20 bytes; unfortunately, it is fragmented into two
chunks of size 10 each. As a result, a request for 15 bytes will fail even
though there are 20 bytes free. And thus we arrive at the problem ad-
dressed in this chapter.
1
2 FREE-SPACE MANAGEMENT
CRUX: HOW TO MANAGE FREE SPACE
How should free space be managed, when satisfying variable-sized re-
quests? What strategies can be used to minimize fragmentation? What
are the time and space overheads of alternate approaches?
17.1 Assumptions
Most of this discussion will focus on the great history of allocators
found in user-level memory-allocation libraries. We draw on Wilson’s
excellent survey [W+95] but encourage interested readers to go to the
source document itself for more details1.
We assume a basic interface such as that provided by malloc() and
free(). Specifically, void *malloc(size t size) takes a single pa-
rameter, size, which is the number of bytes requested by the applica-
tion; it hands back a pointer (of no particular type, or a void pointer in
C lingo) to a region of that size (or greater). The complementary routine
void free(void *ptr) takes a pointer and frees the corresponding
chunk. Note the implication of the interface: the user, when freeing the
space, does not inform the library of its size; thus, the library must be able
to figure out how big a chunk of memory is when handed just a pointer
to it. We’ll discuss how to do this a bit later on in the chapter.
The space that this library manages is known historically as the heap,
and the generic data structure used to manage free space in the heap is
some kind of free list. This structure contains references to all of the free
chunks of space in the managed region of memory. Of course, this data
structure need not be a list per se, but just some kind of data structure to
track free space.
We further assume that primarily we are concerned with external frag-
mentation, as described above. Allocators could of course also have the
problem of internal fragmentation; if an allocator hands out chunks of
memory bigger than that requested, any unasked for (and thus unused)
space in such a chunk is considered internal fragmentation (because the
waste occurs inside the allocated unit) and is another example of space
waste. However, for the sake of simplicity, and because it is the more in-
teresting of the two types of fragmentation, we’ll mostly focus on external
fragmentation.
We’ll also assume that once memory is handed out to a client, it cannot
be relocated to another location in memory. For example, if a program
calls malloc() and is given a pointer to some space within the heap,
that memory region is essentially “owned” by the program (and cannot
be moved by the library) until the program returns it via a correspond-
ing call to free(). Thus, no compaction of free space is possible, which
1It is nearly 80 pages long; thus, you really have to be interested!
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would be useful to combat fragmentation2. Compaction could, however,
be used in the OS to deal with fragmentation when implementing seg-
mentation (as discussed in said chapter on segmentation).
Finally, we’ll assume that the allocator manages a contiguous region
of bytes. In some cases, an allocator could ask for that region to grow;
for example, a user-level memory-allocation library might call into the
kernel to grow the heap (via a system call such as sbrk) when it runs out
of space. However, for simplicity, we’ll just assume that the region is a
single fixed size throughout its life.
17.2 Low-level Mechanisms
Before delving into some policy details, we’ll first cover some com-
mon mechanisms used in most allocators. First, we’ll discuss the basics of
splitting and coalescing, common techniques in most any allocator. Sec-
ond, we’ll show how one can track the size of allocated regions quickly
and with relative ease. Finally, we’ll discuss how to build a simple list
inside the free space to keep track of what is free and what isn’t.
Splitting and Coalescing
A free list contains a set of elements that describe the free space still re-
maining in the heap. Thus, assume the following 30-byte heap:
free used free
0 10 20 30
The free list for this heap would have two elements on it. One entry de-
scribes the first 10-byte free segment (bytes 0-9), and one entry describes
the other free segment (bytes 20-29):
head
addr:0
len:10
addr:20
len:10
NULL
As described above, a request for anything greater than 10 bytes will
fail (returning NULL); there just isn’t a single contiguous chunk of mem-
ory of that size available. A request for exactly that size (10 bytes) could
be satisfied easily by either of the free chunks. But what happens if the
request is for something smaller than 10 bytes?
Assume we have a request for just a single byte of memory. In this
case, the allocator will perform an action known as splitting: it will find
2Once you hand a pointer to a chunk of memory to a C program, it is generally difficult
to determine all references (pointers) to that region, which may be stored in other variables
or even in registers at a given point in execution. This may not be the case in more strongly-
typed, garbage-collected languages, which would thus enable compaction as a technique to
combat fragmentation.
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a free chunk of memory that can satisfy the request and split it into two.
The first chunk it will return to the caller; the second chunk will remain
on the list. Thus, in our example above, if a request for 1 byte were made,
and the allocator decided to use the second of the two elements on the list
to satisfy the request, the call to malloc() would return 20 (the address of
the 1-byte allocated region) and the list would end up looking like this:
head
addr:0
len:10
addr:21
len:9
NULL
In the picture, you can see the list basically stays intact; the only change
is that the free region now starts at 21 instead of 20, and the length of that
free region is now just 93. Thus, the split is commonly used in allocators
when requests are smaller than the size of any particular free chunk.
A corollary mechanism found in many allocators is known as coalesc-
ing of free space. Take our example from above once more (free 10 bytes,
used 10 bytes, and another free 10 bytes).
Given this (tiny) heap, what happens when an application calls free(10),
thus returning the space in the middle of the heap? If we simply add this
free space back into our list without too much thinking, we might end up
with a list that looks like this:
head
addr:10
len:10
addr:0
len:10
addr:20
len:10
NULL
Note the problem: while the entire heap is now free, it is seemingly
divided into three chunks of 10 bytes each. Thus, if a user requests 20
bytes, a simple list traversal will not find such a free chunk, and return
failure.
What allocators do in order to avoid this problem is coalesce free space
when a chunk of memory is freed. The idea is simple: when returning a
free chunk in memory, look carefully at the addresses of the chunk you
are returning as well as the nearby chunks of free space; if the newly-
freed space sits right next to one (or two, as in this example) existing free
chunks, merge them into a single larger free chunk. Thus, with coalesc-
ing, our final list should look like this:
head
addr:0
len:30
NULL
Indeed, this is what the heap list looked like at first, before any allo-
cations were made. With coalescing, an allocator can better ensure that
large free extents are available for the application.
3This discussion assumes that there are no headers, an unrealistic but simplifying assump-
tion we make for now.
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ptr
The header used by malloc library
The 20 bytes returned to caller
Figure 17.1: An Allocated Region Plus Header
size: 20
magic: 1234567
hptr
ptr
The 20 bytes returned to caller
Figure 17.2: Specific Contents Of The Header
Tracking The Size Of Allocated Regions
You might have noticed that the interface to free(void *ptr) does
not take a size parameter; thus it is assumed that given a pointer, the
malloc library can quickly determine the size of the region of memory
being freed and thus incorporate the space back into the free list.
To accomplish this task, most allocators store a little bit of extra infor-
mation in a header block which is kept in memory, usually just before
the handed-out chunk of memory. Let’s look at an example again (Fig-
ure 17.1). In this example, we are examining an allocated block of size 20
bytes, pointed to by ptr; imagine the user called malloc() and stored
the results in ptr, e.g., ptr = malloc(20);.
The header minimally contains the size of the allocated region (in this
case, 20); it may also contain additional pointers to speed up dealloca-
tion, a magic number to provide additional integrity checking, and other
information. Let’s assume a simple header which contains the size of the
region and a magic number, like this:
typedef struct {
int size;
int magic;
} header_t;
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The example above would look like what you see in Figure 17.2. When
the user calls free(ptr), the library then uses simple pointer arithmetic
to figure out where the header begins:
void free(void *ptr) {
header_t *hptr = (header_t *) ptr – 1;
…
After obtaining such a pointer to the header, the library can easily de-
termine whether the magic number matches the expected value as a san-
ity check (assert(hptr->magic == 1234567)) and calculate the to-
tal size of the newly-freed region via simple math (i.e., adding the size of
the header to size of the region). Note the small but critical detail in the
last sentence: the size of the free region is the size of the header plus the
size of the space allocated to the user. Thus, when a user requests N bytes
of memory, the library does not search for a free chunk of size N ; rather,
it searches for a free chunk of size N plus the size of the header.
Embedding A Free List
Thus far we have treated our simple free list as a conceptual entity; it is
just a list describing the free chunks of memory in the heap. But how do
we build such a list inside the free space itself?
In a more typical list, when allocating a new node, you would just call
malloc() when you need space for the node. Unfortunately, within the
memory-allocation library, you can’t do this! Instead, you need to build
the list inside the free space itself. Don’t worry if this sounds a little weird;
it is, but not so weird that you can’t do it!
Assume we have a 4096-byte chunk of memory to manage (i.e., the
heap is 4KB). To manage this as a free list, we first have to initialize said
list; initially, the list should have one entry, of size 4096 (minus the header
size). Here is the description of a node of the list:
typedef struct __node_t {
int size;
struct __node_t *next;
} node_t;
Now let’s look at some code that initializes the heap and puts the first
element of the free list inside that space. We are assuming that the heap is
built within some free space acquired via a call to the system call mmap();
this is not the only way to build such a heap but serves us well in this
example. Here is the code:
// mmap() returns a pointer to a chunk of free space
node_t *head = mmap(NULL, 4096, PROT_READ|PROT_WRITE,
MAP_ANON|MAP_PRIVATE, -1, 0);
head->size = 4096 – sizeof(node_t);
head->next = NULL;
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size: 4088
next: 0
…
head [virtual address: 16KB]
header: size field
header: next field (NULL is 0)
the rest of the 4KB chunk
Figure 17.3: A Heap With One Free Chunk
size: 100
magic: 1234567
. . .
size: 3980
next: 0
. . .
ptr
[virtual address: 16KB]
head
The 100 bytes now allocated
The free 3980 byte chunk
Figure 17.4: A Heap: After One Allocation
After running this code, the status of the list is that it has a single entry,
of size 4088. Yes, this is a tiny heap, but it serves as a fine example for us
here. The head pointer contains the beginning address of this range; let’s
assume it is 16KB (though any virtual address would be fine). Visually,
the heap thus looks like what you see in Figure 17.3.
Now, let’s imagine that a chunk of memory is requested, say of size
100 bytes. To service this request, the library will first find a chunk that is
large enough to accommodate the request; because there is only one free
chunk (size: 4088), this chunk will be chosen. Then, the chunk will be
split into two: one chunk big enough to service the request (and header,
as described above), and the remaining free chunk. Assuming an 8-byte
header (an integer size and an integer magic number), the space in the
heap now looks like what you see in Figure 17.4.
Thus, upon the request for 100 bytes, the library allocated 108 bytes
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size: 100
magic: 1234567
. . .
size: 100
magic: 1234567
. . .
size: 100
magic: 1234567
. . .
size: 3764
next: 0
. . .
sptr
[virtual address: 16KB]
head
100 bytes still allocated
100 bytes still allocated
(but about to be freed)
100-bytes still allocated
The free 3764-byte chunk
Figure 17.5: Free Space With Three Chunks Allocated
out of the existing one free chunk, returns a pointer (marked ptr in the
figure above) to it, stashes the header information immediately before the
allocated space for later use upon free(), and shrinks the one free node
in the list to 3980 bytes (4088 minus 108).
Now let’s look at the heap when there are three allocated regions, each
of 100 bytes (or 108 including the header). A visualization of this heap is
shown in Figure 17.5.
As you can see therein, the first 324 bytes of the heap are now allo-
cated, and thus we see three headers in that space as well as three 100-
byte regions being used by the calling program. The free list remains
uninteresting: just a single node (pointed to by head), but now only 3764
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size: 100
magic: 1234567
. . .
size: 100
next: 16708
. . .
size: 100
magic: 1234567
. . .
size: 3764
next: 0
. . .
[virtual address: 16KB]
head
sptr
100 bytes still allocated
(now a free chunk of memory)
100-bytes still allocated
The free 3764-byte chunk
Figure 17.6: Free Space With Two Chunks Allocated
bytes in size after the three splits. But what happens when the calling
program returns some memory via free()?
In this example, the application returns the middle chunk of allocated
memory, by calling free(16500) (the value 16500 is arrived upon by
adding the start of the memory region, 16384, to the 108 of the previous
chunk and the 8 bytes of the header for this chunk). This value is shown
in the previous diagram by the pointer sptr.
The library immediately figures out the size of the free region, and
then adds the free chunk back onto the free list. Assuming we insert at
the head of the free list, the space now looks like this (Figure 17.6).
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size: 100
next: 16492
. . .
size: 100
next: 16708
. . .
size: 100
next: 16384
. . .
size: 3764
next: 0
. . .
[virtual address: 16KB]
head
(now free)
(now free)
(now free)
The free 3764-byte chunk
Figure 17.7: A Non-Coalesced Free List
Now we have a list that starts with a small free chunk (100 bytes,
pointed to by the head of the list) and a large free chunk (3764 bytes).
Our list finally has more than one element on it! And yes, the free space
is fragmented, an unfortunate but common occurrence.
One last example: let’s assume now that the last two in-use chunks are
freed. Without coalescing, you end up with fragmentation (Figure 17.7).
As you can see from the figure, we now have a big mess! Why? Simple,
we forgot to coalesce the list. Although all of the memory is free, it is
chopped up into pieces, thus appearing as a fragmented memory despite
not being one. The solution is simple: go through the list and merge
neighboring chunks; when finished, the heap will be whole again.
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Growing The Heap
We should discuss one last mechanism found within many allocation li-
braries. Specifically, what should you do if the heap runs out of space?
The simplest approach is just to fail. In some cases this is the only option,
and thus returning NULL is an honorable approach. Don’t feel bad! You
tried, and though you failed, you fought the good fight.
Most traditional allocators start with a small-sized heap and then re-
quest more memory from the OS when they run out. Typically, this means
they make some kind of system call (e.g., sbrk in most UNIX systems) to
grow the heap, and then allocate the new chunks from there. To service
the sbrk request, the OS finds free physical pages, maps them into the
address space of the requesting process, and then returns the value of
the end of the new heap; at that point, a larger heap is available, and the
request can be successfully serviced.
17.3 Basic Strategies
Now that we have some machinery under our belt, let’s go over some
basic strategies for managing free space. These approaches are mostly
based on pretty simple policies that you could think up yourself; try it
before reading and see if you come up with all of the alternatives (or
maybe some new ones!).
The ideal allocator is both fast and minimizes fragmentation. Unfortu-
nately, because the stream of allocation and free requests can be arbitrary
(after all, they are determined by the programmer), any particular strat-
egy can do quite badly given the wrong set of inputs. Thus, we will not
describe a “best” approach, but rather talk about some basics and discuss
their pros and cons.
Best Fit
The best fit strategy is quite simple: first, search through the free list and
find chunks of free memory that are as big or bigger than the requested
size. Then, return the one that is the smallest in that group of candidates;
this is the so called best-fit chunk (it could be called smallest fit too). One
pass through the free list is enough to find the correct block to return.
The intuition behind best fit is simple: by returning a block that is close
to what the user asks, best fit tries to reduce wasted space. However, there
is a cost; naive implementations pay a heavy performance penalty when
performing an exhaustive search for the correct free block.
Worst Fit
The worst fit approach is the opposite of best fit; find the largest chunk
and return the requested amount; keep the remaining (large) chunk on
the free list. Worst fit tries to thus leave big chunks free instead of lots of
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small chunks that can arise from a best-fit approach. Once again, how-
ever, a full search of free space is required, and thus this approach can be
costly. Worse, most studies show that it performs badly, leading to excess
fragmentation while still having high overheads.
First Fit
The first fit method simply finds the first block that is big enough and
returns the requested amount to the user. As before, the remaining free
space is kept free for subsequent requests.
First fit has the advantage of speed — no exhaustive search of all the
free spaces are necessary — but sometimes pollutes the beginning of the
free list with small objects. Thus, how the allocator manages the free list’s
order becomes an issue. One approach is to use address-based ordering;
by keeping the list ordered by the address of the free space, coalescing
becomes easier, and fragmentation tends to be reduced.
Next Fit
Instead of always beginning the first-fit search at the beginning of the list,
the next fit algorithm keeps an extra pointer to the location within the
list where one was looking last. The idea is to spread the searches for
free space throughout the list more uniformly, thus avoiding splintering
of the beginning of the list. The performance of such an approach is quite
similar to first fit, as an exhaustive search is once again avoided.
Examples
Here are a few examples of the above strategies. Envision a free list with
three elements on it, of sizes 10, 30, and 20 (we’ll ignore headers and other
details here, instead just focusing on how strategies operate):
head 10 30 20 NULL
Assume an allocation request of size 15. A best-fit approach would
search the entire list and find that 20 was the best fit, as it is the smallest
free space that can accommodate the request. The resulting free list:
head 10 30 5 NULL
As happens in this example, and often happens with a best-fit ap-
proach, a small free chunk is now left over. A worst-fit approach is similar
but instead finds the largest chunk, in this example 30. The resulting list:
head 10 15 20 NULL
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The first-fit strategy, in this example, does the same thing as worst-fit,
also finding the first free block that can satisfy the request. The difference
is in the search cost; both best-fit and worst-fit look through the entire list;
first-fit only examines free chunks until it finds one that fits, thus reducing
search cost.
These examples just scratch the surface of allocation policies. More
detailed analysis with real workloads and more complex allocator behav-
iors (e.g., coalescing) are required for a deeper understanding. Perhaps
something for a homework section, you say?
17.4 Other Approaches
Beyond the basic approaches described above, there have been a host
of suggested techniques and algorithms to improve memory allocation in
some way. We list a few of them here for your consideration (i.e., to make
you think about a little more than just best-fit allocation).
Segregated Lists
One interesting approach that has been around for some time is the use
of segregated lists. The basic idea is simple: if a particular application
has one (or a few) popular-sized request that it makes, keep a separate
list just to manage objects of that size; all other requests are forwarded to
a more general memory allocator.
The benefits of such an approach are obvious. By having a chunk of
memory dedicated for one particular size of requests, fragmentation is
much less of a concern; moreover, allocation and free requests can be
served quite quickly when they are of the right size, as no complicated
search of a list is required.
Just like any good idea, this approach introduces new complications
into a system as well. For example, how much memory should one ded-
icate to the pool of memory that serves specialized requests of a given
size, as opposed to the general pool? One particular allocator, the slab
allocator by uber-engineer Jeff Bonwick (which was designed for use in
the Solaris kernel), handles this issue in a rather nice way [B94].
Specifically, when the kernel boots up, it allocates a number of object
caches for kernel objects that are likely to be requested frequently (such as
locks, file-system inodes, etc.); the object caches thus are each segregated
free lists of a given size and serve memory allocation and free requests
quickly. When a given cache is running low on free space, it requests
some slabs of memory from a more general memory allocator (the to-
tal amount requested being a multiple of the page size and the object in
question). Conversely, when the reference counts of the objects within
a given slab all go to zero, the general allocator can reclaim them from
the specialized allocator, which is often done when the VM system needs
more memory.
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ASIDE: GREAT ENGINEERS ARE REALLY GREAT
Engineers like Jeff Bonwick (who not only wrote the slab allocator men-
tioned herein but also was the lead of an amazing file system, ZFS) are
the heart of Silicon Valley. Behind almost any great product or technol-
ogy is a human (or small group of humans) who are way above average
in their talents, abilities, and dedication. As Mark Zuckerberg (of Face-
book) says: “Someone who is exceptional in their role is not just a little
better than someone who is pretty good. They are 100 times better.” This
is why, still today, one or two people can start a company that changes
the face of the world forever (think Google, Apple, or Facebook). Work
hard and you might become such a “100x” person as well. Failing that,
work with such a person; you’ll learn more in a day than most learn in a
month. Failing that, feel sad.
The slab allocator also goes beyond most segregated list approaches
by keeping free objects on the lists in a pre-initialized state. Bonwick
shows that initialization and destruction of data structures is costly [B94];
by keeping freed objects in a particular list in their initialized state, the
slab allocator thus avoids frequent initialization and destruction cycles
per object and thus lowers overheads noticeably.
Buddy Allocation
Because coalescing is critical for an allocator, some approaches have been
designed around making coalescing simple. One good example is found
in the binary buddy allocator [K65].
In such a system, free memory is first conceptually thought of as one
big space of size 2N . When a request for memory is made, the search for
free space recursively divides free space by two until a block that is big
enough to accommodate the request is found (and a further split into two
would result in a space that is too small). At this point, the requested
block is returned to the user. Here is an example of a 64KB free space
getting divided in the search for a 7KB block (Figure 17.8, page 15).
In the example, the leftmost 8KB block is allocated (as indicated by the
darker shade of gray) and returned to the user; note that this scheme can
suffer from internal fragmentation, as you are only allowed to give out
power-of-two-sized blocks.
The beauty of buddy allocation is found in what happens when that
block is freed. When returning the 8KB block to the free list, the allocator
checks whether the “buddy” 8KB is free; if so, it coalesces the two blocks
into a 16KB block. The allocator then checks if the buddy of the 16KB
block is still free; if so, it coalesces those two blocks. This recursive coa-
lescing process continues up the tree, either restoring the entire free space
or stopping when a buddy is found to be in use.
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64 KB
32 KB 32 KB
16 KB 16 KB
8 KB 8 KB
Figure 17.8: Example Buddy-managed Heap
The reason buddy allocation works so well is that it is simple to de-
termine the buddy of a particular block. How, you ask? Think about the
addresses of the blocks in the free space above. If you think carefully
enough, you’ll see that the address of each buddy pair only differs by
a single bit; which bit is determined by the level in the buddy tree. And
thus you have a basic idea of how binary buddy allocation schemes work.
For more detail, as always, see the Wilson survey [W+95].
Other Ideas
One major problem with many of the approaches described above is their
lack of scaling. Specifically, searching lists can be quite slow. Thus,
advanced allocators use more complex data structures to address these
costs, trading simplicity for performance. Examples include balanced bi-
nary trees, splay trees, or partially-ordered trees [W+95].
Given that modern systems often have multiple processors and run
multi-threaded workloads (something you’ll learn about in great detail
in the section of the book on Concurrency), it is not surprising that a lot
of effort has been spent making allocators work well on multiprocessor-
based systems. Two wonderful examples are found in Berger et al. [B+00]
and Evans [E06]; check them out for the details.
These are but two of the thousands of ideas people have had over time
about memory allocators; read on your own if you are curious. Failing
that, read about how the glibc allocator works [S15], to give you a sense
of what the real world is like.
17.5 Summary
In this chapter, we’ve discussed the most rudimentary forms of mem-
ory allocators. Such allocators exist everywhere, linked into every C pro-
gram you write, as well as in the underlying OS which is managing mem-
ory for its own data structures. As with many systems, there are many
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trade-offs to be made in building such a system, and the more you know
about the exact workload presented to an allocator, the more you could do
to tune it to work better for that workload. Making a fast, space-efficient,
scalable allocator that works well for a broad range of workloads remains
an on-going challenge in modern computer systems.
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References
[B+00] “Hoard: A Scalable Memory Allocator for Multithreaded Applications” by Emery D.
Berger, Kathryn S. McKinley, Robert D. Blumofe, Paul R. Wilson. ASPLOS-IX, November 2000.
Berger and company’s excellent allocator for multiprocessor systems. Beyond just being a fun paper, also
used in practice!
[B94] “The Slab Allocator: An Object-Caching Kernel Memory Allocator” by Jeff Bonwick.
USENIX ’94. A cool paper about how to build an allocator for an operating system kernel, and a great
example of how to specialize for particular common object sizes.
[E06] “A Scalable Concurrent malloc(3) Implementation for FreeBSD” by Jason Evans. April,
2006. http://people.freebsd.org/˜jasone/jemalloc/bsdcan2006/jemalloc.pdf. A detailed look at
how to build a real modern allocator for use in multiprocessors. The “jemalloc” allocator is in widespread
use today, within FreeBSD, NetBSD, Mozilla Firefox, and within Facebook.
[K65] “A Fast Storage Allocator” by Kenneth C. Knowlton. Communications of the ACM,
Volume 8:10, October 1965. The common reference for buddy allocation. Random strange fact: Knuth
gives credit for the idea not to Knowlton but to Harry Markowitz, a Nobel-prize winning economist.
Another strange fact: Knuth communicates all of his emails via a secretary; he doesn’t send email
himself, rather he tells his secretary what email to send and then the secretary does the work of emailing.
Last Knuth fact: he created TeX, the tool used to typeset this book. It is an amazing piece of software4 .
[S15] “Understanding glibc malloc” by Sploitfun. February, 2015. sploitfun.wordpress.com/
2015/02/10/understanding-glibc-malloc/. A deep dive into how glibc malloc works. Amazingly
detailed and a very cool read.
[W+95] “Dynamic Storage Allocation: A Survey and Critical Review” by Paul R. Wilson, Mark
S. Johnstone, Michael Neely, David Boles. International Workshop on Memory Management,
Scotland, UK, September 1995. An excellent and far-reaching survey of many facets of memory
allocation. Far too much detail to go into in this tiny chapter!
4Actually we use LaTeX, which is based on Lamport’s additions to TeX, but close enough.
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Homework (Simulation)
The program, malloc.py, lets you explore the behavior of a simple
free-space allocator as described in the chapter. See the README for
details of its basic operation.
Questions
1. First run with the flags -n 10 -H 0 -p BEST -s 0 to gener-
ate a few random allocations and frees. Can you predict what al-
loc()/free() will return? Can you guess the state of the free list after
each request? What do you notice about the free list over time?
2. How are the results different when using a WORST fit policy to
search the free list (-p WORST)? What changes?
3. What about when using FIRST fit (-p FIRST)? What speeds up
when you use first fit?
4. For the above questions, how the list is kept ordered can affect the
time it takes to find a free location for some of the policies. Use
the different free list orderings (-l ADDRSORT, -l SIZESORT+,
-l SIZESORT-) to see how the policies and the list orderings in-
teract.
5. Coalescing of a free list can be quite important. Increase the number
of random allocations (say to -n 1000). What happens to larger
allocation requests over time? Run with and without coalescing
(i.e., without and with the -C flag). What differences in outcome do
you see? How big is the free list over time in each case? Does the
ordering of the list matter in this case?
6. What happens when you change the percent allocated fraction -P
to higher than 50? What happens to allocations as it nears 100?
What about as the percent nears 0?
7. What kind of specific requests can you make to generate a highly-
fragmented free space? Use the -A flag to create fragmented free
lists, and see how different policies and options change the organi-
zation of the free list.
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