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Introduction

In this lab you will be writing a dynamic storage allocator for C programs, i.e., your own version of the malloc, free and realloc routines. You are encouraged to explore the design space creatively and implement an allocator that is correct, efficient and fast.

Obtaining the lab

You may work in a team of two or three for this lab. If so, enter the team member information in according to the instructions in Piazza

$ cd cso-labs/ # your team's git repo if you work in a team or your own git repo if you work individually
$ git fetch upstream 
$ git merge upstream/master

This creates a number of new files in subdirectory malloclab/. Note that:

• The only file you should modify is mm.c. You do not need to fill out the team information in the beginning of mm.c

Working on the Lab

In mm.c, you should implement the following five functions (which are declared in mm.h)

int    mm_init(void);
void*  mm_malloc(size_t size);
void   mm_free(void *ptr);
void*  mm_realloc(void *ptr, size_t size);
void   mm_checkheap(int verbose);

The existing code in mm.c implements the simplest but still functionally correct malloc package that we could think of. Using this as a starting place, modify these functions (and possibly define others), so that they obey the following semantics:

• mm_init: Before calling mm_malloc mm_realloc 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 should be -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. The entire allocated block should lie within the heap region and should not overlap with any other allocated chunk.

  We will comparing your implementation to the version of malloc supplied in the standard C library. Since the standard C library malloc always returns payload pointers that are aligned to 16 bytes, your malloc implementation should do likewise and always return 16-byte aligned pointers.

• mm_free: The mm_free routine frees the block pointed to by ptr. It returns nothing. This routine is only guaranteed to work when the passed pointer ptr was returned by an earlier call to mm_malloc or mm_realloc and has not yet been freed.

• mm_realloc: The mm_realloc routine returns a pointer to an allocated region of at least size bytes with the following constraints.

  • if ptr is NULL, the call is equivalent to mm_malloc(size)
  • if size is equal to zero, the call is equivalent to mm_free(ptr)};
  • if ptr is not NULL, it must have been returned by an earlier call to mm_malloc or mm_realloc. The call to mm_realloc changes the size of the memory block pointed to by ptr (i.e.the old block) to size bytes and returns the address of the new block. Notice that the address of the new block might be the same as the old block, or it might be different, depending on your implementation, the amount of internal fragmentation in the old block, and the size of the realloc request.
  • The contents of the new block are the same as those of the old ptr block, up to the minimum of the old and new sizes. Everything else is uninitialized. For example, if the old block is 8 bytes and the new block is 12 bytes, then the first 8 bytes of the new block are identical to the first 8 bytes of the old block and the last 4 bytes are uninitialized. Similarly, if the old block is 8 bytes and the new block is 4 bytes, then the contents of the new block are identical to the first 4 bytes of the old block.

• Lastly, mm_checkheap: The mm_cheapheap routine is an auxilary routine that checks the consistency of your heap. The function argument int verbose can be used to turn on/off verbose debug printing.

The malloc,free, realloc semantics match the the semantics of the C standard library's malloc, realloc, and free routines. Type man malloc for the complete documentation.

When implementing mm_init, mm_free, mm_mallocmm_realloc functions, you need to invoke the following functions which simulate the the memory system for your dynamic memory allocator. They are defined in memlib.c:

• void *mem_sbrk(int incr): Expands the heap by incr bytes, where incr is a positive non-zero integer and returns a generic 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 non-zero integer argument.

• void *mem_heap_lo(void): Returns a generic pointer to the first byte in the heap.

• void *mem_heap_hi(void): Returns a generic pointer to the last byte in the heap.

• size_t mem_heapsize(void): Returns the current size of the heap in bytes.

• size_t mem_pagesize(void): Returns the system's page size in bytes

Checking Heap Consistency

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. We ask you to write mm_checkheap to scan 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 consists of the function void mm_heapcheck(int verbose), to be implemented in mm.c. It will check any invariants or consistency conditions you consider prudent. It returns a nonzero value if and only if your heap is consistent. You are not limited to the listed suggestions nor are you required to check all of them. This consistency checker is for your own debugging during development. When you submit mm.c, make sure to remove any calls to mm_check as they will slow down your throughput. Style points will be given for your mm_check function.

Testing for correctness and performance

We provide you with a tester program mdriver.c that tests your mm.c for correctness, space utilization, and throughput.

The tester mdriver reads a set of trace files, each of which contains a sequence of allocate, reallocate, and free events corresponding to some example application workload. It then calls your mm_malloc, mm_realloc, and mm_free routines according to the sequence of events.

To run the tester, type:

$ ./mdriver -V

The -V turns on verbose printing in the tester.

To run the tester on one specific tracefile instead of all of the default tracefiles, type:

$./mdriver -V -f tracefile

This is helpful when debugging failure with a particular tracefile. Type $./mdriver -h to see a full list of command line options.

Programming Rules

• You should not change any of the interfaces in mm.c.
• You should not invoke any memory-management related library calls or system calls, i.e. you cannot use malloc, calloc, free, realloc, sbrk, brk or any variants of these calls in your code. You may use memcpy and other C library functions that are not related to memory-management. In case of doubt, please check with the staff on Piazza.
• You should not use any dynamically sized data structures such as dynamically-sized arrays, trees, or lists in your program. Yes, you can define a global array of a fixed number of elements of structs, pointers or other scalars.
• For consistency with the C standard library's malloc package, which returns blocks aligned on 16-byte boundaries, your allocator must always return pointers that are aligned to 16-byte boundaries. The tester will enforce this requirement for you.

Programming Advice

• Use simple workloads first to debug correctness: We provide two very simple trace files short{1,2}.rep to simplify debugging. Type mdriver -f short1.rep to test your memory allocator on one of these workloads first.

• Use gdb. It will help you isolate and identify out of bounds memory references.

• 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 is a point of departure. Don't start working on your allocator until you understand everything about the simple implicit list allocator.

• 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. You can reduce the complexity significantly by writing macros for your pointer operations. See the textbook for examples.

• Do your implementation in stages. The first 9 traces contain requests to malloc and free. The last 2 traces contain requests for realloc, malloc, and free. We recommend that you start by getting your malloc and free routines working correctly and efficiently on the first 9 traces. Only then should you turn your attention to the realloc implementation. For starters, build realloc on top of your existing malloc and free implementations. But to get really good performance, you will need to build a stand-alone realloc.

• Use a profiler. You may find the gprof tool helpful for optimizing performance.

Evaluation

You will receive zero points if you break any of the rules or your code is buggy and crashes the tester. Otherwise, your grade will be calculated as follows:

• Correctness (15 points). You will receive full points if your solution passes the correctness tests performed by the tester program. You will receive partial credit for each correct trace.

• Performance (40 points). Two performance metrics will be used to evaluate your solution:

  • Space utilization: The peak ratio between the aggregate amount of memory used by the tester (i.e., allocated via mm_malloc or mm_realloc but not yet freed via mm_free) and the size of the heap used by your allocator. The optimal ratio equals to 1. 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 tester program summarizes the performance of your allocator by computing a performance index, which is a weighted sum of the space utilization (U) and throughput (T)

    Performance index = 0.6*U + 0.4*min(1, T/T_libc)
    

    where U is your space utilization, T is your throughput, and T_libc is the measured throughput of Standard C library's malloc on your system on the default traces.

    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 100%. To receive a good score, you must achieve a balance between utilization and throughput.

• Style (10 points).

  • Please follow this style guide.
  • Your code should be decomposed into functions and use as few global variables as possible.
  • Your code should begin with a header comment that describes the structure of your free and allocated blocks, the organization of the free list, and how your allocator manipulates the free list. each function should be preceeded by a header comment that describes what the function does.
  • Your heap consistency checker mm_checkheap should be thorough.

  You will be awarded 5 points for a good heap consistency checker and 5 points for good program structure and comments.

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