1. Patrick Dussud Technical Fellow Developer Division
FT36
Future of GC
Patrick Dussud
Technical Fellow
Developer Division

2. Why I Am Excited to Work on GC
Large benefit for customer workloads
Sometimes larger than code generation gains
GC contract is crisp and small
Simple principles
The implementation can greatly evolve without compromising compatibility
It’s still a relatively unexplored area, in terms of engineering

3. Contents
GC backgrounder
CLR GC history
Future Directions

4. GC Principles Automatic discovery of live data Two techniques
System can destroy and reclaim the rest
Two techniques
Tracing
Reference Count
In fact there are multiple variants in between

5. GC Factors of Merit Speed of the allocator GC time overhead (% in GC)
GC pause time
Working set for a given workload
Multi – Core scaling (all of preceding F.O.M)

6. Mechanisms and Policy
Often mechanisms are a clever way to implement a policy (see favorite GC research papers)
Better: An architecture including multiple policy-free mechanisms controlled by policy
Key to a long lived implementation

7. Virtual Memory and OS or Hosting layer
CLR GC architecture
.NET FX and program
CLR
Root scanning GC
Policies
Thread suspension
Mechanisms
Runtime Helpers
Virtual Memory and OS or Hosting layer

8. CLR GC V1 Choice of Workstation or server GC Common architecture
Both allocators are MP scalable
Most mechanisms are shared
Policies are tuned to each scenarios
Concurrent GC to reduce pause time
MP scalable GC for servers to reduce GC overhead

9. CLR GC V1 Workload Assumptions
Workstation
Allocation activity is moderate
Low pause time important
Server
Uniform workload on multiple requests
Pause time tolerable

10. CLR V3 GC Workstation Server GC Low latency mode API
Gen 2 notifications API

11. CLR V4 GC
Workstation
Background GC: Allows ephemeral GCs (Gen0-1) while engaged in a concurrent Gen2 GC
Alleviate the assumption that allocation activity is moderate
Net effect: Reduced pause time in full GC for all workloads

12. Technology Trends
More cores
More memory (Larger heaps)
Virtualization

13. GC Scorecard Speed of the allocator GC time overhead (% in GC)
Max GC pause time for large heaps
Working set for a given workload

14. GC Pause Time
Background GC needs more feedback but it seems very good so far
Same approach can be brought to Server GC
Real Time GC not likely for mainstream CLR because of its adverse effect on program execution speed (read barriers)

15. Working Set Large Object Heap isn’t compacting
Background GC isn’t compacting
Will pause executing threads on low memory situations
Considering LOH compaction
Partial compaction for Background GC
Picking a badly fragmented (small)area
Compacting it while doing a gen0-1 GC

16. How About Hardware Assist?
Done in the 80s:TI Explorer and Symbolics Ivory chips
At that time transistors were at a premium, Ivory was too slow for general computing
Now transistors are cheaper
Read Barriers would give the most bang for the buck

17. How About OS assist?
Good research on avoiding paged out object scanning
Paging is becoming less relevant
Some mechanisms canbe improved by having more integration between the Virtual Memory manager and the segment manager of GC
Useful for virtualization
Can delay paging

18. More Ideas Worth Exploring
Tracing is worthwhile when survival is low
Gen2 survival is generally high
Loosely maintained Reference Count on Gen2 objects would allow for a large percentage of dead objects reclamation without tracing the whole heap. There is research literature on the subject

19. A Final Note Thank you for providing so much feedback over the years!
Keep the feedback coming. This is what makes our GC great!

20. YOUR FEEDBACK IS IMPORTANT TO US!
Please fill out session evaluation forms online at
MicrosoftPDC.com

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