Lab 3

Introduction

OptsRus is doing really well now, and has been asked to create a dynamic storage allocator (e.g. malloc, free and realloc routines) for C programs for a new experimental operating system. You are encouraged to explore the design space creatively and implement an allocator that is correct, efficient and fast.

Logistics

You must work individually. This is a difficult lab and you must plan your time wisely. Please ask for any clarifications and revisions to the assignment on Discord and we'll add them to this document.

Setup

Ensure you're in the ece454 directory within VSCode. Make sure you have the latest skeleton code from us by running: git pull upstream main.

This may create a merge, which you should be able to do cleanly. If you don't know how to do this read Pro Git. Be sure to read chapter 3.1 and 3.2 fully. This is how software developers coordinate and work together in large projects. For this course, you should always merge to maintain proper history between yourself and the provided repository. You should never rebase in this course, and in general you should never rebase unless you have a good reason to. It will be important to keep your code up-to-date during this lab as the test cases may change with your help.

The only file you will be modifying and submitting is mm.c. The mdriver 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. When you have completed the lab, only one file (mm.c), will be graded.

Procedure

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:

Note: these semantics match the semantics of the corresponding malloc, realloc, and free routines in libc. Type man malloc to the shell for complete documentation.

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 be 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 16 bytes on the x86_64 architecture, so your malloc implementation should do likewise and always return 16-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);

Otherwise, 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 ptr block, up to the minimum of the old and new sizes. Everything else is uninitialized. For example, if the old block is 16 bytes and the new block is 24 bytes, then the first 16 bytes of the new block are identical to the first 16 bytes of the old block and the last 8 bytes are uninitialized. Similarly, if the old block is 24 bytes and the new block is 16 bytes, then the contents of the new block are identical to the first 16 bytes of the old block.

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. However, (fake) 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 memory system for your dynamic memory allocator. You can invoke the following functions declared in memlib.h:

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 tests your mm.c package for correctness, space utilization, and throughput. The driver program is controlled by a trace file. 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 accepts the following command line arguments:

• -t <tracedir>: Look for the default trace files in directory tracedir instead of the default directory (../traces).
• -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 allocator interfaces declared in mm.h.

• You are not allowed to invoke any memory-management related library calls or system calls in your code, e.g. malloc, calloc, free, realloc, sbrk, brk or any variants of these calls. However, you are allowed to use memcpy and memmove.
• The total size of all defined global and static scalar variables and compound data structures should be minimal. The space is for minimal amount of metadata to bootstrap your memory allocator rather than data storage.
• For consistency with the libc malloc package on x86 64 architecture, which returns blocks aligned on 16-byte boundaries, your allocator must always return pointers that are aligned to 16-byte boundaries. The driver will enforce the requirement for you.

Evaluation

The total grade for this assignment is 100 points. You will receive zero points if you break any of the rules. Otherwise, your grade will be calculated as based on two performance metrics:

• 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 ratio equals to 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.

For each given trace, mdriver outputs the performance of your allocator in terms of utilization and throughput. It summarizes the performance of your allocator by computing a performance index, $P \in [0, 100]$, which is a weighted sum of average space utilization and throughput:

$$P= \lfloor wU \rfloor + \lfloor (100−w) min(1, \frac{T}{T_{libc}}) \rfloor$$

where $U$ is your average utilization, $T$ is your average throughput, and $T_{libc}$ is the estimated throughput of libc malloc on your system on the default traces. The performance index favors space utilization over throughput, by setting $w = 60$.

$P$ is computed based on correct traces only. It is then scaled down (linearly) by the fraction of traces that are correct. For example, if half of the traces pass validation, $P$ will be divided by two.

If your code is buggy and crashes the driver when running all traces together (i.e., without an -f option), you will get zero mark on both Correctness and Performance part. So make sure the code is not breaking the driver.

Note 1: The mark you receive is the mark you see on https://compeng.gg. The traces and compilation settings will be the same as provided. The automarker will be made available within 1 week after the lab is released.

Note 2: The auto-tester may only give a mark close to 100%, but not all the way to 100%. Therefore, after the lab is completed, we will boost everyone’s mark by a constant score such that the top student gets 100% in this lab.

Submission

Simply push your code using git push origin main (or simply git push) to submit it. You need to create your own commits to push, you can use as many as you'd like. You'll need to use the git add and git commit commands. You may push as many commits as you want, your latest commit that modifies the lab files counts as your submission. For submission time we will only look at the timestamp on our server. We will never use your commit times (or file access times) as proof of submission, only when you push your code to GitHub.