Fast Conservative GC
Zaz Brown
Exact vs Conservative
recognizes vs guesses pointers
Exact GC Disadvantages
difficult to implement
prevents code motion
Conservative GC Disadvantages
guesses pointers
- int could be ptr: excess retention 0.01%
- ptr could be int: pin referent 0.03%
was slow
was slow
now fast
Immix Advantage
MCC pins 4 KiB pages, BDW uses free-list
Immix pins 256B lines, uses bump pointer
improves fragmentation & mutator locality
RC Immix
Immix + live object count
combines tracing & counting
RC Immixcons filtering
bitmap to determine valid objects
- collector clears bit for dead object
- ambiguous references are checked against bitmap
10× reduction in pinned objects vs MCC
Performance Robustness
Java
Jikes RVM & MMTk
simulated:
- poor code quality by disabling optimizations
- immature VM by deoptimising runtime code
Time vs Heap Size
Increased Pinning
Why not use conservative GC?
Questions?
References
Fast Conservative Garbage Collection, 2014
High Performance Reference Counting and Conservative Garbage Collection, 2015 - PhD thesis