CS842: Automatic Memory Management and Garbage Collection. Mark and sweep
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1 CS842: Automatic Memory Management and Garbage Collection Mark and sweep 1
2 Schedule M W Sept 14 Intro/Background Basics/ideas Sept 21 Allocation/layout GGGGC Sept 28 Mark/Sweep Mark/Sweep cto 5 Copying GC Ref C cto 12 Thanksgiving Mark/Compact cto 19 Partitioning/Gen Generational cto 26 ther part Runtime Nove 2 Final/weak Conservative Nove 9 wnership Regions etc Nove 16 Adv topics Adv topics Nove 23 Presentations Presentations Nove 30 Presentations Presentations 2
3 Review Memory manager: Allocation and revocation Revocation linked to allocation Scan heap for reachable objects, sweep to free unreachable ones 3
4 Review Mutator yields Collector decides when to collect Collector controls allocation Stop the world : Collector in complete control of heap 4
5 Marking Root bj bj bj Graph reachability problem Roots are not objects! Roots aren t marked Roots don t have headers bj bj bj 5
6 The mark algorithm (one version) markphase(): worklist := new Queue foreach loc in roots: ref := *loc if ref!= NULL and!marked(ref): mark(ref) worklist.push(ref) markworklist() markworklist(): while (ref := worklist.pop()): foreach loc in ref->header.descriptor->ptrs: child := *(ref+loc) if child!= NULL and!marked(child): mark(child) worklist.push(child) 6
7 The mark algorithm (one version) markphase(): worklist := new Queue foreach loc in roots: ref := *loc if ref!= NULL and!marked(ref): mark(ref) worklist.push(ref) markworklist() Root task very different from object task! markworklist(): while (ref := worklist.pop()): foreach loc in ref->header.descriptor->ptrs: child := *(ref+loc) if child!= NULL and!marked(child): mark(child) worklist.push(child) 6
8 Scan order Presented algorithm: Follows root pointers to completion before moving on to another root pointer Is breadth-first for heap objects This should make you angry! 7
9 Scan order bjects often form cliques bject cliques: Are allocated around the same time Mostly point at each other Should be allocated near each other 8
10 Scan order: Address-first? We could sort worklist by ref address Time to sort usually overwhelms saved time scanning 9
11 Mark bit Without mark bit, graph reachability trace may never end! Mark bit can be in header r, can keep a side table If in header: Where to put the bit? 10
12 Mark bit struct bjectheader { struct GCTypeInfo *typeinfo; char markbit; }; void mark(struct bjectheader *hdr) { hdr->markbit = 1; } int ismarked(struct bjectheader *hdr) { return hdr->markbit; } 11
13 Mark bit struct bjectheader { struct GCTypeInfo *typeinfo; char markbit; }; How much larger are objects when this is added? void mark(struct bjectheader *hdr) { hdr->markbit = 1; } int ismarked(struct bjectheader *hdr) { return hdr->markbit; } 11
14 Bit-sneaky There are three 1 wasted bits in our header Pool address bject address Always 0! All pointers have this extra space! 1 n 32-bit systems, two 12
15 Bit-sneaky C struct bjectheader { struct GCTypeInfo *typeinfo; }; void mark(struct bjectheader *hdr) { hdr->typeinfo = (struct GCTypeInfo *) ((size_t) hdr->typeinfo 1); } int ismarked(struct bjectheader *hdr) { return (size_t) hdr->typeinfo & 1; } 13
16 Worth it? If objects are small (hint: they are), every word counts Huge complication: Type info pointer is no longer valid! Must restore type info pointer later 14
17 Sweep Heap parsability is crucial! Consider heap parsability with: Bump-pointer allocation Free-list overallocation Free object type/header 15
18 Sweep algorithm sweep(): freelist := new FreeList foreach ref in heap: if marked(ref): unmark(ref) else: ref *:= new Freebject freelist.push(ref) 16
19 Sweep algorithm sweep(): freelist := new FreeList foreach ref in heap: if marked(ref): unmark(ref) else: ref *:= new Freebject freelist.push(ref) Discard old freelist 16
20 Sweep algorithm sweep(): freelist := new FreeList foreach ref in heap: if marked(ref): unmark(ref) else: ref *:= new Freebject freelist.push(ref) Discard old freelist Must walk entire heap! 16
21 Sweep algorithm sweep(): freelist := new FreeList foreach ref in heap: if marked(ref): unmark(ref) else: ref *:= new Freebject freelist.push(ref) Discard old freelist Must walk entire heap! Perfect chance to unmark 16
22 Sweep algorithm sweep(): freelist := new FreeList foreach ref in heap: if marked(ref): unmark(ref) else: ref *:= new Freebject freelist.push(ref) Discard old freelist Must walk entire heap! Perfect chance to unmark Type of objects change in sweep 16
23 h h h h h h h h h 17
24 h h h h h h h h h 18
25 h h h h h h h h h 19
26 h F h h h h h h h h 20
27 h F F F h h h h h h h h 21
28 h F F F h h h h h h h h 22
29 h F F F h h h h h h h F h F 23
30 Performance Mark: (L) Sweep: (H) Mark-and-sweep: (H) 24
31 Performance Mark: (L) Sweep: (H) Mark-and-sweep: (H) 24
32 Bit-swapping Can avoid cost of clearing bits by swapping meaning: In first collection, 0 = unreachable, 1 = reachable, in second collection, 1 = unreachable, 0 = reachable, etc. Must remember to allocate with correct mark! 25
33 Improving mark Depth-first vs. breadth-first vs. addressordered Bitmapped mark ther tricks beyond scope of course 26
34 Bitmapped mark Connected to bitmap free-list: Bitmap at beginning of pool Clear bitmap before marking ne bit per word If object is alive, mark its words in bitmap Use as bitmap free-list during allocation With bit-swapping, no sweep 27
35 Improving sweep It s not so bad (locality!) Improve by: Even better cache behavior, concurrent/lazy sweeping, or (1) sweep 28
36 Sweep cache behavior Stride of sweep always object size CPUs prefetch bject size varies Segregated blocks: bject size constant, perfect prefetch 29
37 Concurrent sweep Mutator will never touch unmarked objects Sweep in a separate thread Must be careful about allocation/sweep races! 30
38 Lazy sweep Sweep during allocation If free-list is empty, sweep until sufficient free object is found Insufficient objects added to free-list 31
39 Lazy sweep h h h h h h h h h Sweep pointer maintained per pool When allocating, if free-list is empty or has no suitable objects 32
40 Lazy sweep h h h h h h h h h Sweep until a suitable object is found This object is returned to mutator 33
41 Lazy sweep h F h h h h h h h h Unsuitable objects added to free-list during allocation 34
42 Lazy sweep h F h h h h h h h h until a suitable object is found. This object is returned to mutator 35
43 Lazy sweep performance Throughput Responsiveness Resource utilization Fairness Latency 36
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