1. L6: Malloc Lab Writing a Dynamic Storage Allocator October 30, 2006
“The course that gives CMU its Zip!”
L6: Malloc Lab Writing a Dynamic Storage Allocator October 30, 2006
Topics
Memory Allocator (Heap)
L6: Malloc Lab
Reminders
L6: Malloc Lab Due Nov 10, 2006
Section A (Donnie H Kim) recitation8.ppt (some slides from lecture notes)

2. L6: Malloc Lab Things that matter in this lab: Performance goal
Maximizing throughput
Maximizing memory utilization
Implementation Issues (Design Space)
Free Block Organization
Placement Policy
Splitting
Coalescing
And some advice

3. Some sort of useful backgrounds

4. So what is memory allocation?
kernel virtual memory
memory invisible to
user code
stack
%esp
Memory mapped region for
shared libraries
Allocators request
additional heap memory
from the operating system using the sbrk function.
the “brk” ptr
run-time heap (via malloc)
uninitialized data (.bss)
initialized data (.data)
program text (.text)

5. Malloc Package #include <stdlib.h> void *malloc(size_t size)
If successful:
Returns a pointer to a memory block of at least size bytes, (typically) aligned to 8-byte boundary.
If size == 0, returns NULL
If unsuccessful: returns NULL (0) and sets errno.
void free(void *p)
Returns the block pointed at by p to pool of available memory
p must come from a previous call to malloc or realloc.
void *realloc(void *p, size_t size)
Changes size of block p and returns pointer to new block.
Contents of new block unchanged up to min of old and new size.

6. Allocation Examples p1 = malloc(4) p2 = malloc(5) p3 = malloc(6)
free(p2)
p4 = malloc(2)

7. Performance goals Maximizing throughput (Temporal)
Defined as the number of requests that it completes per unit time
Maximizing Memory Utilization (Spatial)
Defined as the ratio of the requested memory size and the actual memory size used
There is a tension between maximizing throughput and utilization! Find an appropriate balance between two goals!
Keep this in mind, we will come back to these issues

8. Implementation Issues
Free Block Organization
How do we keep track of the free blocks?
How do we know how much memory to free just given a pointer?
Placement Policy
How do we choose an appropriate free block?
Splitting
What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in?
Coalescing
How do we reinsert freed block? p0
free(p0)
p1 = malloc(1)

9. Implementation Issues 1: Free Block Organization
Identifying which block is free or allocated
Available design choices of how to manage free blocks
Implicit List
Explicit List
Segregated List
Header, Footer organization
storing information about the block (size, allocated, freed)

10. Keeping Track of Free Blocks
Method 1: Implicit list using lengths -- links all blocks
Method 2: Explicit list among the free blocks using pointers within the free blocks
Method 3: Segregated free list
Different free lists for different size classes
Method 4: Blocks sorted by size
Can use a balanced tree (e.g. Red-Black tree) with pointers within each free block, and the length used as a key 5 4 6 2 5 4 6 2

11. Free Block Organization
Free Block with header
1 word
a = 1: allocated block
a = 0: free block
size: block size
payload: application data
(allocated blocks only)
size a
payload
Format of
allocated and
free blocks
optional
padding

12. Free Block Organization
Free Block with Header and Footer
Header
size a
a = 1: allocated block
a = 0: free block
size: total block size
payload: application data
(allocated blocks only)
payload and
padding
Format of
allocated and
free blocks
Boundary tag
(footer)
size a

13. Implementation Issues 2: Placement Policy
“Placement Policy” choices
First Fit
Search free list from the beginning and chose the first free block
Next Fit
Starts search where the previous search has left off
Best Fit
Examine every free block to find the best free block

14. Implementation Issues 3: Splitting
“Splitting” Design choices
Using the entire free block
Simple, fast
Introduces internal fragmentation (good placement policy might reduce this)
Splitting
Split free block into two parts, when second part can be used for other requests (reduces internal fragmentation)
p1 = malloc(1)

15. Implementation Issues 4: Coalescing
False Fragmentations
Free block chopped into small, unusable free blocks
Coalesce adjacent free blocks to get bigger free block
Coalescing - Policy decision of when to perform coalescing
Immediate coalescing
Merging any adjacent blocks each time a block is freed
Deferred coalescing
Merging free blocks some time later
Ex) when allocation request fails.
Trying “Bidirectional Immediate Coalescing” proposed by Donald Knuth would be good enough for this lab

16. Performance goals Maximizing throughput (Temporal)
Defined as the number of requests that it completes per unit time
Maximizing Memory Utilization (Spatial)
Defined as the ratio of the requested memory size and the actual memory size used
There is a tension between maximizing throughput and utilization! Find an appropriate balance between two goals!

17. Performance goal (1) - Throughput
Throughput is mostly determined by time consumed to search free block
How you keep track of your free block affects search time
Naïve allocator
Never frees block, just extend the heap when you need a new block : throughput is extremely fast, but…?
Implicit Free List
The allocator can indirectly traverse the entire set of free blocks by traversing all of the blocks in the heap, definitely slow.
Explicit Free List
The allocator can directly traverse entire set of free blocks by traversing all of the free blocks in the heap
Segregated Free List
The allocator can directly traverse a particular free list to find an appropriate free block

18. Performance goal (2) – Memory Utilization
Poor memory utilization caused by fragmentation
Comes in two forms: internal and external fragmentation
Internal Fragmentation
Based on previous requests
Causes
Allocator impose minimal size of block (depending on allocator’s choice of block format)
Satisfying alignment requirements
External Fragmenatation
Based on future requests
Aggregate free memory is enough, but no single free block is large enough to handle the request

19. Internal Fragmentation
For some block, internal fragmentation is the difference between the block size and the payload size.
Caused by overhead of maintaining heap data structures, padding for alignment purposes, or explicit policy decisions (e.g., not to split the block).
Depends only on the pattern of previous requests, and thus is easy to measure.
block
Internal
fragmentation
Internal
fragmentation
payload

20. External Fragmentation
Occurs when there is enough aggregate heap memory, but no single
free block is large enough
p1 = malloc(4)
p2 = malloc(5)
p3 = malloc(6)
free(p2)
p4 = malloc(6)
oops!
External fragmentation depends on the pattern of future requests, and
thus is difficult to measure.

21. The Malloc Lab

22. Assumptions Assumptions made in Malloc Lab
Standard C library malloc always returns payload pointer that is aligned to 8 bytes, so should yours
64-bit Architecture
pointers are 8 bytes long!
size_t is now 8 bytes (unsigned long)
But the requested size will be less than 4 bytes
You may use 4 byte headers and footers and get away
Free word
Allocated block
(4 words)
Free block
(2 words)
Allocated word

23. Porting to 64-bit Machine
Porting the code in your CS:APP text book to 64-bit
sizeof(long) == 4 // 32-bit
sizeof(long) == 8 // 64-bit
The only significant difference is in the definitions of the GET and PUT macros.
Changes (To keep our 32-bit header and footers)
#define GET(p) (*(size_t *)(p)) // 32 bits
#define GET(p) (*(unsigned int *)(p)) // 64 bits
#define PUT(p, val) (*(size_t *)(p) = (val)) // 32 bits
#define PUT(p, val) (*(unsigned int *)(p) = (val)) // 64 bits
if ((long)(bp = mem_sbrk(size)) < 0)
if ((int)(bp = mem_sbrk(size)) < 0)

24. Using MACROS – why? #include <stdio.h>
#define GET8(p) (*(unsigned long *)(p))
#define PUT8(p, val) (*(unsigned long *)(p) = (unsigned long)(val))
void test(void *p, void *pval){
unsigned long *newpval;
/* Reading and writing pointers the hard way */
*(unsigned long *)p = (unsigned long) pval;
newpval = (unsigned long *)(*(unsigned long *)p);
printf("pval=%p newpval=%p\n", pval, newpval);
/* Reading and writing pointers the easy way */
PUT8(p, pval);
newpval = (unsigned long *) GET8(p); }
int main() {
char *pval = (char *)0x99;
char buf[128];
test(&buf[0], pval);
return 0;

25. Approach Advice
Start with the implicit list implementation in your text book, and understand every details of it
When you finish your implicit list, start thinking about your heap checker
The more time you spend on this, the more time you will save later
Go on and start implementing explicit list with several placement policies
Modulate, and save each of your placement policy for comparison
When you finish your explicit list, you would like to add more checks in your heap checker, do this right away.
Now when you feel your explicit list is robust, move on to the segregated free list.
We are looking for a good segregated free list implementation. You can go further by trying other schemes such as balanced trees, but a solid segregated free list implementation is good enough for a full credit
You can also try some tweaks on the given trace files

26. Heap Checker (10 pts) Basic Checks Guidelines (5/10 pts)
Check Heap (while working on implicit list)
Check epilogue and prologue blocks
Block’s address alignment (8 bytes)
Heap boundaries
Check your blocks’ header and footer
Size (minimum size , alignment)
prev/next allocate/free bit consistency (explicit list)
header and footer matching each other
Check your coalescing
All blocks are coalesced correctly (no two consecutive free blocks in the heap)

27. Heap Checker (10 pts) Free List Checks Guidelines (5/10 pts)
Check Free List (while working on explicit free list)
All next/prev pointers are consistent (If A’s next pointer points to B, B’s prev pointer should point to A)
All free list pointers points between mem_heap_lo() and mem_heap_high()
Count free blocks by iterating every block, and traversing free list by pointers, see if they match
Recommended to add more as you wish
Check Segregated Free List (segregated free list)
All blocks in each list bucket fall within bucket size range
Be creative

28. Style (10 pts)
It will be some of the most difficult and sophisticated code you have written so far in your career.
Thing we are looking for:
Explain your high level design at front of your code (2 pts)
Each function should be prepared by a header comment (2 pts)
Comment properly inside each functions (2 pts)
Decompose into functions and use as few global variables as possible (2 pts)
Use macros, inline functions, C preprocessors wisely (2 pts)
Please try to write a clean code that is readable and self-explaining!
For you
For your Teaching Staff
And for world peace

29. Debugging Techniques Guidelines for Debugging
Intensively testing your code even though it seems to work is a good programming practice, try to learn the process from this lab
You can print out all the information and monitor it
Do this when you just started
When the trace file is small
You can also print out error messages only when something is wrong
Printing and monitoring becomes painful when trace files are huge
Just print errors

30. Debugging Tips Guidelines for using mdriver’s options
Use ./mdriver –c <file> option to run a particular trace file just once, which only checks correctness
./mdriver runs your allocator multiple times to estimate the throughput of your allocator by using k-best measurement scheme (if you are interested, refer to ch 9 and mdriver source code)
Use ./mdriver –v <level> option to set verbosity level
It is sometimes useful to have layers of debugging depth
Can also use #define, #ifdef, #if
Make sure to turn all checking routines off completely when measuring performance – it does affect performance

31. More Hints? Going further (beyond solid segregated list)
Before trying this, make sure your allocator is doing what you intended, using heap/free list checkers
If you think you have implemented a solid segregated free list, try focus on trace files that gives you less performance results

32. More Hints? Some possible tackle points
In malloc(), you have to adjust the requested size to meet alignment requirements or minimum block size requirements
It turns out that how you adjust size affects the performance of some trace files
And sometimes it is better to force your allocator to avoid splitting the free block by using larger block than the request size
It will obviously increase internal fragmentation, but can also increase throughput by avoiding repeated splitting and coalescing
How large will you extend your heap, when you have to extend your heap?
How do you classify each free list?

33. Questions ?