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4965777: GC changes to support use of discovered field for pending references

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If and when the reference handler thread is able to use the discovered field to link reference objects in its pending list, so will GC. In that case, GC will scan through this field once a reference object has been placed on the pending list, but not scan that field before that stage, as the field is used by the concurrent GC thread to link discovered objects. When ReferenceHandleR thread does not use the discovered field for the purpose of linking the elements in the pending list, as would be the case in older JDKs, the JVM will fall back to the old behaviour of using the next field for that purpose.

Reviewed-by: jcoomes, mchung, stefank
This commit is contained in:
Y. Srinivas Ramakrishna 2011-09-07 13:55:42 -07:00
parent ec13301b32
commit ed72e31727
6 changed files with 229 additions and 88 deletions
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10
hotspot/src/share/vm/memory/referenceProcessor.hpp
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@ -52,6 +52,8 @@ class DiscoveredList;
class ReferenceProcessor : public CHeapObj {
protected:
// Compatibility with pre-4965777 JDK's
static bool _pending_list_uses_discovered_field;
MemRegion _span; // (right-open) interval of heap
// subject to wkref discovery
bool _discovering_refs; // true when discovery enabled
@ -111,7 +113,6 @@ class ReferenceProcessor : public CHeapObj {
return _current_soft_ref_policy;
}
public:
// Process references with a certain reachability level.
void process_discovered_reflist(DiscoveredList refs_lists[],
ReferencePolicy* policy,
@ -297,6 +298,13 @@ class ReferenceProcessor : public CHeapObj {
bool discovery_is_atomic() const { return _discovery_is_atomic; }
void set_atomic_discovery(bool atomic) { _discovery_is_atomic = atomic; }
// whether the JDK in which we are embedded is a pre-4965777 JDK,
// and thus whether or not it uses the discovered field to chain
// the entries in the pending list.
static bool pending_list_uses_discovered_field() {
return _pending_list_uses_discovered_field;
}
// whether discovery is done by multiple threads same-old-timeously
bool discovery_is_mt() const { return _discovery_is_mt; }
void set_mt_discovery(bool mt) { _discovery_is_mt = mt; }