GC Advantage: Improving Program Locality

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GC Advantage Improving Program Locality

• Xianglong Huang, Zhenlin Wang,
• Stephen M Blackburn, Kathryn S McKinley,
• J Eliot B Moss, Perry Cheng

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Motivation

• Memory gap
• How are Java programs affected?

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Marksweep vs. Copying
pseudojbb
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Motivation

• Javac with perfect L1 and L2 cache.
• 16K L1 256K L2
• Appel, GCTk.
• Breadth first

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Motivation

• Copying collector can reorder objects
• Goal take advantage of copying collectors
• reorder objects to improve locality

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Exploring The Space

• Different policies for traversing roots
• Class-oblivious traversal orders
• Which traversing order is the best?
• Class-based traversal orders
• How to find the important data structure?

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Different Root Traversal Policies

• Two different types of roots
• Stack, global variables
• Remember sets (for generational)
• Different traversal orders
• Copy all roots before traversing any children
• Copy each root and its children (root-by-root)
• Split roots
• Stack first and the children
• Remset first and the children

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Experiment Setup

• JikesRVM, JMTk
• Generational copying collector with bounded
  nursery size of 4MB
• PseudoAdaptive 2nd iteration

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Different Root Traversal Policies

• RxR has the best mutator locality

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Different Root Traversal Policies

• Total execution time

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Exploring The Space

• Different policies for traversing roots
• Class-oblivious traversal orders
• Which traversing order is the best?
• Class-based traversal orders
• How to find the important data structure?

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Different Traversal Orders

• Breadth first 1,2,3,4,5,6,7
• Pure depth first 1,2,6,3,4,7,5
• Pure depth first, LIFO 1,5,4,7,3,2,6

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Different Traversal Orders

• Breadth first 1,2,3,4,5,6,7
• Pure depth first 1,2,6,3,4,7,5
• Pure depth first, LIFO 1,5,4,7,3,2,6
• Partial depth first, 2 children 1,2,6,3,4,5,7

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Class Oblivious Type

• Different traversal policies
• Partial DF is the best

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Exploring The Space

• Different policies for traversing roots
• Class-oblivious traversal orders
• Which traversing order is the best?
• Class-based traversal orders
• How to find the important data structure?

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Class-based Traversal

• Class-oblivious traversal orders inflexible
• Class-based object traversal
• Static profiling
• Dynamic sampling

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Static Profiling

• Profile object accesses
• Find hot pairs with strong correlation
• Example
• (1,4), (4,7) and (2,6) have strong correlation
• Order 1,4,7,2,6,3,5

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Online Profiling

• Use the adaptive compiler sampling
• Hot method
• Hot basic block
• Use field accesses to indicate hot fields
• Example (In a hot method)
• Class A a
• a.b

A
b
..
B
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Online Profiling

• Micro benchmark results

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Online Profiling

• Geometric mean

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Reasons

• No advice for most of the objects copied
• For jess, db and raytrace, we only pick ltlt1 of
  the objects as hot objects
• 5 for javac
• The hot fields are within the first 2 pointers
• 90 of the advised objects for javac

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Online Profiling

• PseudoJBB mutator results
• Generate advice for 23 of the copied objects
• 75 of the objects have adviced hot fields other
  than first 2

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Questions

• Have we found all the hot objects?
• Not all hot objects are connected?
• Is class-base good enough?
• For pseudojbb, we need instance-based?
• Locality for the nursery objects?

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Future Work

• Sampling technique
• Catch more hot objects access
• Lower the threshold
• Hot objects that are not connected
• Dynamically change the advice for phase changing
• Nursery locality
• Different traversal orders for cold objects
• Instance-based

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Conclusion

• Reorder objects during copying collection can
  improve locality
• In class-oblivious traversal orders partial depth
  first order is the best
• Online profiling, class-based traversal is
• more flexible, up to 50 better.
• very low overhead, 0
• Still mysteries

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Questions?
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Answers?

• Lower the threshold of the sampling, not only the
  hot methods
• For objects with only 1 or 2 pointers, it maybe
  easier just depth first
• Maybe the nursery locality is more important
• Instance-based advice

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Online Profiling

• Execution overhead

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Online Profiling

• Micro benchmark results for mutator time

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Different Root Traversal Policies
_227_mtrt
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Static Profiling

• Results

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Answers?

• Most objects have only one pointer
• Percentage of objects copied by advice (whether
  it is really hot?)
• For pseudojbb 50, for jess ltlt1, for our micro
  benchmark 16
• Change! Half of the pairs do not form chains
  longer than 2
• Maybe the nursery locality is more important

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Class Oblivious Orderings

• Different traversal policies
• Partial DF is better

pseudoJBB
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Motivation

• MarkSweep vs. Copying Collector

Mutator time of _213_javac
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Motivation
Mutator L2 misses _213_javac