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(short version)
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.
Logistics
You may work in a group of up to two people. Any clarifications and
revisions to the assignment will be posted on the course Moodle.
You must each have your own repository and hand in your own solution.
Hand Out Instructions
Accept the Malloc Lab invitation from Github Classroom.
he 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.)
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 be graded on the version of the lab in Git at the due date.
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. 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 {\em 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.
Type `man malloc` to the shell for complete documentation
or just google `malloc`.
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.
Support Routines
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 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* that are included in the `malloclab-handout.tar`
distribution. 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 and the trace files are the same ones we will use
when we grade your handin `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.
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.
* 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.
Evaluation
You'll be evaluated by having a functioning `malloc`.
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
$$
P = w{U} + (1-w) \min\left(1, \frac{T}{T_{libc}}\right)
$$
where $U$ is your space utilization, $T$ is your throughput, and
$T_{libc}$ is the estimated throughput of `libc` malloc on your
system on the default traces. The value for $T_{libc}$ is a
constant in the driver (600 Kops/s) that we established
when we configured the program. Since we're using so many different
machines, you should take this as a ``nominal'' throughput for
malloc on modern-day machines.
The performance index favors space utilization over throughput.
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.
There is a scoring program, called `./RUN-MM` that will compile
your program and run the test cases. This will report an *average score*.
Your grade on the "does it work" portion of the machine problem
is computed by the RUN-MM script using the average score.
The scoring function is based on using the `https://coding.csel.io` machines and
specific implementations as goals:
* Score of 72, or 68% is the implementation is the book,
fleshed out and implemented. This is a ``first fit'' implicit
list allocator.
* Score of 80, or ~90%, is a ``next fit'' implicit list allocator
* Score of 84, or ~100%, is an ``explicit'' list allocator (see 9.9.13)
* Score of ~86, or ~105%, using ``circular'' lists
* Score of 95, or 110%, is a ``tree based'' allocator
You can earn up to 10\% extra credit based on the score (i.e. a maximum score of 110).
Handin Instructions
You must commit your git repo containing a valid `mm.c`, `Makefile` and other files
needed to automatically build your program. You labs will be run by the TA's
on the evaluation machine and grades recorded using the grading script.
*You must indicate the origin of any code your use or borrow from other
people or sources. Failure to properly attribute the origin of code you
retrieve from any source is grounds for receiving a zero.*
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.
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. That's good for 70\% on
this assignment.
Encapsulate your pointer arithmetic in C preprocessor macros or inline 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 or, better yet, inline functions for your pointer
operations. See the text for examples and look at the handout
for provided samples.
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, we've built a `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 early, and good luck!