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

Activate Write Barrier
Local Root Scans
Central Heap Scan
Deactivate Write Barrier
Process Weak References
Recycle Reference Handles
An Embedded GC Module with Support for Multiple Mutators 29
Fig. 2. GC Cycle Overview
Evacuate
Sparse
Segments
Full
Compaction
Alive Objects
Empty Used
FIFO FIFO
Allocation Engine
Sparse
Fig. 3. Segment Life Cycle
provides the strong advantages of a concise design structure, lower resource demand
and a high achieved clock frequency. In fact, an OpenFIRE solution would
require a second slower clock domain or the reduction of the overall system clock
by 40%. Consequently, it was the ZPU microarchitecture, which we chose to be at
the heart of our memory management.
As illustrated in Fig. 1, the ZPU assumes the central control of the memory
management as memory control unit (MCU). It is surrounded by several
special-purpose hardware components implementing time-critical subtasks. The
connection to the system Wishbone bus enables the slow administrative communication
with the mutator cores. This not only serves the communication of
statistics data but is also used for delivering weak reference proxies to the runtime
system for their possible enqueuing into reference queues so that our design
supports this feature in addition to the CLDC requirements.
The memory access of the mutator cores are prioritized over GC-related accesses.
The garbage collector, thus, operates on cycle stealing, a very fine-grained
utilization of otherwise idle memory bandwidth.
The overall GC cycle is summarized in Fig. 2. It is initiated and supervised
by the MCU. Individual tasks are, however, backed by dedicated hardware components
containing small specialized state machines.
3.2 GC Strategy
The organization of the heap memory must enable both the prompt allocation
of objects as well as a steady recycling of unused memory. We chose a simple
yet efficient bump-pointer allocation scheme. While it guarantees an instant
allocation, it also requires the compaction of the used memory as to re-generate
the continuous allocation region.