18-213 / 15-613 Computer Systems Summer 2024 Malloc Lab: Writing a Dynamic Storage Allocator
18-213 / 15-613, Summer 2024
Malloc Lab: Writing a Dynamic Storage Allocator
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:
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
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1 Introduction 1
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2 Logistics 3
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3 Required Functions 4
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4 Support Routines 6
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5 Programming Rules 7
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6 Driver Programs 9 6.1 Tracefiles............................................ 9 6.2 Command-linearguments.................................... 10
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7 Scoring 11
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7.1 Autogradedscore ........................................ 11 7.1.1 Performanceindex ................................... 11 7.1.2 Utilization ....................................... 11 7.1.3 Throughput....................................... 12 7.1.4 Autogradeddeductions................................. 12
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7.2 HeapConsistencyChecker ................................... 13
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7.3 Style............................................... 13
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7.4 HandinInstructions....................................... 14
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8 Useful Tips 15
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9 Office Hours 16
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10 Strategic Advice 17
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A Performance Evaluation 19 A.1 ApproximateExpectedResultsfromOptimizations ...................... 19 A.2 MachineDependencies..................................... 19 A.3 PerformancePoints....................................... 19
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B Viewing Heap Contents with GDB 21 B.1 Viewingtheheapwithoutahelperfunction .......................... 21 B.2 Viewingtheheapwiththehprobehelperfunction....................... 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/18-x13/malloclab-<YOUR USERNAME>.git
The only file you will turn in is mm.c. 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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Programming Rules
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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.
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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.
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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.
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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.
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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.
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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).
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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:
