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

Concurrent sweep

Like concurrent mark, concurrent sweep gives reduced garbage collection pause times when heap sizes increase. Concurrent sweep starts immediately after a stop-the-world (STW) collection, and must at least finish a certain subset of its work before concurrent mark is allowed to kick off, because the mark map used for concurrent mark is also used for sweeping.

The concurrent sweep process is split into two types of operations:

• Sweep analysis: Sections of data in the mark map (mark bit array) are analyzed for ranges of free or potentially free memory.
• Connection: The analyzed sections of the heap are connected into the free list.

Heap sections are calculated in the same way as for parallel bitwise sweep.

An STW collection initially performs a minimal sweep operation that searches for and finds a free entry large enough to satisfy the current allocation failure. The remaining unprocessed portion of the heap and mark map are left to concurrent sweep to be both analyzed and connected. This work is accomplished by Java™ threads through the allocation process. For a successful allocation, an amount of heap relative to the size of the allocation is analyzed, and is performed outside the allocation lock. In an allocation, if the current free list cannot satisfy the request, sections of analyzed heap are found and connected into the free list. If sections exist but are not analyzed, the allocating thread must also analyze them before connecting.

Because the sweep is incomplete at the end of the STW collection, the amount of free memory reported (through verbose garbage collection or the API) is an estimate based on past heap occupancy and the ratio of unprocessed heap size against total heap size. In addition, the mechanics of compaction require that a sweep is completed before a compaction can occur. Consequently, an STW collection that compacts does not have concurrent sweep active during the next round of execution.

To enable concurrent sweep, use the -Xgcpolicy: parameter optavgpause. It becomes active along with concurrent mark. The modes optthruput, balanced, and gencon do not support concurrent sweep.

Parent: Sweep phase

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