MarkWord | a Markdown editer
kandi X-RAY | MarkWord Summary
kandi X-RAY | MarkWord Summary
a Markdown editer. powered by 墨云软件. Third-party components permissions owned by the original author,If some of the components have the license problem,please inform me,and I will delete as soon as possible ,thank you.
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QUESTION
I am studying the composition of java object header.
Found some documents and some interesting problems.
Under 64 bit vm, the default object head is as follows (Because UseCompressedOops
is on by default)
ANSWER
Answered 2020-Apr-06 at 12:34When using flat pointers, the lowest bits of address pointers are always zero due to alignment and allow marking special state by writing ones into these bits. So the CMS set the lowest bit of the klass
pointer to one when it wants to denote that the particular chunk of memory is not an object (anymore), but free memory.
But the compressed pointer feature utilizes the same property to address more memory via a 32 bit pointer by right shifting the address and leaving no unused lower bits. Therefore, the CMS has to store this bit somewhere else, i.e. the cms_free_bit
in question.
Source: concurrentMarkSweepGeneration.cpp:
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