Some note about GC algorithm

1. WhatwelovetoknowaboutGarbage
Collectoranditsimplementation
@huydx
LINECorp

2. Highlevelview
CollecttheGarbage
Preventsdanglingpointers
ReducechancebutNOTeliminatememoryleak!

3. Somefunfacts

4. HowmanylanguageshaveembededGC?

5. Answer:78/256

7. HowmuchperformancepenaltyGC
created

8. Answer:17%
Whileitistruethatautomaticmemorymanagementdoesimposea
performancepenaltyontheprogram,itisnotasmuchasis
commonlyassumed

9. IsGCalgorithmhard?

10. It'sfuckinghard

11. WhyunderstandGCimplementationis
important?
Presiquisite:Youdon'twanttomanagememoryyourself!
Latencycriticalapplication
Throughputcriticalapplication
Youdon'twantGCtohang(STW)yourapplicationforx0sseconds

12. SomeGCconcepts/term
Heap:arrayofmemorywords,orobjectgraph
Mutator:mutatetheheap(new/delete/update..)
Collector:executeGCcode
Mostlysinglethread:toreducecomplexity
That'swhyweneedtoStopTheWorld(STW)
Liveness:liveobject==accessibleobject
Reachability:reachabledirectlyorbychain

13. GCtasks
Allocatespacefornewobjects
Identifyliveobjects
Reclaimspacebydeadobjects

14. GCalgorithms
Alot
Marksweep
Copying
Markcompact
Refcounting
Ormixbetweenthose

15. MarkSweepGC
Mostcommonone
GoodsampletoshowhowhardGCimplementationis
2phasescollection:
Traverseobjectgraphandmarking(sometimeswecallit
"tracing"
Examineeveryunmarkobjectandreclaimitsspace

16. from(http://www.narihiro.info/)

17. markingphase
markFromRoots():
initialise(worklist)
for each fld in Roots
ref ← *fld
if ref ≠ null && not isMarked(ref)
setMarked(ref)
add(worklist, ref)
mark()
initialise(worklist):
worklist ← empty
mark():
while not isEmpty(worklist)
ref ← remove(worklist)
/* ref is marked */
for each fld in Pointers (ref)
child ← *fld
if child ≠ null && not isMarked(child)
setMarked(child)
add(worklist, child)

18. sweepingphase
sweep(start, end):
scan ← start
while scan < end
if isMarked(scan)
unsetMarked(scan)
else free(scan)
scan ← nextObject(scan)

19. Exploringminigc
https://github.com/authorNari/minigc
Simpleandminimum
MarkandSweep
ConservativeGC
Selfimplementmemoryallocator(K&Rmalloc)

20. ConservativevsPreciseGC
Conservative:knownothingaboutthelanguage,justwalkaround
inmemoryaddressrangeandfindwhatlookslikeobjectaddress
Precise:Knowexactlywhatisobjectaddressornot.
Requirehookwithcompilerphasetogetknowledgeaboutthe
runtime

21. ConservativeGC
Thetricktoconservativecollectionistotreatthestackasan
ordinaryrangeofmemory,andassumethateverywordonthe
stackisapointerintotheheap.Thus,thecollectormarksall
objectswhoseaddressesappearanywhereinthestack,without
knowingforsurehowthatwordismeanttobeinterpreted.
Problem:
intprimitivevsaddress!
thereforefullcopyingcollectorisnotabletoimplemented
(differentstory)

22. minigcgc
void
garbage_collect(void)
{
size_t i;
/* marking machine context */
gc_mark_register();
gc_mark_stack();
/* marking roots */
for (i = 0; i < root_ranges_used; i++) {
gc_mark_range(root_ranges[i].start, root_ranges[i].end)
}
/* sweeping */
gc_sweep();
}

23. markregister:markeverythingincurrentcontext
usingsetjmp
static void
gc_mark_register(void)
{
jmp_buf env;
size_t i;
setjmp(env);
for (i = 0; i < sizeof(env); i++) {
gc_mark(((void **)env)[i]);
}
}

24. markstack:markfromstartofstacktoendofstack
static void
gc_mark_stack(void)
{
set_stack_end();
if (stack_start > stack_end) {
gc_mark_range(stack_end, stack_start);
}
else {
gc_mark_range(stack_start, stack_end);
}
}

25. markingusingheaderbitset
static void
gc_mark(void * ptr)
{
GC_Heap *gh;
Header *hdr;
/* mark check */
if (!(gh = is_pointer_to_heap(ptr))) return;
if (!(hdr = get_header(gh, ptr))) return;
if (!FL_TEST(hdr, FL_ALLOC)) return;
if (FL_TEST(hdr, FL_MARK)) return;
/* marking */
FL_SET(hdr, FL_MARK);
DEBUG(printf("mark ptr : %p, header : %pn", ptr, hdr));
/* mark children */
gc_mark_range((void *)(hdr+1), (void *)NEXT_HEADER(hdr));
}

26. Number.Hard.1
Howanobjectcouldbemarked?
Usingheadermakethewholememoryrangedirty,whichleadto
pagefault
Alternativeoptionbesideheader
Usingsidetabletostoremarkbits

27. Number.Hard.2
Representstateofobjectsduringcollection
Tricolourabstraction:white,gray,black(beingprocessed,
processed,notyetprocessed)

28. Number.Hard.3
K&Rmallocusinglinkedlist
Shouldhavebetterimplementation

29. Tonsofthingscouldbeimproved:
Improvementofmarkingmethod:smallerheader(tagmarkflag
intosizepointer)
Cachehit/miss
Multithreaded
Compactornotcompact(anotherstory)

30. Reference:
GChandbook
SurveyofGCalgorithm
ftp://ftp.cs.utexas.edu/pub/garbage/gcsurvey.ps
http://www.hboehm.info/gc/
https://ruby‑hacking‑guide.github.io/gc.html
https://speakerdeck.com/tenderlove/building‑a‑compacting‑gc