RFR: 8213623: ZGC: Let heap iteration walk all roots

Per Liden per.liden at oracle.com
Fri Nov 9 22:28:44 UTC 2018


On 2018-11-09 21:19, Kim Barrett wrote:
>> On Nov 9, 2018, at 9:49 AM, Per Liden <per.liden at oracle.com> wrote:
>>
>> Let the ZHeapIterator walk all roots, including weak roots that might be logically cleared from the application's point of view. This allows e.g. for a more thorough heap verification.
>>
>> Also the heap iterator should access oops with AS_NO_KEEPALIVE to avoid resurrecting object that would otherwise be garbage collected.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8213623
>> Webrev: http://cr.openjdk.java.net/~pliden/8213623/webrev.0
>>
>> Testing: tier1-4
>>
>> /Per
> 
> ------------------------------------------------------------------------------
> src/hotspot/share/gc/z/zRootsIterator.hpp
>   127   ZConcurrentRootsIterator(bool marking = false);
> 
> Why is this being made optional?  It seems like there is no "safe" or
> "obvious" default value, and that all callers should be required to
> say what they want.

As I see it, this should only be true in one special case, which is when 
we're marking. Other use cases of this iterator should not have to think 
about saying true here.

> 
> ------------------------------------------------------------------------------
> src/hotspot/share/gc/z/zHeapIterator.cpp
>   164 void ZHeapIterator::objects_do(ObjectClosure* cl) {
>   165   // Note that the heap iterator visits all reachable objects, including
>   166   // objects that might be unreachable from the application, such as a
>   167   // not yet cleared JNIWeakGloablRef. However, also note that visiting
>   168   // the JVMTI tag map is a requirement to make sure we visit all tagged
>   169   // objects, even those that might now have become phantom reachable.
>   170   // If we didn't do this the application would have expected to see
>   171   // ObjectFree events for phantom reachable objects in the tag map.
> 
> I think there's a good rationale for walking JNIWeakGlobalRef &etc
> here.  But I think that same rationale applies to the other
> collectors, and I think all the collectors should be consistent about
> this.  Making this semantic change only to ZGC seems wrong to me.

Other collectors typically walk the heap by parsing it from start to 
end, which means it will visit all objects, including those pointed to 
by e.g. JNIWeakGlobalRefs and those that are unreachable. ZGC doesn't 
have a parsable heap, so we instead start from the roots and visit 
everything we can reach from there. Semantically, this change moves ZGC 
a little bit closer to what other collectors do.

cheers,
Per

> 
> ------------------------------------------------------------------------------
> 


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