android - What are the advantages and disadvantages of having mark bits together and separate for Garbage Collection -

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i watching video google io 2008 - dalvik virtual machine internals understand how dalvik vm works , why people has preferred dalvik vm on jvm android. found android uses separate memory garbage information objects , opposed jvm have mark bits(bits telling whether object able garbagfe collection or not) objects.

can tell me in detail advantages , disadvantages of having separate memory marks bits , not having separate memory mark bits ?

i unable difference watching video.

some advantages of separate bitmap:

  • much denser. typical gc needs maybe 8 bits of gc metadata, due alignment in-object header might round memory 32 bits.
  • some operations, in particular around sweeping, become faster. partly because denser (see above) bitmap means less memory traffic , better cache use, because operations (e.g. zeroing mark bits) can vectorized when in format. (other parts of gc needs designed make use of ability.)
  • if fork() on unix system, separate bitmark makes better use of copy-on-write: pages containing objects might remain shared.

some advantages of in-object mark bits:

  • depending on scheme used associate objects bitmaps, getting mark bit object , vice versa can quite complicated and/or slow. in-object header, on other hand, trivial access.
  • easier memory management: no need create separate allocation of right size , keep in sync.
  • many fast schemes finding bitmaps objects , vice versa quite restrictive in other regards. example, if create bitmap every page , store bitmap pointer @ start of page, have problem storing objects larger page.

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