labMalloc

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Due Date

See the calendar for due date.

Objectives:

• Throughly understand malloc(), free() and realloc()
• Use pointers

Description:

For this lab, you get to write your dynamic storage allocator for C programs, i.e., your own version of the malloc(), free() and realloc() routines. Start by copying labMalloc.tgz to a protected directory in which you plan to do your work. Then give the command: tar -xvf labMalloc.tgz. This will cause a number of files to be unpacked into the directory. The 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.) When you have completed this lab, remove the object files with make clean and then make a tarball of the directory.

How to Work on this 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 provided, implements the simplest but still functionally correct malloc package possible. Using this as a starting place, modify these functions (and possibly define other private static functions), so that they obey the following semantics:

• mm_int(): Before calling mm_malloc(), mm_realloc() or mm_free(), the application program (i.e., the trace-driven driver program is used to evaluate your implementation) calls mm_int() 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( size_t size): 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 block. Your implementation will be compared against 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( void *ptr): 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( void *ptr, size_t size): 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 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 semantics of the corresponding libc malloc(), realloc(), and free() routines. Type man malloc() in a terminal 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 tarball, make sure to remove any calls to mm_check() as they will slow down your throughput. 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 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 labMalloc.tgz 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 found in /nfshome/hcarroll/public_html/3240/protected/traces/ on system64). 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 used to grade your lab. 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.

Requirements:

• The only file that you should change in the tarball is mm.c.
• You should not change any of the interfaces in mm.c.
• You should not invoke any memory-management related library calls or system calls (for example, malloc(), calloc, free(), realloc(), sbrk, brk or any variants of these calls in your code).
• You are not allowed to define any global or static compound data structures such as arrays, structs, trees, or lists in your mm.c program. However, you are allowed to declare global scalar variables such as integers, floats, and pointers in mm.c.
• 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.
• Create a file named rubric-yourlastname.txt in your project directory with a completed rubric. Specify estimated points for each entry including the number of hours spent.

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: You will receive points for each trace trace correctly processed.
• Performance: 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 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.
  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/T_libc) 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 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, the formula is adopted 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:
• 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 points for a good heap consistency checker and points for good program structure and comments.

Hints

• Before you write any code, answer the following questions:
  • How do we know how much memory to free given just a pointer?
  • How do we keep track of the free blocks?
  • What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in?
  • How do we pick a block to use for allocation -- many might fit?
  • How do we reinsert freed block?
  • How do we know where the blocks are?
  • How do we know how big the blocks are?
  • How do we know which blocks are free?
• Use the mdriver -f option. During initial development, using tiny trace files will simplify debugging and testing (e.g., short{1,2}-bal.rep).
• 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. (Note: Submitted code needs to significantly improve upon this code to receive a points.)
• Encapsulate your pointer arithmetic in C preprocessor macros. 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 textbook 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(). 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.
• Start early! It is possible to write an efficient malloc package with a few pages of code. However, this will be some of the most difficult and sophisticated code you have written so far in your career. So start early, and good luck!

Don't worry about reducing the heap size. Watch out that some requests might not be word-aligned. Pre-lab exercise (for slide) If you need more space on the heap, increase it in chunks of 1<<12 bytes

Submission

Before submitting your lab, remove all object (.o) files (for example, with a make clean command). Make a gzipped tarball (.tgz) of the directory with your .c, Makefile and rubric-yourlastname.txt files. Submit your tarball at https://3240.cs.mtsu.edu/. For further instructions, please see the Miscellaneous page.

Rubric:

Points       Item
----------   --------------------------------------------------------------
_____ / 10   Style
_____ / 44   Correctness
_____ / 44   Performance
_____ /  0   Compiles and runs on system64
_____ /  2   Completed rubric (estimates for each line including hours spent)
             
_____ /100   Total


_____  Approximate number of hours spent
      

Notes

1. Start small. Write your comments first. Then, think through them, adjusting them until you've accounted for all possible situations. Then, add in code and test it a little at a time. For example, write the supporting code for mm_malloc() a piece at a time.
2. Note: Based off a lab at CMU by the authors of the textbook.