#7.5. Concurrent mark

A full collection may run its mark phase concurrently with mutators. Minor collection, explicit rok_gc_collect, and evacuation are always stop-the-world (7.3).

#7.5.1. When

An unset or empty variable has its default. A value outside those listed prints one warning line on stderr naming the variable, the value and the accepted values, and the variable has its default.

#7.5.2. Phases

1. Initial pause (STW). Run one minor collection and take a snapshot of the roots at a safepoint. Set the global MARKING flag.
2. Concurrent mark. A single marker thread marks starting from the snapshot. Mutators keep running.
3. Final pause (STW). Drain the SATB buffers, rescan the roots, and sweep. Clear MARKING.

#7.5.3. SATB barrier

Snapshot-at-the-beginning (Yuasa), per object. It reuses the generational write barrier (7.4.2) as is.

• The slow path on the first write to an UNLOGGED object, when MARKING is set, records all of that object's current references in the SATB buffer (TLS rok_tls_satb_cursor / rok_tls_satb_limit, 8.3).
• While recording, it sets the header's SATB bit. A competing store to the same object waits until that bit is cleared.
• So the barrier code the compiler emits is the same whether or not concurrent mark is in use.

#7.5.4. Allocation and collection

• Allocation during mark is black at line granularity (objects in that line are treated as live). Objects allocated during mark are young after mark finishes.
• No minor collection happens during mark. If allocation demands a collection, the ongoing mark is finished.
• Evacuation happens only in STW full collections. If the heap is fragmented after a concurrent mark, the next full collection is done as STW with evacuation.

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