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15-213 / 14-513 / 15-513, Fall 2021
Malloc Lab: Writing a Dynamic Storage Allocator
Assigned: October 12, 2021
This lab requires submitting two versions of your code: one as an initial checkpoint, and the second as
your final version. The due dates of each part are indicated in the following table:
Version Due Date Max. Grace Days Last Hand-in Date Weight in Final Grade
Checkpoint Oct. 26 2 Oct. 28 4%
Final Nov. 2 2 Nov. 4 7%
Don’t forget that due dates are at 11:59pm Eastern Time (presently UTC-4). See section 7 for details on how
each section is scored.
1 Introduction
In this lab you will write a dynamic memory allocator which will consist of the malloc, free, realloc, and
calloc functions. Your goal is to implement an allocator that is correct, efficient, and fast.
We strongly encourage you to start early. The total time you spend designing and debugging can easily
eclipse the time you spend coding.
Bugs can be especially pernicious and difficult to track down in an allocator, and you will probably spend
a significant amount of time debugging your code. Buggy code will not get any credit.
This lab has been heavily revised from previous versions. Do not rely on advice or information you may
find on the Web or from people who have done this lab before. It will most likely be misleading or outright
wrong.1 Be sure to read all of the documentation carefully and especially study the baseline implementation
we have provided.
1Not to mention the fact that it would be an academic integrity violation!
1
Contents
1 Introduction 1
2 Logistics 3
3 Required Functions 4
4 Support Routines 6
5 Programming Rules 7
6 Driver Programs 9
6.1 Trace files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2 Command-line arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7 Scoring 11
7.1 Autograded score . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1.1 Performance index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1.2 Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1.3 Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1.4 Autograded deductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.2 Heap Consistency Checker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.3 Style . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.4 Handin Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8 Useful Tips 15
9 Office Hours 16
10 Strategic Advice 17
A Performance Evaluation 19
A.1 Approximate Expected Results from Optimizations . . . . . . . . . . . . . . . . . . . . . . 19
A.2 Machine Dependencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
A.3 Performance Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
B Viewing Heap Contents with GDB 21
B.1 Viewing the heap without a helper function . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.2 Viewing the heap with the hprobe helper function . . . . . . . . . . . . . . . . . . . . . . 22
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2 Logistics
This is an individual project. You should do this lab on one of the Shark machines.
To get your lab materials, click“Download Handout”on Autolab, enter your Andrew ID, and follow the
instructions. Then, clone your repository on a Shark machine by running:
$ git clone https://github.com/cmu15213f2…<YOUR USERNAME>.git
or, if you use SSH keys, All the code for your allocator must be in this file. The rest of the
provided code allows you to evaluate your allocator. Using the command make will generate four driver
programs: mdriver, mdriver-dbg, mdriver-emulate, and mdriver-uninit, as described in section 6.
Your final autograded score is computed by driver.pl, as described in section 7.1.
To test your code for the checkpoint submission, run mdriver and/or driver.pl with the -C flag. To
test your code for the final submission, run mdriver and/or driver.pl with no flags.
These commands will report accurate utilization numbers for your allocator. They will only report
approximate throughput numbers. The Autolab servers will generate different throughput numbers, and
the servers’numbers will determine your actual score. This is discussed in more detail in Section 7.
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3 Required Functions
Your allocator must implement the following functions. They are declared for you in mm.h and you will find
starter definitions in mm.c. Note that you cannot alter mm.h in this lab.
bool mm_init(void);
void *malloc(size_t size);
void free(void *ptr);
void realloc(void ptr, size_t size);
void *calloc(size_t nmemb, size_t size);
bool mm_checkheap(int);
We provide you two versions of memory allocators:
mm.c: A fully-functional implicit-list allocator. We recommend that you use this code as your starting point.
Note that the provided code does not implement block coalescing. The absence of this feature will cause
external fragmentation to be very high, so you should implement coalescing. We strongly recommend
considering all cases you need to implement before writing code for coalesce_block; the lecture
slides should help you identify and reason about these cases.
mm-naive.c: A functional implementation that runs quickly but gets very poor utilization, because it never
reuses any blocks of memory.
Your allocator must run correctly on a 64-bit machine. It must support a full 64-bit address space, even
though current implementations of x86-64 machines support only a 48-bit address space.
Your submitted mm.c must implement the following functions:
bool mm_init(void): Performs any necessary initializations, such as allocating the initial heap area. The
return value should be false if there was a problem in performing the initialization, true otherwise.
You must reinitialize all of your data structures each time this function is called, because the
drivers call your mm_init function every time they begin a new trace to reset to an empty heap.
void *malloc(size_t size): Returns a pointer to an allocated block payload of at least size bytes. The
entire allocated block should lie within the heap region and should not overlap with any other allocated
block.
Your malloc implementation must always return 16-byte aligned pointers, even if size is smaller than
16.
void free(void *ptr) : If ptr is NULL, does nothing. Otherwise, ptr must point to the beginning of a
block payload returned by a previous call to malloc, calloc, or realloc and not already freed. This
block is deallocated. Returns nothing.
void realloc(void ptr, size_t size): Changes the size of a previously allocated block.
If size is nonzero and ptr is not NULL, allocates a new block with at least size bytes of payload,
copies as much data from ptr into the new block as will fit (that is, copies the smaller of size, or the
payload size of ptr, bytes), frees ptr, and returns the new block.
If size is nonzero but ptr is NULL, does the same thing as malloc(size).
If size is zero, does the same thing as free(ptr) and then returns NULL.
Your realloc implementation will have only minimal impact on measured throughput or utilization. A
correct, simple implementation will suffice.
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void *calloc(size_t nmemb, size_t size): Allocates memory for an array of nmemb elements of
size bytes each, initializes the memory to all bytes zero, and returns a pointer to the allocated memory.
Your calloc implementation will have only minimal impact on measured throughput or utilization. A
correct, simple implementation will suffice.
bool mm_checkheap(int line): Scans the entire heap and checks it for errors. This function is called
the heap consistency checker, or simply heap checker.
A quality heap checker is essential for debugging your malloc implementation. Many malloc bugs
are too subtle to debug using conventional gdb techniques. A heap consistency checker can help you
isolate the specific operation that causes your heap to become inconsistent.
Because of the importance of the consistency checker, it will be graded, by hand; section 7.2 describes
the requirements for your implementation in greater detail. We may also require you to write your heap
checker before coming to office hours.
The mm_checkheap function takes a single integer argument that you can use any way you want. One
technique is to use this argument to pass in the line number where it was called, using the LINE
macro:
mm_checkheap(__LINE__);
This allows you to print the line number where mm_checkheap was called, if you detect a problem with
the heap.
The driver will sometimes call mm_checkheap; when it does this it will always pass an argument of 0.
The semantics of malloc, realloc, calloc, and free match the semantics of the functions with the
same names in the C library. You can type man malloc in the shell for more documentation.
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4 Support Routines
To satisfy allocation requests, dynamic memory allocators must themselves request memory from the operating
system, using“primitive”system operations that are less flexible than malloc and free. In this lab, you
will use a simulated version of one such primitive. It is implemented for you in memlib.c and declared in
memlib.h.
void *mem_sbrk(intptr_t incr): Expands the heap by incr bytes, and returns a generic pointer to the
first byte of the newly allocated heap area. If the heap cannot be made any larger, returns (void *)
-1. (Caution: this is different from returning NULL.)
Each time your mm_init function is called, the heap has just been reset to zero bytes long.
mem_sbrk cannot make the heap smaller; it will fail (returning (void *) -1) if size is negative.
(Data type intptr_t is defined to be a signed integer large enough to hold a pointer. On our machines
it is the same size as size_t, but signed.)
This function is based on the Unix system call sbrk, but we have simplified it by removing the ability
to make the heap smaller.
You can also use these helper functions, declared in memlib.h:
void *mem_heap_lo(void): Returns a generic pointer to the first valid byte in the heap.
void *mem_heap_hi(void): Returns a generic pointer to the last valid byte in the heap.
Caution: The definition of“last valid byte”may not be intuitive! If your heap is 8 bytes large, then the
last valid byte will be 7 bytes from the start—not an aligned address.
size_t mem_heapsize(void): Returns the current size of the heap in bytes.
You can also use the following standard C library functions, but only these: memcpy, memset, printf,
fprintf, and sprintf.
Your mm.c code may only call the externally-defined functions that are listed in this section. Otherwise, it
must be completely self-contained.
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5 Programming Rules
• Any allocator that attempts to detect which trace is running will receive a penalty of 20 points. On
the other hand, you should feel free to write an adaptive allocator—one that dynamically tunes itself
according to the general characteristics of the different traces.
• You may not change any of the interfaces in mm.h, or any of the other C source files and headers besides
mm.c. (Autolab only processes your mm.c; it will not see changes you make to any other file.) However,
we strongly encourage you to use static helper functions in mm.c to break up your code into small,
easy-to-understand segments.
• You may not change the Makefile (again, Autolab will not see any changes you make there) and your
code must compile with no warnings using the warnings flags we selected.
• You are not allowed to declare large global data structures such as large arrays, trees, or lists in mm.c.
You are allowed to declare small global arrays, structs, and scalar variables, and you may have as much
constant data (defined with the const qualifier) as you like. Specifically, you may declare no more
than 128 bytes of writable global variables, total. This is checked automatically, as described in
Section 7.1.4.
The reason for this restriction is that global variables are not accounted for when calculating your
memory utilization. If you need a large data structure for some reason, you should allocate space for it
within the heap, where it will count toward external fragmentation.
• Dynamic memory allocators cannot avoid doing operations that the C standard labels as“undefined
behavior.”They need to treat the heap as a single huge array of bytes and reinterpret those bytes as
different data types at different times. It is rarely appropriate to write code in this style, but in this
lab it is necessary.
We ask you to minimize the amount of undefined behavior in your code. For example, instead of
directly casting between pointer types, you should explicitly alias memory through the use of unions.
Additionally, you should confine the pointer arithmetic to a few short helper functions, as we have tried
to do in the handout code.
• In the provided baseline code, we use a zero-length array to declare a payload element in the block
struct. This is a non-standard compiler extension, which, in general, we discourage the use of, but in
this lab we feel it is better than any available alternative.
A zero-length array is not the same as a C99“flexible array member;”it can be used in places where a
flexible array member cannot. For example, a zero-length array can be a member of a union. Using
zero-length arrays this way is our recommended strategy for declaring a block struct that might contain
payload data, or might contain something else (such as free list pointers).
• The practice of using macros instead of function definitions is now obsolete. Modern compilers can
perform inline substitution of small functions, eliminating the overhead of function calls. Use of inline
functions provides better type checking and debugging support.
In this lab, you may only use #define to define constants (macros with no parameters) and debugging
macros that are enabled or disabled at compile time. Debugging macros must have names that begin
with the prefix“dbg_”and they must have no effect when the macro-constant DEBUG is not defined.
Here are some examples of allowed and disallowed macro definitions:
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define DEBUG 1 OK Defines a constant
define CHUNKSIZE (1<<12) OK Defines a constant
define WSIZE sizeof(uint64_t) OK Defines a constant
define dbg_printf(…) printf(__VA_ARGS__) OK Debugging support
define GET(p) ((unsigned int )(p)) Not OK Has parameters
define PACK(size, alloc) ((size)|(alloc)) Not OK Has parameters
When you run make, it will run a program that checks for disallowed macro definitions in your code.
This checker is overly strict—it cannot determine when a macro definition is embedded in a comment
or in some part of the code that has been disabled by conditional-compilation directives. Nonetheless,
your code must pass this checker without any warning messages.
• The code shown in the textbook (Section 9.9.12, and available from the CS:APP website) is a useful
source of inspiration for the lab, but it does not meet the required coding standards. It does not handle
64-bit allocations, it makes extensive use of macros instead of functions, and it relies heavily on lowlevel
pointer arithmetic. Similarly, the code shown in K&R does not satisfy the coding requirements.
You should use the provided mm.c as your starting point.
• It is okay to look at any high-level descriptions of algorithms found in the textbook or elsewhere, but
it is not acceptable to copy or look at any code of malloc implementations found online or in other
sources, except for the allocators described in the textbook, in K&R, or as part of the provided code.
• It is okay to adapt code for useful generic data structures and algorithms (e.g. linked lists, hash tables,
search trees, and priority queues) from any external source (e.g. K&R, Wikipedia, The Art of Computer
Programming) as long as it was not already part of a memory allocator. You must include (as a comment)
an attribution of the origins of any borrowed code.
• Your allocator must always return pointers that are aligned to 16-byte boundaries, even if the allocation
is smaller than 16 bytes. The driver will check this requirement.
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6 Driver Programs
Four driver programs are generated when you run make.
mdriver is used by Autolab to test your allocator’s correctness, utilization, and throughput on a standard set
of benchmark traces.
mdriver-emulate is used by Autolab to test your allocator with a heap spanning the entire 64-bit address
space. In addition to the standard benchmark traces, it will run a set of giant traces that make very large
allocation requests.
As the name implies, this test is an emulation: it does not actually allocate exabytes of memory.
However, it verifies that your allocator could handle allocations that large, if the hardware permitted
them. Failing the checks performed by mdriver-emulate leads to grade penalties, as described in
section 7.1.4.
mdriver-dbg is for you to use when debugging your allocator. It is the same program as mdriver, with
three notable differences:
- It is compiled with DEBUG defined, which enables the dbg_ macros at the top of mm.c. Without
this defined, functions like dbg_printf and dbg_assert will not have any effect. - It is compiled with optimization level -O0, which allows GDB to display more meaningful
debugging information. - It uses the AddressSanitizer instrumentation tool2
to detect several classes of errors that are easy
to make when writing an allocator.
mdriver-uninit is also for you to use when debugging. It uses the MemorySanitizer instrumentation tool3
to detect uses of uninitialized memory.
mdriver-dbg, mdriver-emulate, and mdriver-uninit are much slower than mdriver, so they only
report correctness and the utilization score for each trace. All four programs should report the same utilization
scores for each trace that they all run (only mdriver-emulate runs the giant traces).
6.1 Trace files
The driver programs are controlled by a set of trace files that are included in the traces subdirectory. Each
trace file contains a sequence of commands that instruct the driver to call your malloc, realloc, and free
routines in some sequence. Autolab will use the same trace files to grade your work.
When the driver programs are run, they will process each trace file multiple times: once to make sure
your implementation is correct, once to determine the space utilization, and between 3 and 20 more times to
determine the throughput.
Some of the traces are short traces that are included mainly for detecting errors and debugging. Your
utilization and performance scores on these traces do not count toward your grade. The traces that do count
are marked with a‘*’in the output of mdriver.
2https://clang.llvm.org/docs/A…
3https://clang.llvm.org/docs/M…
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6.2 Command-line arguments
The drivers accept the following command-line arguments.
-C: Apply the scoring standards for the checkpoint, rather than for the final submission.
-f tracefile: Only run the trace tracefile. Correctness, utilization, and performance are all tested.
-c tracefile: Only run the trace tracefile, and only test it for correctness. This still runs the trace
twice, to verify that mm_init correctly resets your heap.
-v level: Set the verbosity level to the specified value. The level can be 0, 1, or 2; the default level is 1.
Raising the verbosity level causes additional diagnostic information to be printed as each trace file is
processed. This can help you determine which trace file is causing your allocator to fail.
-d level: Controls the amount of validity checking performed by the driver. This is separate from the
DEBUG compile-time define.
At debug level 0, very little checking is done, which is useful when testing performance only.
At debug level 1, the driver checks allocation payloads to ensure that they are not overwritten by
unrelated calls into your code. This is the default.
At debug level 2, the driver will also call your implementation of mm_checkheap after each operation.
This mode is slow, but it can help identify the exact point at which an error occurs.
Additional arguments can be listed by running mdriver -h.
10 - Scoring
Malloc Lab is worth 11% of your final grade in the course. This is divided into the Checkpoint and Final
submissions, which have separate due dates.
The checkpoint submission, worth 4% of your final grade, is graded as follows:
Autograded score (7.1) 100 points
Heap checker (7.2) 10 points
Total 110 points
The final submission, worth 7% of your final grade, is graded as follows:
Autograded score (7.1) 100 points
Code style (7.3) 4 points
Total 104 points
7.1 Autograded score
driver.pl is the program that Autolab will use to calculate your score. For the checkpoint submission,
it will be run with the -C flag; for the final submission, it will be run with no flags. This sets the grading
standards, as described later in this section.
driver.pl computes the autograded score in two steps. First, mdriver is run to obtain the performance
index P, which is a number between 0 and 100 (inclusive). If mdriver detects incorrect behavior on any
trace, P will be zero. Otherwise, P is computed from both utilization and throughput, as described below
(Section 7.1.1). Second, mdriver-emulate is run to detect forms of incorrect behavior that mdriver cannot
detect. Incorrect behavior detected by mdriver-emulate will cause deductions from P, as described in
Section 7.1.4.
Your autograded score is P minus any deductions. Separately, your code will be read by TAs and graded
on the thoroughness of your heap checker (see Section 7.2) and for overall style (see Section 7.3).
7.1.1 Performance index
Both memory and CPU cycles are expensive system resources, so the performance index P is a weighted sum
of your allocator’s space utilization and throughput. The weights are different for the checkpoint and the final
submission:
Version Utilization Throughput
Checkpoint 20% 80%
Final 60% 40%
That is, in English, for the checkpoint your score will be computed as 20% utilization and 80% throughput,
and for the final it will be 60% utilization and 40% throughput.
7.1.2 Utilization
The utilization of a single trace is the peak ratio between the total amount of memory used by the driver at
any one moment (i.e. allocated via malloc but not yet freed via free) and the size of the heap used by your
allocator. All allocators have memory overhead: it is not possible to achieve a utilization of 100%. Your goal
is to make the utilization as high as possible.
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The utilization of your allocator, U, will be calculated as the average utilization across all traces. The
associated score will be computed as follows:
• If U ≤ Umin then you will receive no credit for utilization.
• If Umin < U < Umax then your utilization score will scale linearly with U.
• If U ≥ Umax then you will receive full credit for utilization.
The values of Umin and Umax are different for the checkpoint and the final submission:
Version Umin Umax
Checkpoint 55% 58%
Final 55% 74%
7.1.3 Throughput
The throughput of a single trace is measured by the average number of operations completed per second,
expressed in kilo-operations per second or KOPS. A trace that takes T seconds to perform n operations will
have a throughput of n/(1000 · T) KOPS.
Throughput measurements vary according to the type of CPU running the program. We will compensate
for this machine dependency by evaluating the throughput of your implementations relative to those of
reference implementations running on the same machine. For information on how this is done, see Appendix
A.2.
The throughput of your allocator, T, will be calculated as the average throughput across all traces and
then compared to the reference throughput Tref which will change from checkpoint to final, and from machine
to machine. The associated score will be computed as follows:
• If T ≤ Tmin then you will receive no credit for throughput.
• If Tmin < T < Tmax then your throughput score will scale linearly with T.
• If T ≥ Tmax then you will receive full credit for throughput.
Version Tmin Tmax
Checkpoint 0.1 Tref 0.8 Tref
Final 0.5 Tref 0.9 Tref
The throughput standards are set low enough that we expect your allocator will significantly exceed the
requirements for Tmax. If you achieve this, it will also insulate you from run-to-run variations caused by
system load.
Remember that throughput scores printed by mdriver on a Shark machine are only an indication of your
allocator’s performance. Your scores on Autolab are the only scores that count.
7.1.4 Autograded deductions
The driver.pl program will also run the mdriver-emulate program (see Section 6), which emulates a full
64-bit address space. This may deduct points from your autograded score in the following circumstances:
• If mdriver-emulate fails to run successfully, 30 points will be deducted. This can happen if your
code fails to support a full 64-bit address space; for example, if it uses int where a size_t is needed.
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• If the utilization of a trace differs between mdriver and mdriver-emulate, 30 points will be deducted.
• If your program uses more than 128 bytes of global data (see the Programming Rules in Section 5),
then up to 20 points will be deducted.
7.2 Heap Consistency Checker - points will be awarded based on the quality of your implementation of mm_checkheap. The heap checker
will be graded for your checkpoint submission only. It will not be graded in your final submission.
We require that you check all of the invariants of your data structures. Specific items will be dependent
on your design, so after making design changes, think about what changes you need to make to your heap
checker. Some examples of what your heap checker should check:
• Checking the heap (implicit list, explicit list, segregated list):
– Check for epilogue and prologue blocks.
– Check each block’s address alignment.
– Check blocks lie within heap boundaries.
– Check each block’s header and footer: size (minimum size), previous/next allocate/free bit
consistency, header and footer matching each other.
– Check coalescing: no consecutive free blocks in the heap.
• Checking the free list (explicit list, segregated list):
– All next/previous pointers are consistent (if A’s next pointer points to B, B’s previous pointer
should point to A).
– All free list pointers are between mem_heap_lo() and mem_heap_high().
– Count free blocks by iterating through every block and traversing free list by pointers and see if
they match.
– All blocks in each list bucket fall within bucket size range (segregated list).
Your heap checker should run silently until it detects some error in the heap. Then, and only then, should
it print a message and return false. If it finds no errors, it should return true. It is very important that your
heap checker run silently; otherwise, it will produce too much output to be useful on the large traces.
7.3 Style - points will be awarded based on the quality of your code style, following the Style Guidelines on the website
at http://www.cs.cmu.edu/~213/co… Style will be graded for your final submission only. It
will not be graded for your checkpoint submission.
Some points to keep in mind for malloclab in particular:
• Version control. You must commit your code regularly using Git. This allows you to keep track of
your changes, revert to older versions of your code, and regularly remind yourself of what you changed
and why you made those changes. For specific guidelines on Git usage, see the style guideline.
• Modularity. Your code should be decomposed into functions and use as few global variables as
possible. You should use static functions and declared structs and unions to minimize pointer arithmetic
and to isolate it to a few places.
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• Magic numbers. You should avoid sprinkling your code with numeric constants. Instead, use declarations
via #define or static constants. Try, as much as possible, to use C data types, and the operators
sizeof and offsetof to define the sizes of various fields and offsets, rather than using fixed numeric
values.
• Header comment. Your mm.c file must begin with a header comment that gives an overview of the
structure of your free and allocated blocks, the organization of the free list, and how your allocator
manipulates the free list.
• Function comments. In addition to the overview header comment, each function must be preceded by
a header comment that describes what the function does. Make sure to review the course style guide:
we are expecting that for each function, you document at a minimum its purpose, arguments, return
value, and any relevant preconditions or postconditions.
• Inline comments. You will want to use inline comments to explain code flow or code that is tricky.
• Extensibility. Your code should be modular, robust, and easily scalable. You should be able to easily
change various parameters that define your allocator, without any changes in the actual operation of the
program. For example, you should be able to arbitrarily change the number of segregated lists with
minimal modifications.
Study the code in mm.c as an example of the desired coding style.
For formatting your code, we require that you use the clang-format tool, which automatically reformats
your code according to the .clang-format configuration file. To invoke it, run make format. You are
welcome to change the configuration settings to match your desired format. More information is available in
the style guideline.
7.4 Handin Instructions
Make sure your code does not print anything during normal operation, and that all debugging macros have
been disabled. Ensure that you have committed and pushed the latest version of your code to GitHub.
To submit your code, run make submit or upload your mm.c file to Autolab. Only the last version you
submit will be graded.
14 - Useful Tips
• You’ll find debugging macros defined in mm.c that provide contract functions, such as dbg_assert.
We encourage making liberal use of these contracts to verify invariants and ensure correctness of your
code.
• Use the drivers’-c and -f options to run individual traces. During initial development, using short
trace files will simplify debugging and testing.
• Use the drivers’verbose mode. The -V option will also indicate when each trace file is processed, which
will help you isolate errors.
• Use gdb to help you debug. This will help you isolate and identify out-of-bounds memory references.
When debugging, use the mdriver-dbg binary, which is compiled with the -O0 flag to disable
optimizations.
• Use the Clang static analyzer. The Clang static analyzer is able to find and pinpoint some bugs and
provide explanations for them at compile time. To use it, run make clean and then /usr/local/
depot/llvm-7.0/bin/scan-build make and look for errors that it found in your mm.c.
• Use gdb’s watch command to find out what changed some value you did not expect to have changed.
• Reduce obscure pointer arithmetic through the use of struct’s and union’s. Although your data
structures will be implemented in compressed form within the heap, you should strive to make them
look as conventional as possible using struct and union declarations to encode the different fields.
Examples of this style are shown in the baseline implementation.
• Encapsulate your pointer operations in functions. Pointer arithmetic in memory managers is confusing
and error-prone because of all the casting that is necessary. You can significantly reduce the complexity
by writing functions for your pointer operations. See the code in mm.c for examples. Remember: you
are not allowed to define macros—the code in the textbook does not meet our coding standards for this
lab.
• Your allocator must work for a 64-bit address space. The mdriver-emulate will specifically test this
capability. You should allocate a full eight bytes for all of your pointers and size fields. (You can take
advantage of the low-order bits for some of these values being zero due to the alignment requirements.)
Make sure you do not inadvertently invoke 32-bit arithmetic with an expression such as 1«32, rather
than 1L«32.
15 - Office Hours
This lab has a significant implementation portion that is much more involved than the prior labs. Expect to be
debugging issues for several hours — there is no way around this.
TAs will NOT:
• Go line-by-line through your program to determine where things go wrong.
• Read your code to determine if everything you are doing is correct. There are simply too many subtle
issues to go through everyone’s programs.
TAs will:
• Help you use GDB efficiently to monitor the state of your program.
• Go through conceptual discussions of what you are doing.
Here are some useful things to have ready before a TA comes to you:
• A non-trivial heap checker that exhaustively tests your implementation.
• Documentation of your data structures (drawings are great!)
• A detailed description of problems you are experiencing, and what you’ve looked into so far (not“my
malloc doesn’t work and I don’t know why”)
As a reminder, check our office hours schedule on the course website. Start early so you can get help early!
16 - Strategic Advice
You must design algorithms and data structures for managing free blocks that achieve the right balance of
space utilization and speed. This involves a trade-off—it is easy to write a fast allocator by allocating blocks
and never freeing them, or a high-utilization allocator that carefully packs blocks as tightly as possible. You
must seek to minimize wasted space while also making your program run fast.
As described in the textbook and the lectures, utilization is reduced below 100% due to fragmentation,
taking two different forms:
• External fragmentation: Unused space between allocated blocks or at the end of the heap
• Internal fragmentation: Space within an allocated block that cannot be used for storing data, because it
is required for some of the manager’s data structures (e.g., headers, footers, and free-list pointers), or
because extra bytes must be allocated to meet alignment or minimum block size requirements
To reduce external fragmentation, you will want to implement good block placement heuristics. To reduce
internal fragmentation, it helps to reduce the storage for your data structures as much as possible.
Maximizing throughput requires making sure your allocator finds a suitable block quickly. This involves
constructing more elaborate data structures than is found in the provided code. However, your code need
not use any exotic data structures, such as search trees. Our reference implementation only uses singly- and
doubly-linked lists.
Here’s a strategy we suggest you follow in meeting the checkpoint and final version requirements:
• Checkpoint. The provided code already meets the required utilization performance4
, but it has very low
throughput. You can achieve the required throughput by converting to an explicit-list allocator, and then
converting that into a segregated free list allocator. Both of these changes will not affect your utilization
much.
You will want to experiment with allocation policies. The provided code implements first-fit search.
Some allocators attempt best-fit search, but this is difficult to do efficiently. You can find ways to
introduce elements of best-fit search into a first-fit allocator, while keeping the amount of search
bounded.
Depending on whether you place newly freed blocks at the beginning or the end of a free list, you can
implement either a last-in-first-out (LIFO) or a first-in-first-out (FIFO) queue discipline. You should
experiment with both.
• Final Version. Building on the checkpoint version, you must greatly increase the utilization and keep
a high throughput. You must reduce both external and internal fragmentation. Reducing external
fragmentation requires achieving something closer to best-fit allocation. Reducing internal fragmentation
requires reducing data structure overhead. There are multiple ways to do this, each with its own
challenges. Possible approaches and their associated challenges include:
– Eliminate footers in allocated blocks. But, you still need to be able to implement coalescing. See
the discussion about this optimization on page 852 of the textbook.
– Decrease the minimum block size. But, you must then manage free blocks that are too small to
hold the pointers for a doubly linked free list.
– Set up special regions of memory for small, fixed-size blocks. But, you will need to manage these
and be able to free a block when given only the starting address of its payload.
4The baseline code meets the required utilization for the checkpoint given a basic implementation of coalesce_block. However,
you will need to implement coalescing yourself.
17
See Appendix A.1 for approximate expected quantitative results from these optimizations.
Some advice on how to implement and debug your packages will be covered in the lectures and recitations,
as well as the Malloc Lab Bootcamp.
Good luck!
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A Performance Evaluation
A.1 Approximate Expected Results from Optimizations
Optimization Utilization Throughput
Implicit List (Starter Code) 59% 10–100
Explicit Free List −
a 2000–5000
Segregated Free Lists − 11000
Better Fit Algorithm 59% Variable
Eliminating Footers in Allocated Blocks +9% −
Decreasing Block Size/Mini Blocks +6% −20%
Compressing Headers +2% −
a− indicates no change.
A.2 Machine Dependencies
You will find that your program gets different throughput values depending on the model of CPU for the
machine running the program. Our evaluation code compensates for these differences by comparing the
performance of your program to implementations we have written for both the checkpoint and the final
versions. It can determine the reference performance for the machine executing the program in two different
ways. First, it looks in the file throughputs.txt to see if it has a record for the executing CPU. (Linux
machines contain a file /proc/cpuinfo that includes information about the CPU model.) Second, if it does
not find this information, it runs reference implementations that are included as part of the provided files.
Throughput information has been generated for the CPUs in the Shark machines, as well as the CPU
model used by the Autolab servers, which we refer to as“Autolab C”. The different CPU types used are:
Name ID Class Model Clock (GHz)
Shark Intel(R)Xeon(R)CPUE5520@2.27GHz Intel Xeon E5520 2.27
Autolab C Intel(R)Xeon(R)Gold6132CPU@2.60GHz Intel Xeon Gold 6132 2.60
When mdriver runs, it will print out the CPU model ID (a compressed version of the information in
/proc/cpuinfo) and the benchmark throughput for that CPU model.
A.3 Performance Points
Observing that both memory and CPU cycles are expensive system resources, we combine these two measures
into a single performance index P, with 0 ≤ P ≤ 100, computed as a weighted sum of the space utilization
and throughput:
P (U, T) = 100
w · Threshold
U − Umin
Umax − Umin ! - (1 − w) · Threshold
T − Tmin
Tmax − Tmin !!
where U is the space utilization (averaged across the traces), and T is the throughput (harmonic mean across
the traces). Umax and Tmax are the estimated space utilization and throughput of a well-optimized malloc
package, and Umin are Tmin are minimum space utilization and throughput values, below which you will
receive 0 points. The weight w defines the relative weighting of utilization versus performance in the score.
19
The Threshold function is defined by
Threshold(x) =
0, x < 0
x, 0 ≤ x ≤ 1
1, x > 1.
The values of Tmin and Tmax are computed relative to the performance of reference versions of allocators,
with one designed to meet the utilization and throughput goals for the checkpoint, and the other to meet the
goals for the final submission. If the reference version provides throughput Tref, then the throughput values
used in computing the score are given as:
Tmin = Rmin · Tref
Tmax = Rmax · Tref
where the values of Rmin and Rmax differ for the checkpoint and the final versions.
The following table shows the evaluation parameters for the checkpoint and final versions:
Version w Umin Umax Rmin Rmax
Checkpoint 0.2 0.55 0.58 0.1 0.8
Final 0.6 0.55 0.74 0.5 0.9
The following table summarizes the throughput standards, based on CPU model and checkpoint or final
version:
Checkpoint Final
Machine Tmin Tmax Tref Tmin Tmax Tref
Shark 1,211 9,690 12,112 4,004 7,208 8,009
Autolab C 1,319 10,554 13,193 4,630 8,334 9,261
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B Viewing Heap Contents with GDB
The debugging program gdb can be a valuable tool for tracking down bugs in your memory allocator. We
hope by this point in the course that you are familiar with many of the features of gdb. You will want to take
full advantage of them.
Unfortunately, normal gdb commands cannot be used to examine the heap when running mdriver-emulate.
In this appendix, we present a brief tutorial on using gdb with your program and describe a helper
function that can be used to examine the heap with gdb for both mdriver and mdriver-emulate. In this
tutorial, we use the code in mm.c as the reference implementation.
B.1 Viewing the heap without a helper function
A typical gdb session to examine the header of a block on a call to free might go something like this. In the
following, all text in italics was typed by the user. The session has been edited to remove some uninteresting
parts of the printout.
linux> gdb mdriver
(gdb) break mm_free
Breakpoint 1 at 0x4043a1: file mm.c, line 288.
(gdb) run -c traces/syn-array-short.rep
Breakpoint 1, mm_free (bp=bp@entry=0x80000eac0) at mm.c:288
(gdb) print bp
$1 = (void *) 0x80000eac0
(gdb) print /x ((unsigned long ) bp – 1)
$2 = 0x41a1
(gdb) quit
A few things about this session are worth noting:
• The function named“free”in mm.c is known to gdb in its unaliased form as“mm_free.”You can see
that the aliasing is introducted through a macro definition at the beginning of the file. When you use
gdb, you refer to the unaliased function names. The unaliased names of other important functions in
mm.c include: mm_malloc, mm_realloc, mm_calloc, mem_memset, and mem_memcpy.
• The gdb command“print /x ((unsigned long ) bp – 1)”first casts the argument to free
to be a pointer to an unsigned long. It then decrements this pointer to point to the block header and
then prints it in hex format.
• The printed value 0x41a1 indicates that the block is of size 0x41a0 (decimal 16,800), and the lowerorder
bit is set to indicate that the block is allocated. Looking at the trace file, you will see that the
block to be freed has a payload of 16,784 bytes. This required allocating a block of size 16,800 to hold
the header, payload, and footer.
When we try the same method with mdriver-emulate, things don’t work as well. In this case, we use
one of the traces with giant allocations, but the same problem will be encountered with any of the traces.
linux> gdb mdriver-emulate
(gdb) break mm_free
Breakpoint 1 at 0x4043b7: file mm.c, line 285.
(gdb) run -c traces/syn-giantarray-short.rep
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Breakpoint 1, mm_free (bp=bp@entry=0x23368bd380eb2cb0) at mm.c:285
(gdb) print bp
$1 = (void *) 0x23368bd380eb2cb0
(gdb) print /x ((unsigned long ) bp – 1)
Cannot access memory at address 0x23368bd380eb2ca8
(gdb) quit
The problem is that bp is set to 0x23368bd380eb2cb0, or around 2.54 × 1018, which is well beyond the
range of virtual addresses supported by the machine. The mdriver-emulate program uses sparse memory
techniques to provide the illusion of a full, 64-bit address space, and so it supports very large pointer values.
However, the actual addressing has an overall limit of 100 MB of actual memory usage.
B.2 Viewing the heap with the hprobe helper function
To support the use of gdb in debugging both the normal and the emulated version of the memory, we have
created a function with the following prototype:
void hprobe(void *ptr, int offset, size_t count);
This function will print the count bytes that start at the address given by summing ptr and offset. Having
a separate offset argument eliminates the need for doing pointer arithmetic in your queury.
Here’s an example of using hprobe with mdriver:
linux> gdb mdriver
(gdb) break mm_free
Breakpoint 1 at 0x4043a1: file mm.c, line 288.
(gdb) run -c traces/syn-array-short.rep
Breakpoint 1, mm_free (bp=bp@entry=0x80000eac0) at mm.c:288
(gdb) print bp
$1 = (void *) 0x80000eac0
(gdb) print hprobe(bp, -8, 8)
Bytes 0x80000eabf…0x80000eab8: 0x00000000000041a1
(gdb) quit
Observe that hprobe is called with the argument to free as the pointer, an offset of −8 and a count of 8.
The function prints the bytes with the most significant byte on the left, just as for normal printing of numeric
values. The range of addresses is shown as HighAddr…LowAddr. We can see that the printed value is
identical to what was printed using pointer arithmetic, but with leading zeros added.
The same command sequence works for mdriver-emulate:
linux> gdb mdriver-emulate
(gdb) break mm_free
Breakpoint 1 at 0x4043b7: file mm.c, line 285.
(gdb) run -c traces/syn-giantarray-short.rep
Breakpoint 1, mm_free (bp=bp@entry=0x23368bd380eb2cb0) at mm.c:285
(gdb) print bp
$1 = (void *) 0x23368bd380eb2cb0
(gdb) print hprobe(bp, -8, 8)
Bytes 0x23368bd380eb2caf…0x23368bd380eb2ca8: 0x00eb55b00c8f1ed1
(gdb) quit
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The contents of the header indicate a block size of 0xeb55b00c8f1ed0 (decimal 66,240,834,140,315,344),
with the low-order bit set to indicate that the block is allocated. Looking at the trace, we see that the block to
be freed has a payload of 66,240,834,160,315,328 bytes. Sixteen additional bytes were required for the header
and the footer.
Part of being a productive programmer is to make use of the tools available. Many novice programmers
fill their code with print statements. While that can be a useful approach, it is often more efficient to use
debuggers, such as gdb. With the hprobe helper function, you can use gdb on both versions of the driver
program.
