Memory Allocation III CSE 351 Autumn 2016
Memory Allocation III
https://xkcd.com/825/
CMPT 295
L23: Memory Allocation III
Freeing with LIFO Policy (Case 1)
Insert the freed block at the root of the list
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Before
After
Root
Boundary tags not shown, but don’t forget about them!
free( )
Root
CMPT 295
L23: Memory Allocation III
Note: all blue “next” pointers and red “previous” pointers should probably point to either the header or footer of the next/previous block – it shouldn’t matter which (I think), as long as it’s done consistently. The diagrams on this slide and on the next few do not always show pointers to the exact word for the block header/footer.
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Freeing with LIFO Policy (Case 2)
Splice successor block out of list, coalesce both memory blocks, and insert the new block at the root of the list
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Boundary tags not shown, but don’t forget about them!
Before
Root
free( )
After
Root
CMPT 295
L23: Memory Allocation III
Note: all blue “next” pointers and red “previous” pointers should probably point to either the header or footer of the next/previous block – it shouldn’t matter which (I think), as long as it’s done consistently. The diagrams on this slide and on the next few do not always show pointers to the exact word for the block header/footer.
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Freeing with LIFO Policy (Case 3)
Splice predecessor block out of list, coalesce both memory blocks, and insert the new block at the root of the list
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Boundary tags not shown, but don’t forget about them!
Before
Root
free( )
After
Root
CMPT 295
L23: Memory Allocation III
Note: all blue “next” pointers and red “previous” pointers should probably point to either the header or footer of the next/previous block – it shouldn’t matter which (I think), as long as it’s done consistently. The diagrams on this slide and on the next few do not always show pointers to the exact word for the block header/footer.
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Freeing with LIFO Policy (Case 4)
Splice predecessor and successor blocks out of list, coalesce all 3 memory blocks, and insert the new block at the root of the list
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Boundary tags not shown, but don’t forget about them!
Before
Root
free( )
After
Root
CMPT 295
L23: Memory Allocation III
Explicit List Summary
Comparison with implicit list:
Block allocation is linear time in number of free blocks instead of all blocks
Much faster when most of the memory is full
Slightly more complicated allocate and free since we need to splice blocks in and out of the list
Some extra space for the links (2 extra pointers needed for each free block)
Increases minimum block size, leading to more internal fragmentation
Most common use of explicit lists is in conjunction with segregated free lists
Keep multiple linked lists of different size classes, or possibly for different types of objects
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L23: Memory Allocation III
Allocation Policy Tradeoffs
Data structure of blocks on lists
Implicit (free/allocated), explicit (free), segregated (many free lists) – others possible!
Placement policy: first-fit, next-fit, best-fit
Throughput vs. amount of fragmentation
When do we split free blocks?
How much internal fragmentation are we willing to tolerate?
When do we coalesce free blocks?
Immediate coalescing: Every time free is called
Deferred coalescing: Defer coalescing until needed
e.g. when scanning free list for malloc or when external fragmentation reaches some threshold
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CMPT 295
L23: Memory Allocation III
Deferred coalescing: if string of frees in same area of heap, can coalesce just once later. Example: freeing a entire linked list.
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More Info on Allocators
D. Knuth, “The Art of Computer Programming”, 2nd edition, Addison Wesley, 1973
The classic reference on dynamic storage allocation
Wilson et al, “Dynamic Storage Allocation: A Survey and Critical Review”, Proc. 1995 Int’l Workshop on Memory Management, Kinross, Scotland, Sept, 1995.
Comprehensive survey
Available from CS:APP student site (csapp.cs.cmu.edu)
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L23: Memory Allocation III
Memory Allocation
Dynamic memory allocation
Introduction and goals
Allocation and deallocation (free)
Fragmentation
Explicit allocation implementation
Implicit free lists
Explicit free lists (Lab 5)
Segregated free lists
Implicit deallocation: garbage collection
Common memory-related bugs in C
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CMPT 295
L23: Memory Allocation III
Wouldn’t it be nice…
If we never had to free memory?
Do you free objects in Java?
Reminder: implicit allocator
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CMPT 295
L23: Memory Allocation III
Garbage Collection (GC)
Garbage collection: automatic reclamation of heap-allocated storage – application never explicitly frees memory
Common in implementations of functional languages, scripting languages, and modern object oriented languages:
Lisp, Racket, Erlang, ML, Haskell, Scala, Java, C#, Perl, Ruby, Python, Lua, JavaScript, Dart, Mathematica, MATLAB, many more…
Variants (“conservative” garbage collectors) exist for C and C++
However, cannot necessarily collect all garbage
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void foo() {
int* p = (int*) malloc(128);
return; /* p block is now garbage! */
}
(Automatic Memory Management)
CMPT 295
L23: Memory Allocation III
Garbage Collection
How does the memory allocator know when memory can be freed?
In general, we cannot know what is going to be used in the future since it depends on conditionals
But, we can tell that certain blocks cannot be used if they are unreachable (via pointers in registers/stack/globals)
Memory allocator needs to know what is a pointer and what is not – how can it do this?
Sometimes with help from the compiler
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L23: Memory Allocation III
Memory as a Graph
We view memory as a directed graph
Each allocated heap block is a node in the graph
Each pointer is an edge in the graph
Locations not in the heap that contain pointers into the heap are called root nodes (e.g. registers, stack locations, global variables)
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A node (block) is reachable if there is a path from any root to that node
Non-reachable nodes are garbage (cannot be needed by the application)
Root nodes
Heap nodes
not reachable
(garbage)
reachable
CMPT 295
L23: Memory Allocation III
Call this the “reachability graph.”
Specifically: an edge p -> q means that some location in block p points to some location in block q.
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Garbage Collection
Dynamic memory allocator can free blocks if there are no pointers to them
How can it know what is a pointer and what is not?
We’ll make some assumptions about pointers:
Memory allocator can distinguish pointers from non-pointers
All pointers point to the start of a block in the heap
Application cannot hide pointers
(e.g. by coercing them to a long, and then back again)
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L23: Memory Allocation III
Pretty major assumptions in C, but not so much in other languages
Classical GC Algorithms
Mark-and-sweep collection (McCarthy, 1960)
Does not move blocks (unless you also “compact”)
Reference counting (Collins, 1960)
Does not move blocks (not discussed)
Copying collection (Minsky, 1963)
Moves blocks (not discussed)
Generational Collectors (Lieberman and Hewitt, 1983)
Most allocations become garbage very soon, so
focus reclamation work on zones of memory recently allocated.
For more information:
Jones, Hosking, and Moss, The Garbage Collection Handbook: The Art of Automatic Memory Management, CRC Press, 2012.
Jones and Lin, Garbage Collection: Algorithms for Automatic Dynamic Memory, John Wiley & Sons, 1996.
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L23: Memory Allocation III
Mark and Sweep Collecting
Can build on top of malloc/free package
Allocate using malloc until you “run out of space”
When out of space:
Use extra mark bit in the header of each block
Mark: Start at roots and set mark bit on each reachable block
Sweep: Scan all blocks and free blocks that are not marked
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Before mark
root
After mark
Mark bit set
After sweep
free
free
Arrows are NOT free list pointers
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L23: Memory Allocation III
When out of space, or when you periodically decide to run the garbage collector…
Note that the arrows in this example denote memory references, not free list pointers!
Assumptions For a Simple Implementation
Application can use functions to allocate memory:
b=new(n) returns pointer, b, to new block with all locations cleared
b[i] read location i of block b into register
b[i]=v write v into location i of block b
Each block will have a header word (accessed at b[-1])
Functions used by the garbage collector:
is_ptr(p) determines whether p is a pointer to a block
length(p) returns length of block pointed to by p, not including
header
get_roots() returns all the roots
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Non-testable Material
CMPT 295
L23: Memory Allocation III
b is pointer that is used by application: first word in payload
Mark
Mark using depth-first traversal of the memory graph
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ptr mark(ptr p) { // p: some word in a heap block
if (!is_ptr(p)) return; // do nothing if not pointer
if (markBitSet(p)) return; // check if already marked
setMarkBit(p); // set the mark bit
for (i=0; i
head->next = NULL;
// create and manipulate the rest of the list
…
free(head);
return;
}
Error Prog stop Fix:
Type: Possible?
CMPT 295
L23: Memory Allocation III
Memory leak
No program stop, no security flaw
Fix: save next = head->next before free(head), then free next (and all other nodes following it) too…
Dealing With Memory Bugs
Conventional debugger (gdb)
Good for finding bad pointer dereferences
Hard to detect the other memory bugs
Debugging malloc (UToronto CSRI malloc)
Wrapper around conventional malloc
Detects memory bugs at malloc and free boundaries
Memory overwrites that corrupt heap structures
Some instances of freeing blocks multiple times
Memory leaks
Cannot detect all memory bugs
Overwrites into the middle of allocated blocks
Freeing block twice that has been reallocated in the interim
Referencing freed blocks
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Non-testable Material
CMPT 295
L23: Memory Allocation III
Dealing With Memory Bugs (cont.)
Some malloc implementations contain checking code
Linux glibc malloc: setenv MALLOC_CHECK_ 2
FreeBSD: setenv MALLOC_OPTIONS AJR
Binary translator: valgrind (Linux), Purify
Powerful debugging and analysis technique
Rewrites text section of executable object file
Can detect all errors as debugging malloc
Can also check each individual reference at runtime
Bad pointers
Overwriting
Referencing outside of allocated block
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Non-testable Material
CMPT 295
L23: Memory Allocation III
What about Java or ML or Python or …?
In memory-safe languages, most of these bugs are impossible
Cannot perform arbitrary pointer manipulation
Cannot get around the type system
Array bounds checking, null pointer checking
Automatic memory management
But one of the bugs we saw earlier is possible. Which one?
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Non-testable Material
CMPT 295
L23: Memory Allocation III
Memory Leaks with GC
Not because of forgotten free — we have GC!
Unneeded “leftover” roots keep objects reachable
Sometimes nullifying a variable is not needed for correctness but is for performance
Example: Don’t leave big data structures you’re done with in a static field
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Root nodes
Heap nodes
not reachable
(garbage)
reachable
CMPT 295
L23: Memory Allocation III
Freeing with LIFO Policy (Explicit Free List)
Predecessor
Block Successor
Block Change in Nodes in Free List Number of Pointers Updated
Case 1 Allocated Allocated
Case 2 Allocated Free
Case 3 Free Allocated
Case 4 Free Free
CMPT 295
L23: Memory Allocation III