CS61C L07 More Memory Management (1)!Garcia, Spring 2010 © UCB!! !Lecturer SOE Dan Garcia!! !www.cs.berkeley.edu/~ddgarcia inst.eecs.berkeley.edu/~cs61c "CS61C : Machine Structures" Lecture 7 – C Memory Management" 2010-02-03! Flexible plastic displays ⇒ "Phicot has come up with a "way to print silicon electronics onto plastic as they are fed through rollers. The secret was depositing silicon at low enough temperatures that wonʼt melt the plastic. !www.technologyreview.com/computing/24433/CS61C L07 More Memory Management (2)!Garcia, Spring 2010 © UCB!Review!• C has 3 pools of memory!• Static storage: global variable storage, basically permanent, entire program run!• The Stack: local variable storage, parameters, return address!• The Heap (dynamic storage): malloc() grabs space from here, free() returns it. !• malloc() handles free space with freelist. Three different ways to find free space when given a request:!• First fit (find first one thatʼs free)!• Next fit (same as first, but remembers where left off)!• Best fit (finds most “snug” free space)!What programs use what areas? !CS61C L07 More Memory Management (3)!Garcia, Spring 2010 © UCB!Slab Allocator!• A different approach to memory management (used in GNU libc)!• Divide blocks in to “large” and “small” by picking an arbitrary threshold size. Blocks larger than this threshold are managed with a freelist (as before).!• For small blocks, allocate blocks in sizes that are powers of 2!• e.g., if program wants to allocate 20 bytes, actually give it 32 bytes!CS61C L07 More Memory Management (4)!Garcia, Spring 2010 © UCB!Slab Allocator!• Bookkeeping for small blocks is relatively easy: just use a bitmap for each range of blocks of the same size!• Allocating is easy and fast: compute the size of the block to allocate and find a free bit in the corresponding bitmap.!• Freeing is also easy and fast: figure out which slab the address belongs to and clear the corresponding bit.!CS61C L07 More Memory Management (5)!Garcia, Spring 2010 © UCB!Slab Allocator!16 byte blocks:"32 byte blocks:"64 byte blocks:"16 byte block bitmap: 11011000"32 byte block bitmap: 0111"64 byte block bitmap: 00"CS61C L07 More Memory Management (6)!Garcia, Spring 2010 © UCB!Slab Allocator Tradeoffs!• Extremely fast for small blocks.!• Slower for large blocks!• But presumably the program will take more time to do something with a large block so the overhead is not as critical.!• Minimal space overhead!• No fragmentation (as we defined it before) for small blocks, but still have wasted space!!CS61C L07 More Memory Management (7)!Garcia, Spring 2010 © UCB!Internal vs. External Fragmentation!• With the slab allocator, difference between requested size and next power of 2 is wasted!• e.g., if program wants to allocate 20 bytes and we give it a 32 byte block, 12 bytes are unused.!• We also refer to this as fragmentation, but call it internal fragmentation since the wasted space is actually within an allocated block.!• External fragmentation: wasted space between allocated blocks.!CS61C L07 More Memory Management (8)!Garcia, Spring 2010 © UCB!Buddy System!• Yet another memory management technique (used in Linux kernel)!• Like GNUʼs “slab allocator”, but only allocate blocks in sizes that are powers of 2 (internal fragmentation is possible)!• Keep separate free lists for each size!• e.g., separate free lists for 16 byte, 32 byte, 64 byte blocks, etc.!CS61C L07 More Memory Management (9)!Garcia, Spring 2010 © UCB!Buddy System!• If no free block of size n is available, find a block of size 2n and split it in to two blocks of size n !• When a block of size n is freed, if its neighbor of size n is also free, combine the blocks in to a single block of size 2n !• Buddy is block in other half larger block "• Same speed advantages as slab allocator!buddies! NOT buddies!CS61C L07 More Memory Management (10)!Garcia, Spring 2010 © UCB!Allocation Schemes!• So which memory management scheme (K&R, slab, buddy) is best?!• There is no single best approach for every application.!• Different applications have different allocation / deallocation patterns. !• A scheme that works well for one application may work poorly for another application.!CS61C L07 More Memory Management (11)!Garcia, Spring 2010 © UCB!Homework 2?!a) Done!!b) Almost done.!c) Started. Iʼm in the mix.!d) Just basically read it.!e) Havenʼt even started.!CS61C L07 More Memory Management (12)!Garcia, Spring 2010 © UCB!How many hours h on homework 2?!a) 0 ≤ h < 5!b) 5 ≤ h < 10!c) 10 ≤ h < 15!d) 15 ≤ h < 20!e) 20 ≤ h!CS61C L07 More Memory Management (13)!Garcia, Spring 2010 © UCB!Automatic Memory Management!• Dynamically allocated memory is difficult to track – why not track it automatically?!• If we can keep track of what memory is in use, we can reclaim everything else.!• Unreachable memory is called garbage, the process of reclaiming it is called garbage collection.!• So how do we track what is in use?!CS61C L07 More Memory Management (14)!Garcia, Spring 2010 © UCB!Tracking Memory Usage!• Techniques depend heavily on the programming language and rely on help from the compiler.!• Start with all pointers in global variables and local variables (root set).!• Recursively examine dynamically allocated objects we see a pointer to.!• We can do this in constant space by reversing the pointers on the way down!• How do we recursively find pointers in dynamically allocated memory?!CS61C L07 More Memory Management (15)!Garcia, Spring 2010 © UCB!Tracking Memory Usage!• Again, it depends heavily on the programming language and compiler.!• Could have only a single type of dynamically allocated object in memory!• E.g., simple Lisp/Scheme system with only cons cells (61Aʼs Scheme not “simple”)!• Could use a strongly typed language (e.g., Java)!• Donʼt allow conversion (casting) between arbitrary types.!• C/C++ are not strongly typed.!• Here are 3 schemes to collect garbage!CS61C L07 More Memory Management (16)!Garcia, Spring 2010 © UCB!Scheme 1: Reference Counting!• For every chunk of dynamically allocated memory, keep a count of number of pointers that point to it.!• When the count reaches 0, reclaim.!• Simple assignment
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