Malloc Lab

This malloc lab is based on the one by Bryant and O’Hallaron for Computer Systems: A Programmer’s Perspective, Third Edition

Due: Wednesday, November 23, 11:59pm

In this lab, you’ll write a dynamic storage allocator for C programs, i.e., your own version of the malloc and free functions. You are encouraged to explore the design space creatively and implement an allocator that is correct, space-efficient, and fast.

Getting Started

Start by unpacking malloclab-handout.zip. The only file you will be modifying and handing in is "mm.c". The "mdriver.c" program is a driver program that allows you to evaluate the performance of your solution. Use make to generate the driver code and run it as

  $ ./mdriver -V

where the -V flag displays helpful summary information.

When you have completed the lab, you will hand in only one file, "mm.c", which contains your solution.

How to Work on the Lab

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

The "mm.c" file that we have given you implements the simplest but still functionally correct malloc implementation that we could think of. Using this as a starting place, modify these functions (and possibly define other private, static functions), so that they obey the following semantics:

• mm_init: 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 initialization, such as allocating the initial heap area. The return value should be -1 if there was a problem in performing the initialization, 0 otherwise.

  The mm_init function will be called once per benchmark run, so it can be called multiple times in the same run of mdriver. Your mm_init function should reset your implementation to its initial state in each case.

• mm_malloc: The mm_malloc function returns a pointer to an allocated block payload of at least size bytes, where size is less than 232. The entire allocated block should lie within the heap region and should not overlap with any other allocated block.

  We’ll 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 16 bytes, your malloc implementation should do likewise and always return 16-byte aligned pointers.

• mm_free: The mm_free function 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 and has not yet been freed.

These semantics match the corresponding libc malloc and free functions.

Beyond correctness, your goal is to produce an allocator that performs well in time and space. That is, the mm_malloc and mm_free functions should work as quickly as possible, and the total amount of memory used by your allocator should stay as close as possible to the amount of memory needed to hold the payload of mm_malloc calls not yet balanced by mm_free calls.

Support Functions

The "memlib.c" module provides a thin wrapper on the operating system’s virtual-memory system. Your allocator will need to use these functions to obtain pages of memory that it can allocate from. When mdriver runs your allocator, it resets the memory system (i.e., frees all pages) before calling mm_init to start a new benchmark run.

You can invoke the following functions from memlib.c:

• size_t mem_pagesize(void);

  Returns the miminum granularity of allocation via mem_map and deallocation via mem_unmap.

  You can assume that the allocation granularity is a power of 2 and at least 4096.

• void *mem_map(size_t amt);

  Returns a pointer to the start of group of newly allocated, contiguous pages that occupy exactly amt bytes.

  The given amt must be a multiple of the value produced by mem_pagesize, and the returned address is always a multiple of the value produced by mem_pagesize. Separate calls to mem_map may not return pages that are contiguous to pages from previous calls.

  The result is NULL if memory cannot be allocated.

• void mem_unmap(void *addr, size_t amt);

  Releases back to the operating system a group of contiguous pages that start at addr and cover exactky amt bytes.

  The given addr must be page-aligned (i.e., it must be a multiple of the value produced by mem_pagesize), and amt must be a multiple of the value produced by mem_pagesize. The given addr must also identify pages formerly allocated with mem_map and since released by mem_unmap. A single mem_unmap can release pages allocated by multiple previous mem_map calls, and multiple different mem_unmap calls can release pages allocated by a single mem_map call—as long as each individual page’s allocation is balanced by exactly one deallocation.

• size_t mem_heapsize(void);

  Returns the total size of all currently allocated pages (i.e., mapped and not yet freed).

While your allocator will obviously need to call mem_map to obtain memory for allocation, space-efficiency for this assignment also means using mem_unmap to avoid retaining pages that are not needed as allocated blocks are freed.

Tip: Heap Consistency Checker

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. You will find it very 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?

You may find it useful to insert a call to your consistency checking just before your mm_malloc or mm_free function returns. Before you submit "mm.c", however, make sure to remove any calls to your checker, since it will slow down throughput.

Trace-based Driver Program

The driver program "mdriver.c" tests your "mm.c" implementation for correctness, space utilization, and throughput. The driver program is controlled by a set of trace files that are included in the malloclab-handout.zip archive. Each trace file contains a sequence of allocate, free, and reallocate (i.e., allocate plus free) directions that instruct the driver to call your mm_malloc and mm_free functions in some sequence. The driver and the trace files are the same ones we will use when we grade your handin "mm.c" file, although we may change the order in which the traces are used.

The provided makefile combines "mdriver.c" with your "mm.c" to create mdriver, which accepts the following command line arguments:

• -t ‹tracedir› — Look for the default trace files in directory ‹tracedir› instead of the default directory that is 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 "mm.c" malloc implementation.

• -v — Verbose output, printing a performance breakdown for each tracefile in a compact table.

• -V — More verbose output, printing additional diagnostic information as each trace file is processed. This flags is useful during debugging to determine which trace file is causing your malloc implementation to fail.

You may find it useful to see the shape of memory use created by a trace file. Run

  $ racket plot.rkt ‹tracefile›

to see a plot of allocated memory over time for ‹tracefile›.

Programming Rules

• Your "mm.c" must be implemented in ANSI standard C, as always.

• You must not change any of the interfaces in "mm.c". We will compile your "mm.c" with a fresh copy of the driver files.

• You must not invoke any memory-management related library calls or system calls, including mmap, malloc, calloc, free, realloc, sbrk, brk or any variants.

• You must not define any global or static compound data structures such as arrays, structs, trees, or lists in your "mm.c" program. However, you are allowed to declare types (including struct types) in "mm.c", and you are allowed to declare global scalar variables such as integers, floats, and pointers in "mm.c".

• For consistency with the libc malloc implementation, which returns blocks aligned on 16-byte boundaries, your allocator must always return pointers that are aligned to 16-byte boundaries. The driver will enforce this requirement for you.

Evaluation

Your grade will be calculated as follows:

• Correctness — Your performance grade will be scaled by the percentage of traces that your allocator handles correctly.

• Performance — Three performance metrics will be used to evaluate your solution:

  • Space utilization: The ratio between the peak aggregate amount of memory used by the driver (i.e., allocated via mm_malloc but not yet freed via mm_free) and the peak total size of pages used by your allocator. The optimal ratio is 1. You should find good policies to minimize fragmentation in order to make this ratio as close as possible to the optimal.

  • Instantaneous space utilization: The geometric mean of the ratio between the amount of memory used by the driver at after each step of the trace and and the total size of pages used by your allocator at that point. The optimal ratio is 1. Since the amount of memory allocated by a trace goes goes down as well as up during the trace, an optional solution will release memory pages as they are no longer needed. At the same time, beware that constantly mapping and unmapping pages can take too much time.

  • 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 space utilization U, instantaneous space utilization Ui, and throughput T relative to a baseline through Tlibc:

  P = 0.3U + 0.3Ui + 0.4T/Tlibc

  The value of Tlibc is 7500K ops/second. We will run your code on a CADE machine, as usual, and we’ll also supply the -l flag to warm up the processor and cache before measuring your functions.

  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%. Since each metric will contribute at most a fraction to the performance index, you should not go to extremes to optimize either the memory utilization or the throughput only. To receive a good score, you must achieve a balance between utilization and throughput. Note that the performance index overall favors space utilization over throughput.

  An overall P of 0.52 is the threshold for a guaranteed check (80%), and an overall P of 0.75 is the threshold for a guaranteed check+ (100%). Partial credit will be given for solutions that are not simply the naive allocator and still pass tests. Substiantial partial credit will be given for implementations that include strategies needed for a performant solution, such as using an explicit free list, coalescing free blocks, and unmapping unused pages. Solutions that nearly reach a threshold will receive a grade that is close to the threshold’s grade.

More Tips

• Start early! It is possible to write an efficient malloc package with a few pages of code. However, an allocator could easily be the most difficult and sophisticated code you have written so far.

• 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.

• Use the mdriver -l options to warm up the processor and cache before running your functions.

• Compile with gcc -g and use a debugger. A debugger will help you isolate and identify out of bounds memory references.

• Understand every line of the malloc implementations in the textbook and slides. The textbook and slides both have a detailed example of a simple allocator based on an implicit free list. Use them as a point of departure. Don’t start working on your allocator until you understand everything about the implicit-list allocator. That simple allocator will not work as-is for this assignment, however, since it’s based on sbrk instead of mem_map.

• Encapsulate your pointer arithmetic in functions or 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 and slides for examples.

• If you use macros, put each use of a macro argument in parentheses within the macro definition, and always put parentheses around the right-hand side of a macro definition. Otherwise, it’s easy to write macros that parse differently than you expect when the macro is textually expanded.

• Complete your implementation in stages. Get a basic implementation working, and then modify it to improve performance

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

• Keep in mind that mmap (especially) takes time, so you’ll want to make as few calls to mem_map as possible while achieving good utilization.