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

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32 T.B. Preußer, P. Reichel, and R.G. Spallek 31 30 29 0 S W Reference Handle 0 0 1 1 0 1 1 0 Tag Strong Soft Weak undefined Strength Fig. 4. Reference Strength Tags * 0..1 Reference ReferenceQueue SoftReference WeakReference PhantomReference Fig. 5. Hierarchy of Reference Proxy Classes scanned. The heap scan is completed when all reachable and, thus, marked references have assumed the cycle color. After disposing of the remaining unmarked and, thus, unreachable references, the meaning of the colors is exchanged and the initial condition that all references are of the opposite color is re-established for the next GC cycle. While the completion of the scan could be detected by another walk through the mark table that does not produce any marked reference without cycle color, we implemented a simple but effective optimization. Each time a previously unmarked reference is marked in the mark table, it is also copied into a mark FIFO. This FIFO is read to determine objects that still have to be scanned during the heap scan. Only when it runs empty, the mark table is re-walked to search for additional work but only if the walk just completed did produce a FIFO overflow. As all work has been safely finished when the FIFO sufficed, a final unproductive walk of the table is no longer needed. The heap scan must account for conccurent modifications of the object graph by the mutators. In particular, it must be ensured that no mutator ever gets hold of a reference that then remains unmarked. This situation might occur when a reference field of a still unscanned object is read and overwritten by a different value before it is scanned. To keep the effected subgraph safely alive, a write barrier is implemented to intercept reference writes and to enter the overwritten references into the mark table. In the context of multiple mutators, the write barrier is implemented using an atomic swap operation on the backing memory as to outrule a race condition among possibly concurrent read-write sequences on the same storage location. Although only required for the heap scan, we chose to activate the write barrier even prior to the initiation of the local root scans. This choice relaxes the phase transition from root to heap scan and avoids its system-wide synchronization throughanatomicrendezvous. 3.5 Weak Reference Support In contrast to their regular strong counterparts, weak references do not keep their referents alive. An object may become eligible for garbage collection in spite of the existence of paths via weak references to it. If the collector decides to discard the referent of a weak reference, the reference must be deprived of
An Embedded GC Module with Support for Multiple Mutators 33 its capability to retrieve the referent, which is usually achieved by clearing it to null. Reference queues may establish an additional notification scheme allowing cleared references to be enqueued for processing by an application thread. The Java 2 Standard Edition knows of several different strengths of weak references, which gives rise to the class hierarchy shown in Fig. 5. While the CLDC 1.1 only requires the WeakReference, our architecture additonally supports their soft variant and reference queuing. Weak references are implemented as proxy objects containing a special reference member initialized from a strong reference provided to the constructor. The garbage collector must be enabled to recognize these special references in order to treat them appropriately in the heap scan and to clear them when it is about to collect their referents. Instead of providing the GC with information about the hierarchy of the reference classes, we designated two bits of the reference word to tag the supported weak reference types as shown in Fig. 4. While these bits are generally cleared, they are set by the constructors of reference objects. This approach even enables a more flexible use of weak references outside the hierarchy of the reference classes. During the heap scan, weak references are not followed. The treatment of soft references is decided once at the beginning of a GC cycle according to the current memory utilization so that they are followed as long as free storage is available. The objects containing unfollowed references are, however, enlisted during the scan. After its completion, this list of proxies is scanned for references to unreached referents. Any one found will be cleared, and the effected proxy will be communicated to the mutators for its possible enqueuing into a reference queue. For this purpose, the SHAP runtime system forks a designated service thread that maintains the communication with the memory manager via its Wishbone connection. Special care is to be taken upon the retrieval of a strong reference from a weak proxy. Assume that an object only remains reachable through a weak reference. As long as the garbage collector does not discover this condition, the reference is not cleared but totally valid. The invocation of the get() method on the WeakReference proxy object will return with a normal strong reference to the referent effectively resurrecting the object from the brink of death. A race condition may, however, arise when such a resurrection interferes with an ongoing heap scan. It must be ensured that the reference to be returned by get() is either entered into the mark table prior to the completion of the heap scan or invalidated by returning null. Not knowing whether an ongoing heap scan will render the referent strongly-reachable or not, a consistent resurrection with a mark table entry is attempted first. Only if the scan has finished in the meanwhile, the actually determined reachability is evaluated. If necessary, null will be returned in conformance to the inevitable clearing of the original reference field. 4 Evaluation in SHAP The described design was implemented for SHAP and integrated into the runtime system through adapted microcode implementations accessing the port to the
Page 2 and 3: Lecture Notes in Computer Science 5
Page 4 and 5: Volume Editors Christian Müller-Sc
Page 6 and 7: General Chair Organization Christia
Page 8 and 9: Organization IX Hartmut Schmeck Kar
Page 10 and 11: Keynote Table of Contents HyVM - Hy
Page 12 and 13: Table of Contents XIII JetBench:AnO
Page 14 and 15: How to Enhance a Superscalar Proces
Page 16 and 17: 4 J. Mische et al. The Real-time Vi
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Page 38 and 39: 26 T.B. Preußer, P. Reichel, and R
Page 50 and 51: 38 P. Bellasi, W. Fornaciari, and D
Page 62 and 63: 50 J. Zeppenfeld and A. Herkersdorf
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82 M. Bonn and H. Schmeck done rate
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84 M. Bonn and H. Schmeck Fig. 8. J
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86 M. Bonn and H. Schmeck tells the
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88 J.-P. Steghöfer et al. � �
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90 J.-P. Steghöfer et al. mechanis
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92 J.-P. Steghöfer et al. resource
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94 J.-P. Steghöfer et al. Choose a
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96 J.-P. Steghöfer et al. 1. Defin
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98 J.-P. Steghöfer et al. and all
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100 J.-P. Steghöfer et al. 19. Kim
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102 K. Kloch et al. large-scale sys
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104 K. Kloch et al. a�t� 1.0 0.
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106 K. Kloch et al. constant. This
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108 K. Kloch et al. Relative number
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110 K. Kloch et al. (a) infection r
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112 K. Kloch et al. (ii) Phase of a
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114 P. Petoumenos et al. Studying t
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116 P. Petoumenos et al. % of Misse
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118 P. Petoumenos et al. IQ: n 4-in
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120 P. Petoumenos et al. downsizing
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122 P. Petoumenos et al. As long as
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124 P. Petoumenos et al. comparable
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Exploiting Inactive Rename Slots fo
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128 M. Kayaalp et al. In a supersca
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130 M. Kayaalp et al. INSTRUCTION 1
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132 M. Kayaalp et al. time. Alterna
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134 M. Kayaalp et al. Fig. 3. Numbe
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136 M. Kayaalp et al. The results o
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Efficient Transaction Nesting in Ha
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140 Y. Liu et al. HTMs include TCC
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142 Y. Liu et al. rollback T0 begin
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144 Y. Liu et al. Processor core Pr
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146 Y. Liu et al. 5.2 Results and A
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148 Y. Liu et al. decreases, partia
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Decentralized Energy-Management to
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152 B. Becker et al. Furthermore, t
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154 B. Becker et al. An optimizing
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156 B. Becker et al. smart-home man
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158 B. Becker et al. freedom like w
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160 B. Becker et al. Power [W] 4500
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EnergySaving Cluster Roll: Power Sa
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164 M.F. Dolz et al. Themodulequeri
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166 M.F. Dolz et al. This daemon al
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168 M.F. Dolz et al. been submitted
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170 M.F. Dolz et al. On the other h
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172 M.F. Dolz et al. at the inactiv
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Effect of the Degree of Neighborhoo
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176 T. Abdullah et al. A zone based
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178 T. Abdullah et al. A consumer/p
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180 T. Abdullah et al. Messages 140
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182 T. Abdullah et al. (except when
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184 T. Abdullah et al. % Matchmakin
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186 T. Abdullah et al. show that th
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188 M. Schindewolf, D. Kramer, and
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200 R. Plyaskin and A. Herkersdorf
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212 M.Y. Qadri, D. Matichard, and K
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A Tightly Coupled Accelerator Infra
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224 F. Nowak and R. Buchty where A
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226 F. Nowak and R. Buchty Fig. 3.
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228 F. Nowak and R. Buchty Table 2.
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230 F. Nowak and R. Buchty Table 4.
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232 F. Nowak and R. Buchty 5.3 Comp
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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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238 F. Xudong et al. stencil comput
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240 F. Xudong et al. threads in the
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242 F. Xudong et al. Speedup 14 12
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244 F. Xudong et al. 5 Related Work
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Abdullah, Tariq 174 Alima, Luc Onan
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