The document discusses garbage collection performance tips for .NET, highlighting its complexity and the various generations involved. It emphasizes minimizing allocations and managing large objects efficiently while avoiding finalization issues. Recommendations include profiling for allocations, pooling temporary objects, and understanding the implications of different garbage collection modes.

1. Sasha Goldshtein
CTO
Sela Group
@goldshtn
blog.sashag.net
Garbage Collection
Performance Tips

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GARBAGE COLLECTION PERFORMANCE TIPS
WHY THIS TALK?
 The .NET GC is a complex beast
 Many performance optimizations over the
years
 Still many ways to shoot yourself in the foot
Mark and Sweep Generations LOH
Fragmentation Hoarding Segments
Finalization Background GC
VMMap PerfView Server GC
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GARBAGE COLLECTION PERFORMANCE TIPS
REFRESHER: TRACING GC
.NET GC is not based on
reference counting
GC starts from roots and
marks reachable
objects as “live”
The rest of the heap is
swept as garbage
Usually holes are
compacted and objects
are moved around
Allocations are very
cheap
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GARBAGE COLLECTION PERFORMANCE TIPS
PROBLEMS?
Naïve implementation blocks mutators
Result: very long application pauses
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GARBAGE COLLECTION PERFORMANCE TIPS
GC FLAVORS
CLR 2.0
Workstation GC
•Concurrent, non-
concurrent
Server GC
•Parallel GC
CLR 4.0
Background
workstation GC
CLR 4.5
Concurrent and
background
server GC
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GARBAGE COLLECTION PERFORMANCE TIPS
GENERATIONS
 Full GC is slow and very
inefficient – most
objects are live
 New objects die
quickly, old objects die
hard
 Divide the heap into
regions, enabling
partial collections
 Tricky to deal with
cross-generation
references
 Special area for large
objects (>85K)
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GARBAGE COLLECTION PERFORMANCE TIPS
MEASUREMENT
Profile apps for allocations, not just time
Look for large allocations, long-lived objs
Pool temporary large objects
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GARBAGE COLLECTION PERFORMANCE TIPS
GC AND VIRTUAL MEMORY
Windows VirtualAlloc
has 64KB granularity in
user-mode
CLR allocates consecutive
16MB-64MB segments (x86)
of memory
On x86, easy to fragment
the address space
VMMap to detect, x64/VM
hoarding to fix
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GARBAGE COLLECTION PERFORMANCE TIPS
FINALIZATION
 Finalizer (cleanup) runs automatically
after object becomes unreachable
 Performance problems and bugs:
 Allocation/finalization rate
 Race conditions, deadlocks
 The finalizer may run while a method is still
executing on an instance
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GARBAGE COLLECTION PERFORMANCE TIPS
CONCLUSIONS
Minimize the number and size of
allocations
Strive to quickly kill temporary objects
Pool temporary large objects
Don’t use finalization. Just don’t
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GARBAGE COLLECTION PERFORMANCE TIPS
ADDITIONAL REFERENCES

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GARBAGE COLLECTION PERFORMANCE TIPS
THANK YOU!
Sasha Goldshtein @goldshtn
sashag@sela.co.il blog.sashag.net
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