Malloc Lab: Writing a Dynamic Storage Allocator Solution

(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!