Page. Harsh Reality. Dynamic Memory Allocation. Malloc Package. Process Memory Image. Assumptions. Malloc Example

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1 Hrsh Relity Memory Mtters Memory is not unbounded It must be llocted nd mnged 1 Mny lictions re memory dominted Esecilly those bsed on comlex, grh lgorithms Memory referencing bugs esecilly ernicious Effects re distnt in both time nd sce Memory erformnce is not uniform Cche nd virtul memory effects cn gretly ffect rogrm erformnce Adting rogrm to chrcteristics of memory system cn led to mjor seed imrovements Dynmic Memory Alloction Exlicit vs. Imlicit Memory Alloctor Exlicit: liction lloctes nd frees sce E.g., mlloc nd free in C Imlicit: liction lloctes, but does not free sce E.g. grbge collection in Jv, ML or Lis Alloction In both cses the memory lloctor rovides n bstrction of memory s set of blocks Doles out free memory blocks to liction Will discuss simle exlicit memory lloction tody Aliction Dynmic Memory Alloctor He Memory Process Memory Imge %es Alloctors request dditionl he memory from the oerting system using the sbrk function. kernel virtul memory stck Memory med region for shred librries run-time he (vi mlloc) uninitilized dt (.bss) initilized dt (.dt) rogrm text (.text) memory invisible to user code the brk tr Mlloc Pckge #include <stdlib.h> void *mlloc(_t ) If successful: Returns ointer to memory block of t lest bytes, (tyiclly) ligned to 8-byte boundry. If == 0, returns NULL If unsuccessful: returns NULL (0) nd sets errno. void free(void *) Returns the block ointed t by to ool of vilble memory must come from revious cll to mlloc or relloc. void *relloc(void *, _t ) Chnges of block nd returns ointer to new block. Contents of new block unchnged u to min of old nd new. 3 0 Mlloc Exmle void foo(int n, int m) { int i, *; /* llocte block of n ints */ if (( = (int *) mlloc(n * of(int))) == NULL) { error("mlloc"); exit(0); for (i=0; i<n; i++) [i] = i; /* dd m bytes to end of block */ if (( = (int *) relloc(, (n+m) * of(int))) == NULL) { error("relloc"); exit(0); for (i=n; i < n+m; i++) [i] = i; Assumtions Assumtions mde in this lecture Memory is word ddressed (ech word cn hold ointer) Allocted block ( words) Free block (3 words) Free word Allocted word 5 /* rint new rry */ for (i=0; i<n+m; i++) rintf("%d\n", [i]); free(); /* return to vilble memory ool */ 6

2 Alloction Exmles 7 1 = mlloc() = mlloc(5) 3 = mlloc(6) free() = mlloc() Constrints Alictions: Cn issue rbitrry sequence of lloction nd free requests Free requests must corresond to n llocted block Alloctors Cn t control number or of llocted blocks Must resond immeditely to ll lloction requests i.e., cn t reorder or buffer requests Must llocte blocks from free memory i.e., cn only lce llocted blocks in free memory Must lign blocks so they stisfy ll lignment requirements 8 byte lignment for GNU mlloc (libc mlloc) on Linux boxes Cn only mniulte nd modify free memory Cn t move the llocted blocks once they re llocted i.e., comction is not llowed 8 Gols of Good mlloc/free Primry gols 9 Good time erformnce for mlloc nd free Idelly should tke constnt time (not lwys ossible) Should certinly not tke liner time in the number of blocks Good sce utiliztion User llocted structures should be lrge frction of the he. Wnt to minimize frgmenttion. Some other gols Good loclity roerties Structures llocted close in time should be close in sce Similr objects should be llocted close in sce Robust Cn check tht free(1) is on vlid llocted object 1 Cn check tht memory references re to llocted sce Performnce Gols: Throughut Given some sequence of mlloc nd free requests: 10 R 0, R 1,..., R k,..., R n-1 Wnt to mximize throughut nd ek memory utiliztion. These gols re often conflicting Throughut: Number of comleted requests er unit time Exmle: 5,000 mlloc clls nd 5,000 free clls i0 seconds Throughut is 1,000 oertions/second. Performnce Gols: Pek Memory Utiliztion Given some sequence of mlloc nd free requests: 11 R 0, R 1,..., R k,..., R n-1 Def: Aggregte ylod P k : mlloc() results in block with ylod of bytes.. After request R k hs comleted, the ggregte ylod P k is the sum of currently llocted ylods. Def: Current he is denoted by H k Assume tht H k is monotoniclly nondecresing Def: Pek memory utiliztion: After k requests, ek memory utiliztion is: U k = ( mx i<k P i ) / H k 1 Internl Frgmenttion Poor memory utiliztion cused by frgmenttion. Comes in two forms: internl nd externl frgmenttion Internl frgmenttion For some block, internl frgmenttion is the difference between the block nd the ylod. block Internl frgmenttion ylod Internl frgmenttion Cused by overhed of mintining he dt structures, dding for lignment uroses, or exlicit olicy decisions (e.g., not to slit the block). Deends only on the ttern of revious requests, nd thus is esy to mesure.

3 Externl Frgmenttion 13 Occurs when there is enough ggregte he memory, but no single free block is lrge enough 1 = mlloc() = mlloc(5) 3 = mlloc(6) free() = mlloc(6) oos! Externl frgmenttion deends on the ttern of future requests, nd thus is difficult to mesure. Imlementtion Issues How do we know how much memory to free just given ointer? How do we kee trck of the free blocks? Wht do we do with the extr sce when llocting structure tht is smller thn the free block it is lced in? How do we ick block to use for lloction -- mny might fit? How do we reinsert freed block? 1 free(0) 1 = mlloc(1) 0 Knowing How Much to Free Stndrd method Kee the length of block in the word receding the block. This word is often clled the heder field or heder Requires n extr word for every llocted block Keeing Trck of Free Blocks Method 1: 1 Imlicit list using lengths -- links ll blocks 5 6 Method : Exlicit list mong the free blocks using ointers within the free blocks 0 = mlloc() free(0) 5 Block dt Method 3: 3 Segregted free list Different free lists for different clsses Method : Blocks sorted by Cn use blnced tree (e.g. Red-Blck tree) with ointers within ech free block, nd the length used s key Method 1: Imlicit List 17 Need to identify whether ech block is free or llocted Cn use extr bit Bit cn be ut in the sme word s the if block s re lwys multiles of two (msk out low order bit when reding ). Formt of llocted nd free blocks 1 word ylod otionl dding = 1: llocted block = 0: free block : block ylod: liction dt (llocted blocks only) Imlicit List: Finding Free Block First fit: 18 Serch list from beginning, choose first free block tht fits Cn tke liner time in totl number of blocks (llocted nd free) In rctice it cn cuse slinters t beginning of list Next fit: Like first-fit, but serch list from loction of end of revious serch Reserch suggests tht frgmenttion is worse Best fit: = strt; while (( < end) (* & 1) (* <= len)); \\ not ssed end \\ lredy llocted \\ too smll Serch the list, choose the free block with the closest tht fits Kees frgments smll --- usully hels frgmenttion Will tyiclly run slower thn first-fit

4 Imlicit List: Allocting in Free Block Allocting in free block - slitting Since llocted sce might be smller thn free sce, we might wnt to slit the block 6 Imlicit List: Freeing Block Simlest imlementtion: Only need to cler llocted flg void free_block(tr ) { * = * & - But cn led to flse frgmenttion void ddblock(tr, int len) { int new = ((len + 1) >> 1) << 1; // dd 1 nd round u int old = * & -; // msk out low bit * = new 1; // set new length if (new < old) *(+new) = old - new; // set length in remining // rt of block free() mlloc(5) Oos! 19 ddblock(, ) 0 There is enough free sce, but the lloctor won t be ble to find it Imlicit List: Colescing Join (colesce( colesce) ) with next nd/or revious block if they re free Colescing with next block free() void free_block(tr ) { * = * & -; // cler llocted flg next = + *; // find next block if ((*next & 1) == 0) * = * + *next; // dd to this block if // not llocted Imlicit List: Bidirectionl Colescing Boundry tgs [Knuth73] Relicte /llocted word t bottom of free blocks Allows us to trverse the list bckwrds, but requires extr sce Imortnt nd generl technique! Formt of llocted nd free blocks Heder Boundry tg (footer) 1 word ylod nd dding = 1: llocted block = 0: free block : totl block ylod: liction dt (llocted blocks only) 1 6 But how do we colesce with revious block? 6 6 Constnt Time Colescing Constnt Time Colescing (Cse 1) block being freed Cse 1 Cse Cse 3 Cse llocted llocted free free llocted free llocted free m 1 n 0 n 0 m 1 m 1 m 1 3

5 Constnt Time Colescing (Cse ) Constnt Time Colescing (Cse 3) n+ n+m 0 m 0 m 1 n+ m 1 m 0 n+m 0 m 1 m Constnt Time Colescing (Cse ) 7 m 0 m 0 n+m1+m 0 n+m1+m 0 Summry of Key Alloctor Policies Plcement olicy: 8 First fit, next fit, best fit, etc. Trdes off lower throughut for less frgmenttion Interesting observtion: segregted free lists (next lecture) roximte best fit lcement olicy without hving the serch entire free list. Slitting olicy: When do we go hed nd slit free blocks? How much internl frgmenttion re we willing to tolerte? Colescing olicy: Immedite colescing: colesce djcent blocks ech time free is clled Deferred colescing: try to imrove erformnce of free by deferring colescing until needed. e.g., Colesce s you scn the free list for mlloc. Colesce when the mount of externl frgmenttion reches some threshold. Imlicit Lists: Summry Imlementtion: very simle Allocte: liner time worst cse Free: constnt time worst cse -- even with colescing Memory usge: will deend on lcement olicy First fit, next fit or best fit Not used in rctice for mlloc/free becuse of liner time llocte. Used in mny secil urose lictions. However, the concets of slitting nd boundry tg colescing re generl to ll lloctors. 9