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 summer I've
decided to make the implementation of realloc optional). You are encouraged to
explore the design space creatively and implement an allocator that is correct,
efficient and fast.
For this machine problem you'll be working in the mps/05 directory.
As before, don't forget to commit your previous work and pull the latest changes from the central repository before starting!
The only file you will be modifying is mm.c. The mdriver.c program
is a driver program that allows you to evaluate the performance of your
solution. Use the command make to generate the driver code and run it
with the command ./mdriver -V. (The -V flag displays helpful summary
information.)
Your dynamic storage allocator will consist of the following four
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);
void *mm_realloc(void *ptr, size_t size);
The mm.c file we have given you 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 other
private static functions), 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 (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.
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 (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.
These semantics match the the semantics of the corresponding libc malloc,
realloc, and free routines. Refer to the
malloc manpage for complete documentation.
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:
Your heap checker will consist of the function int mm_check(void) 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. You are encouraged to print out error
messages when mm_check fails.
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.
The memlib.c package simulates the memory system for your dynamic
memory allocator. You can invoke the following functions 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
(4K on Linux systems).
The driver program mdriver.c tests your mm.c package for correctness, space
utilization, and throughput. The driver program is controlled by a set of trace
files, each of which contains a sequence of allocate, reallocate, and free
directions that instruct the driver to call your mm_malloc, mm_realloc, 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 driver mdriver.c 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.
You should not change any of the interfaces in mm.c.
You should not invoke any memory-management related library calls or system
calls. This excludes the use of malloc, calloc, free, realloc,
sbrk, brk or any variants of these calls in your code.
You are not allowed to allocate any global or static compound data
structures such as arrays, structs, trees, or lists in your code. However,
you are allowed to declare global scalar variables such as integers, floats,
and pointers.
For consistency with the libc malloc package, 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.
While you may certainly refer to the book's implicit list based implementation for inspiration or help, you may not plagiarize it!
You will receive zero points if you break any of the rules or your code is buggy and crashes the driver. Otherwise, your grade will be calculated as follows:
Correctness (35 points). You will receive full points if your solution passes the correctness tests performed by the driver program. You will receive partial credit for each correct trace.
Performance (45 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 driver (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 driver program summarizes the performance of your allocator by computing a performance index, (P), which is a weighted sum of the space utilization ((U)) and throughput ((T)):
[ P = w \cdot U + (1 - w) \cdot \mbox{min}\left( 1, \frac{T}{T_{libc}} \right) ]
where (T_{libc}) is the estimated average throughput of libc malloc on your
system on the default traces. The value for (T_{libc}) is a constant in the
driver (9000 Kops/s), based on a conservative estimate of libc malloc
throughput on fourier. The performance index favors space utilization over
throughput, with a default of (w = 0.6).
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. Since each metric will contribute at most w and 1-w to the performance index, respectively, 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.
While the ideal value for your performance index is 1 (100%), a score of 0.9 (90%) or higher will net you the full 45 points for performance. A lower index will be used to compute your score according to the formula (\mbox{min}(45, (P + (1-0.9)) \times 45)).
Use the mdriver -f option. During initial development, using tiny trace
files will simplify debugging and testing. We have included two such trace
files (short{1,2-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 gdb! A debugger 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.
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.
Start early! It is possible to write an efficient malloc package with a few pages of code. However, we can guarantee that it will be some of the most difficult and sophisticated code you have written so far in your career. So start right away, and good luck!