RFR(S): 8173013: JVMTI tagged object access needs G1 pre-barrier
kim.barrett at oracle.com
Wed Feb 8 07:00:41 UTC 2017
> On Feb 6, 2017, at 10:44 PM, David Holmes <david.holmes at oracle.com> wrote:
>>> Does the SATB occur at a global safepoint?
>> SATB is the approach G1 uses for a part of concurrent collection. It
>> defines the invariants that must be maintained by the mutator so that
>> the collector can find all the live objects. The barriers in question
>> are executed by the mutator concurrently with the collector, in order
>> to inform the collector of reachability graph modifications.
> Is that a "no"? :) I don't understand how the barriers operate in relation to concurrent actions by the GC. To take the current case, what prevents the GC from deciding that the referent is garbage and can deleted, before the enqueue is called? I assumed a safepoint had to be involved somewhere so that the sequence of actions: extract, enqueue, store, was atomic with respect to the GC.
To be more precise, the barriers in question are executed by the
mutator concurrently with the marking phase of the collector. Once
marking is (mostly) complete, G1 safepoints and performs various
tasks, including reference processing. The barrier must be atomic
wrto that safepoint (which is accomplished by not allowing any
safepoints during the barrier execution). So yes, the sequence of
extract and enqueue are atomic wrto the relevant part of the GC.
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