Collect garbage that has accumulated in the memory and makes your computer slow Your Benefits at a Glance  Suitable for systems with hard real-time requirements (in particular embedded systems)  Low garbage collection overhead  No interruptions caused by garbage collection  Extremely robust due to object-based memory protection  Fully operational prototype (including a Java compiler)

1. A Novel RISC Processor Architecture For
Garbage Collection in Embedded Systems
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2. Buffer Overflows are Responsible for a
Large Number of Today’s Security and Safety Problems
 In a standard computer system, dynamically growing data
structures can overwrite unrelated data (buffer overflow)
 The standard processor architecture lacks protection mechanisms
against buffer overflows
 Buffer overflow errors are a common cause for critical security
vulnerabilities
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3. Garbage Collection Helps Reduce Buffer Overflows,
But Causes High Overhead & Unpredictable Pauses
 Automatic dynamic memory management automatically releases and
compacts dynamically allocated memory after its last use
 Such “garbage collection” reduces common error sources
for buffer overflows
 Existing garbage collection is mostly software-based, demands a high
overhead and causes unpredictable pauses in the program execution
 The limited resources of embedded systems typically do not allow
for efficient garbage collection in real time
© TLB GmbH, Karlsruhe 2012

4. A Novel Approach Enables Parallel Garbage Collection
And Parallel Synchronization in Real-Time
 The novel RISC processor architecture is optimized for security:
 Strict separation of pointers from ordinary non-pointer data by using
distinct register sets for pointers and data
 The dedicated coprocessor performs the garbage collection:
 The coprocessor uses an optimized Baker-style copying collector
algorithm that runs in parallel to the main processor
 A new garbage collection cycle is started by the coprocessor when the
available memory falls below a chosen threshold
 Simple hardware extensions to the processor pipeline support the
synchronization between garbage collector and main processor
 Key for the efficient implementation to avoid unbounded pauses
© TLB GmbH, Karlsruhe 2012

5. This Novel Approach Improves Performance
By Leaving The Cache Largely Unaffected
 Software garbage collectors usually repeatedly
displace the entire contents of the cache
 Examine the entire heap during a single cycle
 The coprocessor directly connects to the memory controller
 Does not access memory through the main processor’s cache
 The cache remains largely unaffected by the garbage collection
 The coprocessor ensures cache coherency
 Inspects and selectively flushes single cache lines through a dedicated
cache port (resembles snoop port)
 The coprocessor eliminates unnecessary memory traffic
 Invalidates all cache lines with dead objects
at the end of a garbage collection cycle
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6. A Fully Functional Prototype Exists
And Has Been Used For Performance Measurements
 Main processor & GC Coprocessor modeled at register transfer level
in VHDL, synthesized for Altera APEX 20K1000C (@ 25MHz)
 Pipelined RISC processor, statically scheduled
 up to 3 instructions per clock cycle (3-way multiple issue, “in order”)
 16 pointer registers, 16 data registers, 8 Praedikatregister
 8K execution cache, 8K data cache, 2K attribute cache
 two-way set-associative copy-back cache
 Micro-coded garbage collection coprocessor
 256 x 80 bit on-chip microcode memory
 Uses less than 20% of the chip surface area
 Software
 Native Java bytecode compiler developed for the architecture.
An included code scheduler rearranges instructions to take advantage of
the processor’s parallel execution units and to hide instruction latencies
 Subset of the Java class libraries supporting text-based apps in order to
facilitate the execution of representative programs (includes NFS client)
© TLB GmbH, Karlsruhe 2012

7. An Experimental Computer System Was Assembled
Based On The Garbage-Collection Processor
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8. Pauses Caused By Garbage Collection Do
Not Exceed 500 Clock Cycles
Frequency distribution of synchronization pauses (shown for javac)
Frequency
Pause Duration in Clock Cycles
© TLB GmbH, Karlsruhe 2012

9. The Runtime Overhead For The Hardware-Based
Garbage Collection Is Small
© TLB GmbH, Karlsruhe 2012

10. The Advantages Of This Approach Could Enable
Real-Time Garbage Collection in Embedded Systems
 Limits pauses from garbage collection to 500 clock cycles
 Efficient synchronization
 No code overhead
 Low total runtime overhead of only a few percent
 Undisturbed cache locality
 Exact (non-conservative) garbage collection
 Compiler & code are independent from garbage collector
© TLB GmbH, Karlsruhe 2012

11. BACKUP
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12. Efficient Implementation
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13. Coprocessor Architecture
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14. Synchronization I
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15. Synchronization II
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16. Synchronization III
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17. Synchronization IV
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18. The Runtime Overhead Is Small - I
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19. The Runtime Overhead Is Small - II
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20. The Runtime Overhead Is Small - III
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