CSE 351 Spring 2011 Lab 5
Writing a Dynamic Storage Allocator
Assigned: Wednesday, May 25, 2011
Wednesday, June 1, Friday, June 3, 2011 at 11:59 PM
In this lab you will be writing a dynamic storage allocator for C
programs, i.e., your own version of the malloc and free
Start by extracting /projects/instr/11sp/cse351/lab5.tar
to a directory on attu in which you plan to do your work, by typing:
tar xvf /projects/instr/11sp/cse351/lab5.tar
(In the following instructions, we will assume that you are executing
programs in your local directory on attu
For this lab, you can work anywhere
there's a C compiler and make
, but make sure your allocator
works on attu
, where we'll be testing it.)
This will cause a number of files to be unpacked in a directory called lab5.
The only file you will be
modifying and handing in is mm.c.
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.
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's basically an
unsigned integer.) The entire allocated block should lie within
the heap region and should not overlap with any other allocated
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.
- 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
and has not
yet been freed.
These semantics match the the semantics of the corresponding
malloc and free routines in libc.
Type man malloc to the shell for complete documentation.
Provided (Supporting) Code
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(int reqSize) returns a block of
at least the requested size if one exists (and NULL
- void insertFreeBlock(BlockInfo* blockInfo) inserts the
given block in the free list in LIFO manner.
- void removeFreeBlock(BlockInfo* blockInfo) removes the
given block from the free list.
- Helper functions for implementing list functions:
- BlockInfo* getFreeListHead() returns a pointer to the
first block in the free list.
- void getFreeListHead(BlockInfo* newHead) takes a pointer
to a block and sets the head of the free list to point to the block
referenced by the pointer.
In addition, we implement mm_init and provide two helper
functions implementing important parts of the allocator:
- void requestMoreSpace(int 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 use macros as well, though the ones already included in
mm.c are the only ones we used in our sample solution, so it's
possible without more. For more info on macros, check the GCC manual
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 non-zero 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 non-zero 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).
The Trace-driven Driver Program
The driver program mdriver.c in the lab5.tar
distribution 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 are posted on attu (a copy is included in the
lab5.tar distribution in case you want to work on another computer; 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
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.
Print a summary of the command line arguments.
Run and measure libc malloc in addition to the student's malloc package.
Verbose output. Print a performance breakdown for each tracefile
in a compact table.
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 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.
Your grade will be calculated (as a percentage) out of a total of 60 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
Your code should be as clear and concise as possible.
Since some of the unstructured pointer manipulation inherent to
allocators can be confusing, short inline comments on steps of the
allocation algorithms are also recommended. (These will also help us
give you partial credit if you have a partially working
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:
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
- 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. You should find good policies to minimize
fragmentation in order to make this ratio as close as possible to the
Throughput: The average number of operations completed per second.
where U is your space utilization, T is your throughput, and
Tlibc is the estimated throughput of libc malloc on your
system on the default traces.1 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 0.7 and 0.8; it
would receive 5 points.
P = 0.6U + 0.4 ||
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
- 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. To use
the -g flag with the Makefile, edit the CFLAGS
variable in the Makefile OR just run make like this:
make CFLAGS="-Wall -g"
This has the effect of replacing the value of CFLAGS as
defined in the Makefile with whatever you type on the command
line instead. (Just be sure to quote it if it contains spaces, and
do not put spaces around the = sign.) In general, when debugging,
you want to turn off the -O2 (that's a capital o, not a
zero), since it tells the compiler to perform optimizations that can
occasionally make following in the debugger confusing. 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.
- 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.
- 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, 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 early, and good luck!
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. 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
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
- 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
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.
Submitting Your Work
When you have completed the lab, you will hand in only one file (mm.c
), which contains your solution. Turn in mm.c
using the regular
1The value for Tlibc is a
constant in the driver (1800 Kops/s) that is close to the average
throughput of the libc allocator on the same traces, measured
on attu. The performance index will vary from system to
system based on the local libc allocator throughput, so run on
attu for a good idea of where you stand.