Architecture of Computing Systems (Lecture Notes in Computer ...

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30 T.B. Preußer, P. Reichel, and R.G. Spallek The classical compaction approaches are two-space copying [17] and markand-compact, which is used by JOP’s hardware-assisted GC [12]. We, instead, propose a novel mark-and-copy approach operating on a segmented memory. This approach avoids both the tremendous space overhead of two-space copying as well as the compaction race of a concurrent mark-and-compact where each allocation shrinks the allocation area while also adding to the compaction work. The segment life cycle depicted in Fig. 3 is managed by the MCU. Designated hardware modules assist the allocation and the compaction and operate on segments assigned by the MCU. The time-critical allocation is, thus, fully decoupled from the MCU, and even the exchange of the allocation segment is buffered by FIFOs. The compaction process, on the other hand, is mostly controlled by the MCU itself. Merely, the object movement is implemented as hardware service provided by the memory access management. This low-level integration enables transparent mutator access even to objects being copied. The MCU regularly initiates heap scans to detect dead objects. While their references handles are recycled immediately, their occupied memory is only marked as such. Segment utilization statistics are maintained to identify sparsely used segments whose surviving objects are evacuated into an evacuation segment before the segment is returned to the pool of empty ones. Similar to the allocation segment, the evacuation segment is populated compactly using bump-pointer allocation and is only exchanged upon the first unsuccessful migration. This proposed algorithm enables continuous allocation and concurrent garbage collection. A race between allocation and collection has been avoided as both are operating in distinct segments. The copying effort is reduced to surviving objects co-residing with garbage in the same segment. Segments with only short-lived operational objects are freed as a whole without any copying work. Segments with accumulated old long-lived objects will remain untouched. In addition to the spontaneous formation of object generations, the collector can accelerate this trend by the use of generational evacuation segments. The critical parameter of the proposed approach is the segment size. Firstly, it restricts the size of the largest allocatable object. Secondly, small segments increase the management overhead in terms of segment exchanges and state information. On the other hand, large segments force a coarse-grain memory management with a potentially significant space overhead approaching the behavior of a copying collector. Hence, a set of well over 4 segments should be, at least, available. Having decided for a moving GC, measures must be taken to ensure the stable identification of each object throughout its life cycle even in the possibility of its displacement. This is achieved through fully-transparent handles, which are the only identifications of objects ever known to a mutator. The memory manager internally maps a handle to a state record comprising the current storage location of the referenced object, the sizes of its reference and data areas as well as some GC information.
An Embedded GC Module with Support for Multiple Mutators 31 3.3 Exact Garbage Collection Targetting the employment in constraint embedded devices, it is essential to provide an exact GC. A conservative collection is simply no option as its impreciseness would have to be paid for with additional usually spare memory resources. The exact garbage collection must be capable of a clear distinction between references and primitive data. This may be achieved either by the direct tagging of data words or by administrative metadata. The latter option is the more economical on the heap where only instances of well-defined class layouts reside. Adopting the bidirectional class layout of the SableVM [18] further allows to reduce the required metadata to the sizes of the primitive storage growing with positive offsets and of the reference storage growing with negative offsets from the object’s base address. Our GC architecture features distributed tapping modules to collect the root set of references from the mutators. These must also be able to identify references precisely within the tapped storage elements. In our adaptation for SHAP, we chose the tagging approach for the internal stack and registers as the data contained therein does not follow a only handful of structural blueprints. 3.4 Object Graph Marking Our architecture builds upon a decentralized collection of the root set. Each mutator core is extended by its own root scan unit, which collects the references contained in the local registers and stack into a mark table. As shown in Fig. 1, all mutator cores are arranged in a ring on a GC bus, which is mastered by the MMU. This unit issues commands onto the ring and receives their acknowledgements as they return on the other end. This ring also serves the gradual merger of the contents of the individual root tables into the global root set. While the MMU transmits an empty table, each core ORs the received table with its own table contents before forwarding it along the ring. The final result is entered into the central mark table inside the MMU. The local root scans operate concurrently to their associated mutator core. Once finished, they are deactivated for the remainder of the GC cycle so that further modifications of the local root sets are not logged. This is safe as the mutators only gain access to other references by loading them from objects – which were, thus, reachable from the original root set – or by creating new ones. Objects allocated during the GC cycle are already implicitly marked by the allocation engine. After the completion of all root scans, the actual computation of the reachable subgraph is performed by the heap scan. Our optimized variant of the wellestablished tri-color marking uses the mark table to tell reached from unreached objects. The colors red and green further distinguish the reachable objects into scanned and unscanned ones. The meaning of these colors alternates and is determined by the color of the active GC cycle. Initially, all references are unscanned and have the color opposite to the current cycle. Upon being scanned, a reference assumes the cycle color. References to newly-allocated objects are immediately assigned the active color as they do not contain valid references and need not be
Page 2 and 3: Lecture Notes in Computer Science 5
Page 4 and 5: Volume Editors Christian Müller-Sc
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Page 8 and 9: Organization IX Hartmut Schmeck Kar
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Exploiting Inactive Rename Slots fo
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130 M. Kayaalp et al. INSTRUCTION 1
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Efficient Transaction Nesting in Ha
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144 Y. Liu et al. Processor core Pr
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Decentralized Energy-Management to
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Optimizing Stencil Application on M
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236 F. Xudong et al. compared to th
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242 F. Xudong et al. Speedup 14 12
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Abdullah, Tariq 174 Alima, Luc Onan
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