Allocator Basics. Dynamic Memory Allocation in the Heap (malloc and free) Allocator Goals: malloc/free. Internal Fragmentation

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Margaret Manning
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1 Alloctor Bsics Dynmic Memory Alloction in the Hep (mlloc nd free) Pges too corse-grined for llocting individul objects. Insted: flexible-sized, word-ligned blocks. Allocted block (4 words) Free block (3 words) Free word Allocted word Now: Explicit lloctors (.k.. mnul memory mngement) Lter: implicit lloctors (.k.. utomtic memory mngement) pointer to newly llocted block of t lest tht size void* mlloc(size_t size); void free(void* ptr); number of contiguous bytes required pointer to llocted block to free 3 Alloctor Gols: mlloc/free 1. Progrmmer does not decide loctions of distinct objects. Progrmmer decides: wht size, when needed, when no longer needed Internl Frgmenttion pylod smller thn block block 2. Fst lloction. mllocs/second or bytes mlloc'd/second pylod 3. High memory utiliztion. Most of hep contins necessry progrm dt. Little wsted spce. Enemy: frgmenttion unused memory tht cnnot be llocted. Cuses metdt lignment policy decisions Internl frgmenttion 6

2 Externl Frgmenttion (64-bit words) Totl free spce lrge enough, but no contiguous free block lrge enough p1 = mlloc(32); p2 = mlloc(40); p3 = mlloc(48); free(p2); p4 = mlloc(48); Implementtion Issues 1. Determine how much to free given just pointer. 2. Keep trck of free blocks. 3. Pick block to llocte. 4. Choose wht do with extr spce when llocting structure tht is smller thn the free block used. 5. Mke freed block vilble for future reuse. Depends on the pttern of future requests. 7 8 Knowing How Much to Free Keep length of block in heder word preceding block Tkes extr spce! Keeping Trck of Free Blocks Method 1: Implicit list of ll blocks using length p0 = mlloc(32); free(p0); 48 p0 metdt dt pylod Method 2: Explicit list of free blocks using pointers Method 3: Seglist Different free lists for different size blocks More methods tht we will skip 9 10

3 Implicit Free List: Block Formt Implicit Free List: Hep Lyout Block metdt: 1. Block size 2. Alloction sttus Store in one heder word. 1 word pylod (ppliction dt, when llocted) Stel LSB for sttus flg. LSB = 1: llocted LSB = 0: free Strt of hep Block Heder (metdt) block llocted? Specil end-hep word Looks like heder of zero-size llocte block optionl pdding -byte ligned sizes hve 4 zeroes in low-order bits Initil word cn't be prt of block. My force internl frgmenttion. Pylods strt t -byte (2-word) lignment. Blocks sizes re multiples of bytes. Free word Allocted word Allocted word wsted Implicit Free List: Finding Free Block First fit: Serch list from beginning, choose first free block tht fits Implicit Free List: Allocting Free Block 48 Next fit: Do first-fit strting where previous serch finished p = mlloc(24); Allocted spce free spce. Use it ll? Split it up? Best fit: Serch the list, choose the best free block: fits, with fewest bytes left over 32 p Block Splitting 13 Now showing lloction sttus flg implicitly with shding. 14

4 Implicit Free List: Freeing Block Colescing Free Blocks 32 p free(p) p Colesce with following free block. free(p); Cler llocted flg logiclly gone mlloc(40); Externl frgmenttion! Enough spce, not one block. 15 Colesce with preceding free block? Bidirectionl Colescing: Boundry Tgs [Knuth73] Constnt-Time Colescing: 4 cses Heder Boundry tg (footer) pylod (ppliction dt, when llocted) optionl pdding n 0 n 0 n+ n+ n+ n+m n+ 17 n+m1+ 19

5 Summry: Implicit Free Lists Implementtion: Allocte: Free: simple O(blocks in hep) O(1) Memory utiliztion: depends on plcement policy Not widely used in prctice some specil purpose pplictions Explicit Free Lists Allocted block: pylod (ppliction dt, when llocted) (sme s implicit free list) optionl pdding Free block: next pointer prev pointer Explicit list of free blocks rther thn implicit list of ll blocks. Splitting, boundry tgs, colescing re generl to ll lloctors Explicit Free Lists: List vs. Memory Order Explicit Free Lists: Allocting Free Block Abstrctly: doubly-linked lists Next A B C Previous Before Concretely: free list blocks in ny memory order After (with splitting) A Next C B Previous List Order Memory Order 22 = mlloc( ) 23

6 Explicit Free Lists: Freeing Block Insertion policy: Where in the free list do you dd freed block? LIFO (lst-in-first-out) policy Pro: simple nd constnt time Con: studies suggest frgmenttion is worse thn ddress ordered Freeing with LIFO Policy: between free blocks Before Hed free( ) Address-ordered policy Con: liner-time serch to insert freed blocks Pro: studies suggest frgmenttion is lower thn LIFO Splice out predecessor nd successor blocks, colesce ll 3 memory blocks nd insert the new block t the hed of the list. After LIFO Exmple: 4 cses of freed block neighbor sttus. Hed Summry: Explicit Free Lists Summry: Alloctor Policies Implementtion: firly simple All policies offer trde-offs in frgmenttion nd throughput. Allocte: O(free blocks) vs. O(ll blocks) Free: O(1) vs. O(1) Memory utiliztion: depends on plcement policy lrger minimum (next/prev) vs. implicit list Used widely in prctice, often with more optimiztions. Plcement policy: First-fit, next-fit, best-fit, etc. Seglists pproximte best-fit in low time Splitting policy: Alwys? Sometimes? Size bound? Colescing policy: Immedite vs. deferred Splitting, boundry tgs, colescing re generl to ll lloctors

Outlines. Dynamic Memory Dynamic Memory Dynamic Memory The malloc Package. malloc Example

Outlines. Dynamic Memory Dynamic Memory Dynamic Memory The malloc Package. malloc Example Outlines Dynmic Memory Alloc@on CSCI 01: Mchine Architecture nd Orgniz@on Bsic concepts Implicit free lists Explicit free lists Pen- Chung Yew Deprtment Computer Science nd Engineering University of Minnesot