Concurrent mark

IBM User Guide for Java V7 on Windows > IBM SDK for Java > The collector and the allocator > Global garbage collection > Mark phase
Concurrent mark

Concurrent mark gives reduced and consistent garbage collection pause times when heap sizes increase.
The GC starts a concurrent marking phase before the heap is full. In the concurrent phase, the GC scans the heap, inspecting "root" objects such as stacks, JNI references, and class static fields. The stacks are scanned by asking each thread to scan its own stack. These roots are then used to trace live objects concurrently. Tracing is done by a low-priority background thread and by each application thread when it does a heap lock allocation.
While the GC is marking live objects concurrently with application threads running, it must record any changes to objects that are already traced. It uses a write barrier that is run every time a reference in an object is updated. The write barrier flags when an object reference update has occurred. The flag is used to force a rescan of part of the heap.
The heap is divided into 512 byte sections. Each section is allocated a single-byte card in the card table. Whenever a reference to an object is updated, the card that corresponds to the start address of the object that has been updated with the new object reference is marked with the hex value 0x01. A byte is used instead of a bit to eliminate contention, by allowing cards to be marked using non-atomic operations. A stop-the-world (STW) collection is started when one of the following events takes place:

An allocation failure occurs.
A System.gc call is made.
Concurrent mark finishes all the possible marking.

The GC tries to start the concurrent mark phase so that it finishes at the same time as the heap is exhausted. The GC identifies the optimum start time by constant tuning of the parameters that govern the concurrent mark time. In the STW phase, the GC rescans all roots, then uses the marked cards to see what else must be retraced. The GC then sweeps as normal. It is guaranteed that all objects that were unreachable at the start of the concurrent phase are collected. It is not guaranteed that objects that become unreachable during the concurrent phase are collected. Objects which become unreachable during the concurrent phase are known as "floating garbage".
Reduced and consistent pause times are the benefits of concurrent mark, but they come at a cost. Application threads must do some tracing when they are requesting a heap lock allocation. The processor usage needed varies depending on how much idle processor time is available for the background thread. Also, the write barrier requires additional processor usage.
The -Xgcpolicy command-line parameter is used to enable and disable concurrent mark:

-Xgcpolicy: <gencon | optavgpause | optthruput | subpool | balanced>

The -Xgcpolicy options have these effects:

gencon Enable concurrent mark, and uses it in combination with generational garbage collection to help minimize the time that is spent in any garbage collection pause. gencon is the default setting. If you are having problems with erratic application response times that are caused by normal garbage collections, you can reduce those problems, reduce heap fragmentation, and still maintain good throughput, by using the gencon option. This option is particularly useful for applications that use many short-lived objects.
optavgpause Enables concurrent mark with its default values. If you are having problems with erratic application response times that are caused by normal garbage collections, you can reduce those problems at the cost of some throughput, by using the optavgpause option.
optthruput Disables concurrent mark. If you do not have pause time problems (as seen by erratic application response times), you get the best throughput with this option.
subpool Deprecated. Now an alias for optthruput. If used, the effect is the same as optthruput.
balanced Disables concurrent mark. This policy does use concurrent garbage collection technology, but not in the way that concurrent mark is implemented here. For more information, see Global Mark Phase.

Parent: Mark phase

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gencon	Enable concurrent mark, and uses it in combination with generational garbage collection to help minimize the time that is spent in any garbage collection pause. gencon is the default setting. If you are having problems with erratic application response times that are caused by normal garbage collections, you can reduce those problems, reduce heap fragmentation, and still maintain good throughput, by using the gencon option. This option is particularly useful for applications that use many short-lived objects.
optavgpause	Enables concurrent mark with its default values. If you are having problems with erratic application response times that are caused by normal garbage collections, you can reduce those problems at the cost of some throughput, by using the optavgpause option.
optthruput	Disables concurrent mark. If you do not have pause time problems (as seen by erratic application response times), you get the best throughput with this option.
subpool	Deprecated. Now an alias for optthruput. If used, the effect is the same as optthruput.
balanced	Disables concurrent mark. This policy does use concurrent garbage collection technology, but not in the way that concurrent mark is implemented here. For more information, see Global Mark Phase.