Homework3: Dynamic Memory Allocation solution

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1 Introduction
OptsRus is doing really well now, and has been asked to create a dynamic storage allocator (eg. 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.
2 Logistics
You may work in a group of up to two people. Any clarifications and revisions to the assignment will be posted on
the Piazza Web page.
3 Setup
You will first login the UG EECG machines and start with assn3-malloc.tar.gz:
mkdir /hw3; cd /hw3
cp /cad2/ece454f/hw3/assn3-malloc.tar.gz ˜/hw3/
tar -xf assn3-malloc.tar.gz
cd assn3-malloc/assn
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.
Looking at the file mm.c youll 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 submit only one file (mm.c), which contains your solution.
3.1 Procedure
Your dynamic storage allocator will consist of the following four functions, which are declared in mm.h and defined
in mm.c.
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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 tracedriven
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);
– 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.
These semantics match the semantics of the corresponding malloc, realloc, and free routines in libc. Type
man malloc to the shell for complete documentation.
3.2 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?
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• 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, style points will be given for
your mm check function. Make sure to put in comments and document what you are checking.
3.3 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 systems page size in bytes (4K on Linux systems).
3.4 The Trace-driven Driver Program
The driver program mdriver in the assn3-malloc.tar distribution 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 students 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.
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3.5 Programming Rules
• You should not change any of the allocator interfaces declared in mm.h.
• You are not allowed to invoke any memory-menagement 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 must not exceed
256 bytes.
• 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.
3.6 Evaluation
The total grade for this homework is 66 points. You will receive zero points if you break any of the rules. Otherwise,
your grade will be calculated as follows:
• Correctness (11 points). You will receive full points if your solution passes the correctness tests performed by
the driver program. You will receive partial credit (1 point) for each correct trace.
• Performance (35 points). 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.
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 ∈ [0, 100], which
is a weighted sum of average space utilization and throughput:
P = ⌊wU⌋ + ⌊(100 − w)min(1,
T
Tlibc
)⌋
where U is your average utilization, T is your average throughput, and Tlibc 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.
Note that 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.
• Style (20 points).
– 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 function
should be preceded by a header comment that describes what the function does.
– Each subroutine should have a header comment that describes what it does and how it does it.
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– Your heap consistency checker mm check should be thorough and well-documented.
You will be awarded 10 points for a good heap consistency checker and 10 points for good program structure
and comments.
3.7 Submit Instructions
Submit your assignment by typing submitece454f 3 mm.c on one of the UG EECG machines. Once again,
do not submit any other files as you will be marked solely on your mm.c file. Submit only one file per two person
group from either one of your two accounts.
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