Assigned | Wednesday, November 28, 2012 |
---|---|
Due Date | Friday, December 7, 2012 at 5:00p |
Files | lab5.tar.gz |
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.
Start by
extracting lab5.tar.gz
to a
directory on attu
in which you plan to do your work, by typing:
wget cs.washington.edu/education/courses/cse351/12sp/labs/5/lab5.tar.gz
tar xzvf lab5.tar.gz
This will cause a number of files to be unpacked in a directory
called lab5. The only file you will modify and turn in is mm.c
.
(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.)
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's an
unsigned integer that can represent a size spanning all of memory, so
on x86_64 it is a 64-bit integer.) 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 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
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.
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
otherwise).
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.
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.
FREE_LIST_HEAD
: returns a pointer to
the first block in the free list (the head of the free list).
POINTER_ADD
and POINTER_SUB
: useful
for doing pointer arithmetic without worrying about the size
of BlockInfo struct.
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 driver program mdriver.c
in
the lab5.tar.gz
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
(if you want to work on another
computer, you can copy these files and then 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.
mm.c
.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.)
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.)
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:
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
optimal.
P = 0.6U + 0.4 min (1, T/Tlibc)
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. 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.
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.
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.
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.
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.
POINTER_ADD
and
POINTER_SUB
.
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
.
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:
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.
Submit your mm.c
file to
the Catalyst
Drop Box for this assignment.