Assignment #4: Dynamic Storage Allocator
Trivia email to TA by 11:59pm, Monday, April 6.
Full assignment due by 11:59pm, Monday, April 13.
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.
The managing TA for this assignment is Brandon Allard
(ballard@u.rochester.edu).
Please first direct your questions about this assignment to the managing TA who
has the most intimate knowledge about the assignment setup and specific details.
The TA's office hours for this assignment are:
- 3-5pm on Friday, April 3
- 6-8pm on Monday, April 6
- 6-8pm on Wednesday, April 8
- 3-5pm on Friday, April 10
- 10am-noon, Monday, April 13
The office hours are held at Hylan 301.
You can form a group of two for this assignment. You can also choose to
work alone. You are able to choose to work in a group of two
for four assignments (assignments #2/#3/#4/#5).
It is our policy that you
can NOT work with the same team partner for more than two assignments.
If you do, we will apply 50% penalty on your third and four assignments
with the same grouping.
Hand Out Instructions:
Start by copying /u/cs252/labs15/malloclab-handout.tar to a
protected directory in which you plan to do your work. Then give
the command:
tar xvf malloclab-handout.tar
This will cause a bunch of files to be unpacked into the directory.
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 the command
make to generate the driver code and run it with the command
./mdriver –V. (The –V flag displays
helpful summary information.)
Looking at the file mm.c you'll notice a C structure team
into which you should insert the requested identifying information
about the one or two individuals comprising your programming team.
Do this right away so you don't forget.
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 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.
All payload pointers returned by mm_malloc should be aligned to a 16-byte boundary.
- 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
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 (garbage). 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. Type
man malloc to the shell for complete documentation.
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?
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.
Style points will be given for your mm_check function.
Make sure to put in comments and document what you are checking.
Support Routines:
The memlib.c package simulates the OS portion of 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
based on the Unix sbrk function, with one notable exception:
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 (4KB on Linux
systems).
The Trace-driven Driver Program:
The driver program mdriver.c in the malloclab-handout.tar
distribution tests your mm.c package for correctness, space
utilization, and throughput. The driver program is controlled by a
set of trace files. Some small traces are included in the
malloclab-handout.tar distribution. The larger traces that we will
test your file with are located at /u/cs252/labs15/malloctraces/.
These larger traces are automatically run if you execute mdriver
without a -f argument. Each trace file 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 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.
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. 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 have any global or static data
elements of compound
data structures such as arrays or structs in your
mm.c program. However, you are allowed to have
global pointers to data elements of compound data structures.
Beyond pointers, you are also allowed to
declare global scalar variables such as integers and floats.
- Your allocator must
always return pointers that are aligned to 16-byte boundaries.
The driver will enforce this requirement for you.
Evaluation:
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 (20 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 (35 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 (unachievable) ratio would
be 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 and throughput
P = wU + (1−w) 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 value for
Tlibc is a constant in the driver (4000 Kops/s) that
the TA established when he configured the release). 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. Ideally, the performance
index will reach P = w + (1−w) = 1 or 100%.
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.
- Style (10 points).
-
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 subroutine should have a header comment that describes
what it does and how it does it.
-
Your heap consistency checker mm_check should be
thorough and well documented.
You will be awarded 5 points for a good heap consistency checker and
5 points for good program structure and comments.
Hints:
- 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.
Other trace files are located in /u/cs252/labs15/malloctraces/.
- 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 a debugger.
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 or gcc in-line functions.
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 text 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.
“Trivia” Assignment:
By Monday, April 6, send email to TA Brandon Allard with the subject
line "[CSC252] Assign#4 Trivia - uname1 uname2" (without the quotes,
where uname is your login name) containing answers to the following
questions (a single email per team is expected):
-
Are you working alone or in a team of two?
If the latter, who is your partner?
-
After you have inputed your team information, what output do you get
when you run the following command?
mdriver -V -f short2-bal.rep
-
What is the value of ALIGNMENT defined in mm.c? How many
words is this?
-
What does the mem_sbrk function do? How does it indicate
an allocation error?
-
How do the default implementations of mm_malloc, mm_free,
and mm_realloc work? What is obviously bad about these
implementations?
Turn-in:
The “trivia” assignment will be submitted via email.
You should electronically turn in your main assignment through Blackboard.
You only need to turn in your completed mm.c. Before turn-in, be sure that you have
- included your full name and email address in the comment at the top
of mm.c
- removed any extraneous print statements
- included any appropriate commentary on your code in a separate
README file or as C comments in mm.c
Late Turn-in Policy:
Late turn-ins will be accepted for up to three days, with 10% penalty for each
late day.
No turn-ins more than three-day late will be accepted.