Lab 5: Writing a Dynamic Memory Allocator

Learning Objectives and Overview

• Implement a memory allocator using an explicit free list.
• Examine how algorithm choice impacts tradeoffs between utilization and throughput.
• Read and modify a substantial C program.
• Improve your C programming skills including gaining more experience with structs, pointers, macros, and debugging.

In this lab, you will be writing a dynamic storage allocator for C programs, i.e., your own version of the malloc and free routines. This is a classic implementation problem with many interesting algorithms and opportunities to put several of the skills you have learned in this course to good use. It is quite involved. Start early!

Code for this lab

• Browser (click to download): lab5.tar
• Terminal: wget https://courses.cs.washington.edu/courses/cse351/18sp/labs/lab5.tar

Running tar xvf lab5.tar from the terminal will extract the lab files to a directory called lab5.

Instructions

The only file you will modify and turn in is mm.c (unless you decide to do extra credit). You may find the short README file useful to read.

(In the following instructions, we will assume that you are executing programs on the CSE VM or in your local directory on attu. For this lab, you can work anywhere there's a C compiler and make.)

Your dynamic storage allocator will consist of the following three functions (and several helper functions), which are declared in mm.h and defined in mm.c:

int   mm_init(void);
void* mm_malloc(size_t size);
void  mm_free(void* ptr);

The mm.c file we have given you partially implements an allocator using an explicit free list. Your job is to complete this implementation by filling out mm_malloc and mm_free. The three main memory management functions should work as follows:

• mm_init (provided): Before calling mm_malloc or mm_free, the application program (i.e., the trace-driven driver program that you will use to evaluate your implementation) calls mm_init to perform any necessary initializations, such as allocating the initial heap area. The return value is -1 if there was a problem in performing the initialization, 0 otherwise.
• mm_malloc: The mm_malloc routine returns a pointer to an allocated block payload of at least size bytes. (size_t is a type for describing sizes; it is an unsigned integer that can represent a size spanning all of memory, so on x86_64 it is a 64-bit unsigned value.) The entire allocated block should lie within the heap region and should not overlap with any other allocated block.
• mm_free: The mm_free routine frees the block pointed to by ptr. It returns nothing. This routine is guaranteed to work only when the passed pointer (ptr) was returned by an earlier call to mm_malloc and has not yet been freed. These semantics match the semantics of the corresponding malloc and free routines in libc. Type man malloc in the shell for complete documentation.

We will compare your implementation to the version of malloc supplied in the standard C library (libc). Since the libc malloc always returns payload pointers that are aligned to 8 bytes, your malloc implementation should do likewise and always return 8-byte aligned pointers.

Provided Code for the Allocator

We define a BlockInfo struct designed to be used as a node in a doubly-linked explicit free list, and the following functions for manipulating free lists:

• BlockInfo* searchFreeList(size_t reqSize): returns a block of at least the requested size if one exists (and NULL otherwise)
• void insertFreeBlock(BlockInfo* blockInfo): inserts the given block in the free list in a LIFO manner
• void removeFreeBlock(BlockInfo* blockInfo): removes the given block from the free list

In addition, we implement mm_init and provide two helper functions implementing important parts of the allocator:

• void requestMoreSpace(size_t incr): enlarges the heap by incr bytes (if enough memory is available on the machine to do so)
• void coalesceFreeBlock(BlockInfo* oldBlock): coalesces any other free blocks adjacent in memory to oldBlock into a single new large block and updates the free list accordingly

Finally, we use a number of C Preprocessor macros to extract common pieces of code (constants, annoying casts/pointer manipulation) that might be prone to error. Each is documented in the code. You are welcome to create your own macros as well, though the ones already included in mm.c are the only ones we used in our sample solution, so it is possible without more. For more info on macros, check the GCC manual and/or two of the videos posted at the top of this page.

• FREE_LIST_HEAD: returns a pointer to the first block in the free list (the head of the free list)
• UNSCALED_POINTER_ADD and UNSCALED_POINTER_SUB: useful for calculating pointers without worrying about the size of struct BlockInfo
• Other short utilities for extracting the size field and determining block size

Additionally, for debugging purposes, you may want to print the contents of the heap. This can be accomplished with the provided examine_heap() function.

Provided Code for the Underlying Memory System

The memlib.c package simulates the memory system for your dynamic memory allocator. In your allocator, you can call the following functions (if you use the provided code for an explicit free list, most uses of the memory system calls are already covered).

• void* mem_sbrk(int incr): Expands the heap by incr bytes, where incr is a positive nonzero integer and returns a pointer to the first byte of the newly allocated heap area. The semantics are identical to the Unix sbrk function, except that mem_sbrk accepts only a positive nonzero integer argument. (Run man sbrk if you want to learn more about what this does in Unix.)
• void* mem_heap_lo(): Returns a pointer to the first byte in the heap
• void* mem_heap_hi(): Returns a pointer to the last byte in the heap.
• size_t mem_heapsize(): Returns the current size of the heap in bytes.
• size_t mem_pagesize(): Returns the system's page size in bytes (4K on Linux systems).

Provided Code for a Trace-Driven Driver Program

The driver program mdriver.c tests your mm.c package for correctness, space utilization, and throughput. Use the command make to generate the driver code and run it with the command ./mdriver -V (the -V flag displays helpful summary information as described below).

The driver program is controlled by a set of trace files that it will expect to find in a subdirectory called traces. The .tar. file provided to you should unpack into a directory structure that places the traces subdirectory in the correct location relative to the driver. (If you want to move the trace files around, you can update the TRACEDIR path in config.h). Each trace file contains a sequence of allocate and free directions that instruct the driver to call your mm_malloc and mm_free routines in some sequence. The driver and the trace files are the same ones we will use when we grade your submitted mm.c file.

The mdriver executable accepts the following command line arguments:

• -t <tracedir>: Look for the default trace files in directory tracedir instead of the default directory defined in config.h.
• -f <tracefile>: Use one particular tracefile for testing instead of the default set of tracefiles.
• -h: Print a summary of the command line arguments.
• -l: Run and measure libc malloc in addition to the student's malloc package.
• -v: Verbose output. Print a performance breakdown for each tracefile in a compact table.
• -V: More verbose output. Prints additional diagnostic information as each trace file is processed. Useful during debugging for determining which trace file is causing your malloc package to fail.

Programming Rules

• You should not change any of the interfaces in mm.c (e.g. names of functions, number and type of parameters, etc.).
• You should not invoke any memory-management related library calls or system calls. This means you cannot use malloc, calloc, free, realloc, sbrk, brk or any variants of these calls in your code. (You may use all the functions in memlib.c, of course.)
• You are not allowed to define any global or static compound data structures such as arrays, structs, trees, or lists in your mm.c program. You are allowed to declare global scalar variables such as integers, floats, and pointers in mm.c, but try to keep these to a minimum. (It is possible to complete the implementation of the explicit free list without adding any global variables.)
• For consistency with the malloc implementation in libc, which returns blocks aligned on 8-byte boundaries, your allocator must always return pointers that are aligned to 8-byte boundaries. The driver will enforce this requirement for you.

Collaboration

This is all a reminder, but an important one: In general we encourage students to discuss ideas from the labs and homeworks. But the code you submit must be your own. You must not refer to solutions from previous quarters or from a similar course at another university -- that is cheating. You must not consult a fellow student's solution -- that is also cheating.

We encourage you to start early and ask questions as needed. The course staff is here to help you succeed. But even if you do end up struggling and not completing the assignment to your full satisfaction, submitting a partial solution is much better than cheating.

Evaluation

Your grade will be calculated (as a percentage) out of a total of 64 points as follows:

• Correctness (45 points). You will receive 5 points for each test performed by the driver program that your solution passes. (9 tests)
• Style (10 points).
  • Your code should use as few global variables as possible (ideally none!).
  • Your code should be as clear and concise as possible.
  • You should use provided macros whenever they do what is needed and make your code more readable. You will lose points if you reimplement code or use “magic numbers” where you should have used a macro instead.
  • Since some of the unstructured pointer manipulation inherent to allocators can be confusing, we expect to see short inline comments on steps of the allocation algorithms. (These will also help us give you partial credit if you have a partially working implementation.)
  • Each function should have a header comment that describes what it does and how it does it.
• Performance (5 points). Performance represents a small portion of your grade. We are most concerned about the correctness of your implementation. For the most part, a correct implementation will yield reasonable performance. Two performance metrics will be used to evaluate your solution:
  • Space utilization: The peak ratio between the aggregate amount of memory used by the driver (i.e., allocated via mm_malloc but not yet freed via mm_free) and the size of the heap used by your allocator. The optimal ratio is 1, although in practice we will not be able to achieve that ratio. You should find good policies to minimize fragmentation in order to make this ratio as close as possible to the optimal.
  • Throughput: The average number of operations completed per second.
  The driver program summarizes the performance of your allocator by computing a performance index, P, which is a weighted sum of the space utilization and throughput:

  P = 0.6U + 0.4 min (1, T/Tlibc)

  where U is your space utilization, T is your throughput, and T_libc is the estimated throughput of libc malloc on your system on the default traces. The performance index favors space utilization over throughput. You will receive 5(P+ 0.1) points, rounded up to the closest whole point. For example, a solution with a performance index of 0.63 or 63% will receive 4 performance points. Our complete version of the explicit free list allocator has a performance index between just over 0.7 and 0.8; it would receive a full 5 points. Thus if you have a performance index GREATER THAN 0.7 (mdriver prints this as "70/100") then you will get the full 5 points for Performance. Observing that both memory and CPU cycles are expensive system resources, we adopt this formula to encourage balanced optimization of both memory utilization and throughput. Ideally, the performance index will reach P = 1 or 100% . To receive a good performance score, you must achieve a balance between utilization and throughput.
• Reflection (4 points).

Hints

Getting Started

• Read these instructions.
• Read over the provided code.
• Take notes while doing the above.
• Draw some diagrams of what the data structures should look like before and after various operations.
• Understand every line of the malloc implementation in the textbook. The textbook has a detailed example of a simple allocator based on an implicit free list. Use this as a point of departure. Don't start working on your allocator until you understand everything about the simple implicit list allocator.

Debugging

• Use the mdriver -f option. During initial development, using tiny trace files will simplify debugging and testing. We have included two such trace files (short1-bal.rep and short2-bal.rep) that you can use for initial debugging.
• Use the mdriver -v and -V options. The -v option will give you a detailed summary for each trace file. The -V will also indicate when each trace file is read, which will help you isolate errors.
• Compile with gcc -g and use gdb. The -g flag tells gcc to include debugging symbols, so gdb can follow the source code as it steps through the executable. The Makefile should already be set up to do this. A debugger will help you isolate and identify out of bounds memory references. You can specify any command line arguments for mdriver after the run command in gdb e.g. run -f short1-bal.rep.
• Write a function that treats the heap as an implicit list, and prints all header information from all the blocks in the heap. Using fprintf to print to stderr is helpful here because standard error is not buffered so you will get output from your print statements even if the next statement crashes your program.
• Encapsulate your pointer arithmetic in C preprocessor macros. Pointer arithmetic in memory managers is confusing and error-prone because of all the casting that is necessary. We have supplied maros that do this: see UNSCALED_POINTER_ADD and UNSCALED_POINTER_SUB.
• Use a profiler. You may find the gprof tool helpful for optimizing performance. (man gprof or searching online for gprof documentation will get you the basics.) If you use gprof, see the hint about debugging above for how to pass extra arguments to GCC in the Makefile.
• Start early! It is possible to write an efficient malloc package with a few pages of code. However, it will be some of the most difficult and sophisticated code you have written so far in your career. So start early, and good luck!

Heap Consistency Checker

This is an optional, but recommended, addition that will help you check to see if your allocator is doing what it should (or figure out what it's doing wrong if not). Dynamic memory allocators are notoriously tricky beasts to program correctly and efficiently. They are difficult to program correctly because they involve a lot of untyped pointer manipulation. In addition to the usual debugging techniques, you may find it helpful to write a heap checker that scans the heap and checks it for consistency.

Some examples of what a heap checker might check are:

• Is every block in the free list marked as free?
• Are there any contiguous free blocks that somehow escaped coalescing?
• Is every free block actually in the free list?
• Do the pointers in the free list point to valid free blocks?
• Do any allocated blocks overlap?
• Do the pointers in a heap block point to valid heap addresses?

Your heap checker will consist of the function int mm_check(void) in mm.c. Feel free to rename it, break it into several functions, and call it wherever you want. It should check any invariants or consistency conditions you consider prudent. It returns a nonzero value if and only if your heap is consistent. This is not required, but may prove useful. When you submit mm.c, make sure to remove any calls to mm_check as they will slow down your throughput.

Extra Credit

As optional extra credit, implement a final memory allocation-related function: mm_realloc. Write your implementation in mm-realloc.c.

The signature for this function, which you will find in your mm.h file, is:

extern void* mm_realloc(void* ptr, size_t size);

Similarly, you should find the following in your mm-realloc.c file:

void* mm_realloc(void* ptr, size_t size) {
	// ... implementation here ...
}

To receive credit, you should follow the contract of the C library's realloc exactly (pretending that malloc and free are mm_malloc and mm_free, etc.). The man page entry for realloc says:

The realloc() function changes the size of the memory block pointed to by
ptr to size bytes.  The contents will be unchanged in the range from the
start of the region up to the minimum of the old and new sizes.  If the
new size is larger than the old size, the added memory will not be
initialized.  If ptr is NULL, then the call is equivalent to
malloc(size), for all values of size; if size is equal to zero, and ptr
is not NULL, then the call is equivalent to free(ptr).  Unless ptr is
NULL, it must have been returned by an earlier call to malloc(), calloc()
or realloc().  If the area pointed to was moved, a free(ptr) is done.

A good test would be to compare the behavior of your mm_realloc to that of realloc, checking each of the above cases. Your implementation of mm_realloc should also be performant. Avoid copying memory if possible, making use of nearby free blocks. You should not use memcpy to copy memory; instead, copy WORD_SIZE bytes at a time to the new destination while iterating over the existing data.

To run tracefiles that test mm_realloc, compile using make mdriver-realloc. Then, run mmdriver-realloc with the -f flag to specify a tracefile, or first edit config.h to include additional realloc tracefiles (realloc-bal.rep and realloc2-bal.rep) in the default list.

Don't forget to submit your finished mm-realloc.c along with mm.c.

Extra Extra Credit

Do NOT spend time on this part until you have finished and turned in the core assignment.

In this extra credit portion, you will implement a basic mark and sweep garbage collector. Write your implementation in mm-gc.c.

Some additional notes:

• To test the current garbage collector, use make mdriver-garbage which generates an executable called mdriver-garbage.
• The driver assumes that you have a correctly working mm_malloc and mm_free implementation.
• The tester checks that all of the blocks that should have been freed are freed and that all of the others remain allocated. On success it prints "Success! The garbage collector passed all of the tests". You can look in GarbageCollectorDriver.c to see what the test code does.
• This implementation assumes that the alignment is 8 bytes because the third bit of sizeAndTags is used as the mark bit to mark the block.
• The function is_pointer only looks for pointers that point to the beginning of a payload (like Java), and will return false if it points to a free block.
• Pointers will always be word aligned in the data block.

Don't forget to submit your finished mm-gc.c along with mm.c.

Lab 5 Reflection

#include <stdlib.h>
#include <stdio.h>

// Assume called with one argument that is a positive number
int main(int argc, char**argv) {
  int *p, *q;
  p = (int*)malloc(sizeof(int));
  *p = 42;
  free(p);
  q = (int*)malloc(atoi(argv[1])*sizeof(int));
  *q = 43;
  *p = 44;
  printf("%d\n",*q);
  free(q);
}

1. This program has a bug. Explain what it is. [1pt]
2. If you run this buggy program with argument 2 (i.e., ./hmm 2), what happens? Give a plausible description of what malloc and free might be doing that leads to this behavior. [1pt]
3. If you run this buggy program with argument 20 (i.e., ./hmm 20), what happens? Give a plausible description of what malloc and free might be doing that leads to this behavior. [1pt]
4. If you run this buggy program with argument 40 (i.e., ./hmm 40), what happens? Give a plausible description of what malloc and free might be doing that leads to this behavior. [1pt]

Submission Instructions

Submit the following files:

1. mm.c
2. mm-realloc.c [if you did the extra credit]
3. mm-gc.c [if you did the extra extra credit]
4. lab5reflect.txt

Once you're satisfied with your solutions, submit them via Canvas (link is at the top of this page).