Program address space

Transcription

1 Program address space What does the OS loader do? Creates new process Sets p address space/segments Read exectable ile, load instrctions, init global data Mapped rom ile into green segments Libraries loaded on demand Set p stack Reserve stack segment, init %rsp, call main malloc written in C, will init sel on se Asks OS or large memory region, parcels ot to service reqests 0x x x x00000 Stack Shared library text/data Heap Global data Text (machine code) 8MB reserved Sized or library Grows on demand Sized or exectable

2 Global allocation +/- Convenient, somewhat sae Thanks or the memory! Atomatic alloc/dealloc on program start/exit Can access by name rom anywhere No encapslation, hard to track se/dependencies - Size ixed at declaration, no option to resize +/- Scope/lietime is global/whole program One shared namespace, mst manally avoid conlicts Stack allocation + Eicient Fast to allocate/deallocate, ok to oversize + Convenient, mostly sae Atomatic alloc/dealloc on nction entry/exit (can mistakenly retrn address o stack variable) Reasonable type saety - Size ixed at declaration, no option to resize +/- Scope/lietime dictated by control low

3 Thanks or the memory (con t) Heap allocation + Moderately eicient Have to search or available space, pdate record-keeping + Very plentil Heap enlarges on demand to limits o address space + Versatile, nder programmer control Can precisely determine scope, lietime Can be resized - Mch opportnity or error void* means eectively no type saety Possible to allocate wrong size, se ater ree, doble ree, - Leaks mch less critical in grand scheme o things, bt or long-rnning programs may be isse Do we need all three options (globals/stack/heap)?

4 Heap allocator correctness Service arbitrary seqence o malloc/realloc/ree reqests Malloc retrns pointer to memory block >= reqested size (or NULL i cannot satisy) Payload contents nspeciied (client can se calloc to zero i desired) Client error reslts in ndeined behavior (ree non-malloc address, se reed memory, etc) Sbject to constraints Can t control nmber, size, lietime o allocated blocks Mst respond immediately to each malloc reqest i.e., cannot reorder/ber malloc reqests Can deer/ignore/reorder reqests to ree Mst align blocks so they satisy all alignment reqirements Rond p sizes (typically to mltiple o 8 or 1) Allocated payload mst be maintained as-is Cannot move allocated blocks, sch as to compact/coalesce ree, why not? Can maniplate and modiy memory not crrently in se

5 Non-negotiable: correctness Well-ormed reqests mst be properly serviced Highly desirable: perormance Fast service o reqests Allocator goals Ideally constant-time, active/large heap shold not bog down into linear behavior Tight space tilization Minimize ragmentation, allocated blocks groped together, small overhead relative to payload Possible tradeos: Ease o implementation/maintenance Code oten complex, be sre eorts are worthwhile (measre!) Robst Client errors generally blndered throgh, what is reqired to detect/report them? worth attempting?

6 Tracing a "bmp" allocator Empty heap segment, each sqare represents one 8-byte word a = malloc(3) b = malloc(0) c = malloc(8) ree(b) d = malloc(1) Does not recycle!

7 Code sketch: bmp allocator static void *segment_start; static size_t segment_size, nsed = 0; // global variables segment_start/size track total heap segment void *malloc(size_t nbytes) { nbytes = rondp(nbytes, 1); i (nsed + nbytes > segment_size) // not enogh space retrn NULL; void *ptr = (char *)segment_start + nsed; nsed += nbytes; retrn ptr; } void ree(void *ptr) { // no-op! does not recycle sed memory }

8 Recycling Mst track block inormation to be able to recycle on ree Separate hosekeeping Free/in-se inormation maintained in list/table Given address, how to look p inormation? How to pdate to service malloc/ree reqest? How mch overhead per-block? Seems reasonable approach, bt not oten sed in practice Special-case allocators Tools like Valgrind Block header Block inormation stored in memory that precedes payload Given address, how to look p inormation? How to pdate to service malloc/ree reqest? How mch overhead per-block? Most common approach in crrent se

9 Tracing block header, recycling Each sqare represents one 8-byte word, size in block header expressed in nmber o 8-byte words a = malloc(3) 19 b = malloc(0) 13 c = malloc() ree(b) Implicit list d = malloc(10)

10 realloc can also recycle a = malloc(3) 19 b = malloc(0) 13 c = malloc() b = realloc(b, 8) a = realloc(a, 0) 7 What is the advantage to an in-place realloc?

11 Code sketch: block header #deine FREE_BIT 1 strct header { nsigned long stats; // bit mash size+ree, ree stored in lsb }; strct header *ptr_to_header(void *ptr) { retrn (strct header *)((char *)ptr - sizeo(strct header)); } void ree(void *ptr) { strct header *hdr = ptr_to_header(ptr); hdr->stats = FREE_BIT; }

12 Adding an explicit ree list 3 Traversing an implicit list bogs down as heap gets large/ll Ideally, malloc only examines reed blocks Adding another data strctre? hmmm Idea: payload o reed blocks is available! reelist 3 0

13 strct header *reelist; Code sketch: explicit list void ree(void *ptr) { strct header *hdr = ptr_to_header(ptr); hdr->stats = FREE_BIT; *(strct header **)ptr = reelist; reelist = hdr; } 3 0 reelist 3 0 reelist

14 Implicit list Managing ree list Size in each block header allows traverse rom block to block Search visits all blocks to ind ree ones, becomes slow as heap ills p Explicit list Chain ree blocks into linked list Why allowed/desirable to se the payload to store the links? Search looks only ree blocks! Can be sorted or segregated (by size) Qickly access appropriate blocks or reqested size why valable? I sorted, what data strctres to se needs to qick to pdate I segregated, how many/what size classes to se? Tradeos Additional overhead (minimm payload size) More complex code to maintain/pdate

15 Placement policy First-it, next-it, best-it Trades throghpt or tilization Splitting policy Policy decisions When to leave excess and when to split into separate node (In my grandmother s attic: "Pieces o string too short to save" ) Coalescing policy Immediate coalescing: coalesce each time ree() is called Deerred coalescing: try to improve perormance o ree() by deerring coalescing ntil needed. Examples: Coalesce as yo scan the ree list or malloc() Coalesce when the amont o external ragmentation reaches some threshold Tension between split and coalesce may do/ndo or no beneit

16 How to make operations ast? malloc is generally abot search Make it aster by more qickly identiying which block to se Examine ewer blocks Be less picky abot which block to se ree is mostly abot pdate Ideal data strctre can be modiied in constant-time Possibly postpone work till clearly needed (immediate vs deerred coalesce) realloc generally rides on malloc/ree, resize in place i possible! Big win i avoid copy payload data What is necessary to allow resize in place? Is it worth it to anticipate that? How prominent is realloc in mix o operations? Heap allocator coding reqires "scrappy" mindset Pare down to tens o instrctions per-reqest, every instrction conts!