Memory Allocator

Objectives

• Learn the basics of memory management by implementing minimal versions of malloc(), calloc(), realloc(), and free().
• Accommodate with the memory management syscalls in Linux: brk(), mmap(), and munmap().
• Understand the bottlenecks of memory allocation and how to reduce them.

Statement

Build a minimalistic memory allocator that can be used to manually manage virtual memory. The goal is to have a reliable library that accounts for explicit allocation, reallocation, and initialization of memory.

Support Code

The support code consists of three directories:

• src/ will contain your solution
• tests/ contains the test suite and a Python script to verify your work
• utils/ contains osmem.h that describes your library interface, block_meta.h which contains details of struct block_meta, and an implementation for printf() function that does NOT use the heap

The test suite consists of .c files that will be dynamically linked to your library, libosmem.so. You can find the sources in the tests/snippets/ directory. The results of the previous will also be stored in tests/snippets/ and the reference files are in the tests/ref/ directory.

The automated checking is performed using run-tests.py. It runs each test and compares the syscalls made by the os_* functions with the reference file, providing a diff if the test failed.

API

1. void *os_malloc(size_t size)

   Allocates size bytes and returns a pointer to the allocated memory.

   Chunks of memory smaller than MMAP_THRESHOLD are allocated with brk(). Bigger chunks are allocated using mmap(). The memory is uninitialized.

   • Passing 0 as size will return NULL.

2. void *os_calloc(size_t nmemb, size_t size)

   Allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allocated memory.

   Chunks of memory smaller than page_size are allocated with brk(). Bigger chunks are allocated using mmap(). The memory is set to zero.

   • Passing 0 as nmemb or size will return NULL.

3. void *os_realloc(void *ptr, size_t size)

   Changes the size of the memory block pointed to by ptr to size bytes. If the size is smaller than the previously allocated size, the memory block will be truncated.

   If ptr points to a block on heap, os_realloc() will first try to expand the block, rather than moving it. Otherwise, the block will be reallocated and its contents copied.

   When attempting to expand a block followed by multiple free blocks, os_realloc() will coalesce them one at a time and verify the condition for each. Blocks will remain coalesced even if the resulting block will not be big enough for the new size.

   Calling os_realloc() on a block that has STATUS_FREE should return NULL. This is a measure to prevent undefined behavior and make the implementation robust, it should not be considered a valid use case of os_realloc().

   • Passing NULL as ptr will have the same effect as os_malloc(size).
   • Passing 0 as size will have the same effect as os_free(ptr).

4. void os_free(void *ptr)

   Frees memory previously allocated by os_malloc(), os_calloc() or os_realloc().

   os_free() will not return memory from the heap to the OS by calling brk(), but rather mark it as free and reuse it in future allocations. In the case of mapped memory blocks, os_free() will call munmap().

5. General

   • Allocations that increase the heap size will only expand the last block if it is free.
   • You are allowed to use sbrk() instead of brk(), in view of the fact that on Linux sbrk() is implemented using the brk().
   • Do NOT use mremap()
   • You must check the error code returned by every syscall. You can use the DIE() macro for this.

Implementation

An efficient implementation must keep data aligned, keep track of memory blocks and reuse freed blocks. This can be further improved by reducing the number of syscalls and block operations.

Memory Alignment

Allocated memory should be aligned (i.e. all addresses are multiple of a given size). This is a space-time trade-off because memory blocks are padded so each can be read in one transaction. It also allows for atomicity when interacting with a block of memory.

All memory allocations should be aligned to 8 bytes as required by 64 bit systems.

Block Reuse

struct block_meta

We will consider a block to be a continuous zone of memory, allocated and managed by our implementation. The structure block_meta will be used to manage the metadata of a block. Each allocated zone will comprise of a block_meta structure placed at the start, followed by data (payload). For all functions, the returned address will be that of the payload (not of the block_meta structure).

struct block_meta {
    size_t size;
    int status;
    struct block_meta *prev;
    struct block_meta *next;
};

Note: Both the struct block_meta and the payload of a block should be aligned to 8 bytes.

Note: Most compilers will automatically pad the structure, but you should still align it for portability.

Split Block

Reusing memory blocks improves the allocator's performance, but might lead to Internal Memory Fragmentation. This happens when we allocate a size smaller than all available free blocks. If we use one larger block the remaining size of that block will be wasted since it cannot be used for another allocation.

To avoid this, a block should be truncated to the required size and the remaining bytes should be used to create a new free block.

The resulting free block should be reusable. The split will not be performed if the remaining size (after reserving space for block_meta structure and payload) is not big enough to fit another block (block_meta structure and at least 1 byte of usable memory).

Note: Do not forget the alignment!

Coalesce Blocks

There are cases when there is enough free memory for an allocation, but it is spread across multiple blocks that cannot be used. This is called External Memory Fragmentation.

One technique to reduce external memory fragmentation is block coalescing which implies merging adjacent free blocks to form a contiguous chunk.

Coalescing will be used before searching for a block and in os_realloc() to expand the current block when possible.

Note: You might still need to split the block after coalesce.

Find Best Block

Our aim is to reuse a free block with a size closer to what we need in order to reduce the number of future operations on it. This strategy is called find best. On every allocation we need to search the whole list of blocks and choose the best fitting free block.

In practice, it also uses a list of free blocks to avoid parsing all blocks, but this is out of the scope of the assignment.

Note: For consistent results, coalesce all adjacent free blocks before searching.

Heap Preallocation

Heap is used in most modern programs. This hints at the possibility of preallocating a relatively big chunk of memory (i.e. 128 kilobytes) when the heap is used for the first time. This reduces the number of future brk() syscalls.

For example, if we try to allocate 1000 bytes we should first allocate a block of 128 kilobytes and then split it. On future small allocations, we should proceed to split the preallocated chunk.

Note: Heap preallocation happens only once.

Building Memory Allocator

To build libosmem.so, run make in the src/ directory:

student@os:~/.../mem-alloc$ cd src/
student@os:~/.../mem-alloc/src$ make
gcc -fPIC -Wall -Wextra -g -I../utils  -c -o osmem.o osmem.c
gcc -fPIC -Wall -Wextra -g -I../utils  -c -o ../utils/printf.o ../utils/printf.c
gcc -shared -o libosmem.so osmem.o helpers.o ../utils/printf.o

Testing and Grading

The testing is automated and performed with the run-tests.py script from the tests/ directory.

Before running run-tests.py, you first have to build libosmem.so in the src/ directory and generate the test binaries in tests/snippets. You can do so using the all-in-one Makefile rule from tests/: make check.

student@os:~/.../mem-alloc$ cd tests/
student@os:~/.../mem-alloc/tests$ make check
gcc -fPIC -Wall -Wextra -g -I../utils  -c -o osmem.o osmem.c
gcc -fPIC -Wall -Wextra -g -I../utils  -c -o helpers.o helpers.c
gcc -fPIC -Wall -Wextra -g -I../utils  -c -o ../utils/printf.o ../utils/printf.c
[...]
gcc -I../utils -fPIC -Wall -Wextra -g -o snippets/test-all snippets/test-all.c -L../src -losmem
gcc -I../utils -fPIC -Wall -Wextra -g -o snippets/test-calloc-arrays snippets/test-calloc-arrays.c -L../src -losmem
gcc -I../utils -fPIC -Wall -Wextra -g -o snippets/test-calloc-block-reuse snippets/test-calloc-block-reuse.c -L../src -losmem
gcc -I../utils -fPIC -Wall -Wextra -g -o snippets/test-calloc-coalesce-big snippets/test-calloc-coalesce-big.c -L../src -losmem
gcc -I../utils -fPIC -Wall -Wextra -g -o snippets/test-calloc-coalesce snippets/test-calloc-coalesce.c -L../src -losmem
gcc -I../utils -fPIC -Wall -Wextra -g -o snippets/test-calloc-expand-block snippets/test-calloc-expand-block.c -L../src -losmem
[...]
test-malloc-no-preallocate       ........................ passed ...   2
test-malloc-preallocate          ........................ passed ...   3
test-malloc-arrays               ........................ passed ...   5
test-malloc-block-reuse          ........................ passed ...   3
test-malloc-expand-block         ........................ passed ...   2
test-malloc-no-split             ........................ passed ...   2
test-malloc-split-one-block      ........................ passed ...   3
test-malloc-split-first          ........................ passed ...   2
test-malloc-split-last           ........................ passed ...   2
test-malloc-split-middle         ........................ passed ...   3
test-malloc-split-vector         ........................ passed ...   2
test-malloc-coalesce             ........................ passed ...   3
test-malloc-coalesce-big         ........................ passed ...   3
test-calloc-no-preallocate       ........................ passed ...   1
test-calloc-preallocate          ........................ passed ...   1
test-calloc-arrays               ........................ passed ...   5
test-calloc-block-reuse          ........................ passed ...   1
test-calloc-expand-block         ........................ passed ...   1
test-calloc-no-split             ........................ passed ...   1
test-calloc-split-one-block      ........................ passed ...   1
test-calloc-split-first          ........................ passed ...   1
test-calloc-split-last           ........................ passed ...   1
test-calloc-split-middle         ........................ passed ...   1
test-calloc-split-vector         ........................ passed ...   2
test-calloc-coalesce             ........................ passed ...   2
test-calloc-coalesce-big         ........................ passed ...   2
test-realloc-no-preallocate      ........................ passed ...   1
test-realloc-preallocate         ........................ passed ...   1
test-realloc-arrays              ........................ passed ...   3
test-realloc-block-reuse         ........................ passed ...   3
test-realloc-expand-block        ........................ passed ...   2
test-realloc-no-split            ........................ passed ...   3
test-realloc-split-one-block     ........................ passed ...   3
test-realloc-split-first         ........................ passed ...   3
test-realloc-split-last          ........................ passed ...   3
test-realloc-split-middle        ........................ passed ...   2
test-realloc-split-vector        ........................ passed ...   2
test-realloc-coalesce            ........................ passed ...   3
test-realloc-coalesce-big        ........................ passed ...   1
test-all                         ........................ passed ...   5

Total:                                                            90/100

NOTE: By default, run-test.py checks for memory leaks, which can be time-consuming. To speed up testing, use the -d flag or make check-fast to skip memory leak checks, but remember to run make check before submitting your assignment to ensure it meets all criteria.

Debugging

run-tests.py uses ltrace to capture all the libcalls and syscalls performed.

The output of ltrace is formatted to show only top level library calls and nested system calls. For consistency, the heap start and addresses returned by mmap() are replaced with labels. Every other address is displayed as <label> + offset, where the label is the closest mapped address.

run-tests.py supports three modes:

• verbose (-v), prints the output of the test
• diff (-d), prints the diff between the output and the ref
• memcheck (-m), prints the diff between the output and the ref and announces memory leaks

If you want to run a single test, you give its name or its path as arguments to run-tests.py:

student@os:~/.../mem-alloc/tests$ python run-tests.py test-all
OR
student@os:~/.../mem-alloc/tests$ python run-tests.py snippets/test-all

Debugging in VSCode

If you are using Visual Studio Code, you can use the launch.json configurations to run tests.

Setup the breakpoints in the source files or the tests and go to Run and Debug (F5). Select Run test script and press F5. This will enter a dialogue where you can choose which test to run.

You can find more on this in the official documentation: Debugging with VSCode.

If VSCode complains about MAP_ANON argument for mmap() change C_Cpp.default.cStandard option to gnu11.

Resources

• "Implementing malloc" slides by Michael Saelee
• Malloc Tutorial