Request for review: 8008079 G1: Add nextObject routine to CMBitMapRO and replace nextWord

John Cuthbertson john.cuthbertson at oracle.com
Mon Feb 25 23:06:15 UTC 2013


Hi Tao,

Looks fine except for the following nits....

concurrentMark.hpp:100
Can you remove the line:

// XXX Fix these so that offsets are size_t's...

concurrentMark.hpp:111
It might be safer to return:

return offsetToHeapWord(heapWordToOffset(addr + obj->size()));

or assert something like:

HeapWord* res = addr + obj->size();
assert(offsetToHeapWord(heapWordToOffset(res)) == res, "sanity");
return res;

JohnC

On 2/21/2013 6:16 PM, Tao Mao wrote:
> 8008079 G1: Add nextObject routine to CMBitMapRO and replace nextWord
> https://jbs.oracle.com/bugs/browse/JDK-8008079
>
> webrev:
> http://cr.openjdk.java.net/~tamao/8008079/webrev.00/
>
> changeset:
> When concurrent marking scans an object, the task local finger is 
> updated to point to the start of the object. If the marking task is 
> asked to abort, the local finger is updated to the next word where 
> another could possibly start. When the marking task is restarted, we 
> restart scanning the marking bitmap from the updated local finger.
>
> This is a safe implementation but has not been fully exploited for 
> efficiency because the contents of the marking bitmap should be all 
> 0's until the offset associated with the actual start of the next 
> object. When the marking task is restarted, we will scan these 0's 
> looking for the first set bit.
>
> We can avoid this redundant scanning by updating the finger to and 
> re-starting the scan at the actual offset where the next object starts.
>
> testing:
> passed gc-test-suite with stressing marking cycles
> script:
> ./run.sh /home/tamao/jdk1.8.0 --bits 64 --args -tamao -XX:+UseG1GC 
> -XX:InitiatingHeapOccupancyPercent=10 -XX:+UnlockDiagnosticVMOptions 
> -XX:+VerifyDuringGC

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