RFR: 8213623: ZGC: Let heap iteration walk all roots (original) (raw)

Kim Barrett kim.barrett at oracle.com
Fri Nov 9 20:19:42 UTC 2018

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 ASNOKEEPALIVE 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.

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.

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