net garbage collection performance tips

Sasha Goldshtein

CTOSela Group

@goldshtnblog.sashag.net

Garbage Collection Performance Tips

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GARBAGE COLLECTION PERFORMANCE TIPS

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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

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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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PROBLEMS?

Naïve implementation blocks mutators Result: very long application pauses

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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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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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MEASUREMENT

Profile apps for allocations, not just time Look for large allocations, long-lived objs

Pool temporary large objects

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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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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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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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THANK YOU!

Sasha Goldshtein@goldshtn

sashag@sela.co.ilblog.sashag.net