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JetBrains Australia 2019 - Exploring .NET’s memory management – a trip down memory lane


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The .NET Garbage Collector (GC) helps provide our applications with virtually unlimited memory, so we can focus on writing code instead of manually freeing up memory. But how does .NET manage that memory? What are hidden allocations? Can we do without allocations? Are strings evil? It still matters to understand when and where memory is allocated.

In this talk, we’ll go over the base concepts of .NET memory management and explore how .NET helps us and how we can help .NET – making our apps better. Expect profiling, Intermediate Language (IL), ClrMD and more!

Published in: Technology
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JetBrains Australia 2019 - Exploring .NET’s memory management – a trip down memory lane

  1. 1. Exploring .NET memory management A trip down memory lane Maarten Balliauw @maartenballiauw —
  2. 2. Garbage Collector
  3. 3. The goal of managed memory “Virtually unlimited memory for our applications” .NET CLR manages a big chunk of pre-allocated memory Allocations in that chunk Garbage Collector (GC) reclaims unused memory
  4. 4. .NET memory management 101 Memory allocation Objects allocated in “managed heap” (big chunk of memory) Cheap and fast, it’s just adding a pointer Memory release or “Garbage Collection” (GC) Generations Large Object Heap
  5. 5. .NET memory management 101 Memory allocation Memory release or “Garbage Collection” (GC) GC releases memory that’s no longer in use Expensive! Pause application Build a graph of objects Remove unreachable objects Compact memory Generations Large Object Heap
  6. 6. .NET memory management 101 Memory allocation Memory release or “Garbage Collection” (GC) Generations Large Object Heap Generation 0 Generation 1 Generation 2 Short-lived objects (e.g. Local variables) In-between objects Long-lived objects (e.g. App’s main form)
  7. 7. .NET memory management 101 Memory allocation Memory release or “Garbage Collection” (GC) Generations Large Object Heap (LOH) Large objects (>85KB) Collected only during full garbage collection Not compacted (by default) Fragmentation can cause OutOfMemoryException
  8. 8. The .NET garbage collector Runs often for gen0, less often for higher generations Background GC (enabled by default) Concurrent with application threads, pauses usually just for one thread When does it run? Vague… But usually: Out of memory condition – when the system fails to allocate or re-allocate memory After some significant allocation – if X memory is allocated since previous GC Profiler, forced, internal events GC is not guaranteed to run
  9. 9. Throughput More allocations, more garbage collections Garbage collection means pauses happen Pauses are bad Less powerful devices (mobile) Games (lag) Server (“resource sharing” with others) Trading …
  10. 10. – © Age of Ascent
  11. 11. Can we help the GC avoid pauses? Allocating is cheap, collecting is expensive Use struct when it makes sense, Span<T>, ValueTuple<T>, object pooling, … Make use of IDisposable / using statement & clean up manually Weak references Allow the GC to collect these objects, no need for checks Finalizers Beware! Moved to finalizer queue -> always gen++
  12. 12. Helping the GC DEMO
  13. 13. Allocations
  14. 14. Types REFERENCE TYPES Variable is a pointer to an object Passed around by reference Assignment copies the reference Allocated on heap GC involved, plenty of space VALUE TYPES Variable is the value Passed around by value (copied) Assignment copies the value Allocated on stack No GC involved, limited space int, bool, struct, decimal, enum, float, byte, long, …class, string, new
  15. 15. Hidden allocations! Boxing Lambda’s/closures Allocate compiler-generated DisplayClass to capture state Params arrays (depending on compiler version) Async/await ... int i = 42; // boxing - wraps the value type in an "object box" // (allocating a System.Object) object o = i;
  16. 16. How to find them? Intermediate Language (IL) Profiler “Heap allocations viewer” ReSharper Heap Allocations Viewer plugin Roslyn’s Heap Allocation Analyzer
  17. 17. Hidden allocations DEMO
  18. 18. Measure! Don’t do premature optimization – measure! Allocations don’t always matter (that much) Performance counters: How frequently are we allocating? How frequently are we collecting? What generation do we end up on? Are our allocations introducing pauses? Or use a profiler: (and
  19. 19. Always Be Measuring DEMO
  20. 20. [ { ... }, { "name": "Westmalle Tripel", "brewery": "Brouwerij der Trappisten van Westmalle", "votes": 17658, "rating": 4.7 }, { ... } ]
  21. 21. Object pools / object re-use Re-use objects / collections (when it makes sense) Fewer allocations, fewer objects for the GC to clean Less memory traffic (maybe no need for a full GC) Object pooling - object pool pattern System.Buffers.ArrayPool “Borrow objects that have been allocated before”
  22. 22. Garbage Collector summary Don’t fear allocations! GC is optimized for high memory traffic in short-lived objects Don’t optimize what should not be optimized… Know when allocations happen GC is awesome Gen2 collection that stop the world not so much… Measure!
  23. 23. Strings
  24. 24. Strings are objects .NET tries to make them look like a value type, but they are a reference type Read-only collection of char Length property A bunch of operator overloading Allocated on the managed heap var a = new string('-', 25); var b = a.Substring(5); var c = httpClient.GetStringAsync("");
  25. 25. String literals Are all strings on the heap? Are all strings duplicated? var a = "Hello, World!"; var b = "Hello, World!"; Console.WriteLine(a == b); Console.WriteLine(Object.ReferenceEquals(a, b)); Prints true twice. So “Hello World” only in memory once?
  26. 26. Portable Executable (PE) #UserStrings DEMO
  27. 27. String literals in #US Compile-time optimization Store literals only once in PE header metadata stream ECMA-335 standard, section II.24.2.4 Reference literals (IL: ldstr) var a = Console.ReadLine(); var b = Console.ReadLine(); Console.WriteLine(a == b); Console.WriteLine(Object.ReferenceEquals(a, b));
  28. 28. String duplicates Any .NET application has them (System.Globalization duplicates quite a few) Are they bad? .NET GC is fast for short-lived objects, so meh. Don’t waste memory with string duplicates on gen2 (but: it’s okay to have strings there)
  29. 29. String interning Store (and read) strings from the intern pool Simply call String.Intern when “allocating” or reading the string Scans intern pool and returns reference var url = ""; var stringList = new List<string>(); for (int i = 0; i < 1000000; i++) { stringList.Add(string.Intern(url + "/")); }
  30. 30. String interning caveats Why are not all strings interned by default? CPU vs. memory Not on the heap but on intern pool No GC on intern pool – all strings in memory for AppDomain lifetime! Rule of thumb Lot of long-lived, few unique -> interning good Lot of long-lived, many unique -> no benefit, memory growth Lot of short-lived -> trust the GC Measure!
  31. 31. Exploring the heap for fun and profit
  32. 32. How would you... …build a managed type system, store in memory, CPU/memory friendly Probably: Store type info (what’s in there, what’s the offset of fieldN, …) Store field data (just data) Store method pointers Inheritance information
  33. 33. Managed Heap (scroll down for more...)
  34. 34. .Managed heap is like a database Pointer to an “instance” Instance Pointer to Runtime Type Information (RTTI) Field values (which can be pointers in turn) RunTime Type Information Interface addresses Instance method addresses Static method addresses …
  35. 35. Theory is nice... Microsoft.Diagnostics.Runtime (ClrMD) “ClrMD is a set of advanced APIs for programmatically inspecting a crash dump of a .NET program much in the same way that the SOS Debugging Extensions (SOS) do. This allows you to write automated crash analysis for your applications as well as automate many common debugger tasks. In addition to reading crash dumps ClrMD also allows supports attaching to live processes.” “LINQ-to-heap” Maarten’s definition
  36. 36. ClrMD DEMO
  37. 37. But... Why? Programmatic insight into memory space of a running project Unit test critical paths and assert behavior (did we clean up what we expected?) Capture memory issues in running applications Other (easier) options in this space dotMemory Unit (JetBrains) Benchmark.NET
  38. 38. dotMemory Unit DEMO
  39. 39. Conclusion
  40. 40. Conclusion Garbage Collector (GC) optimized for high memory traffic + short-lived objects Don’t fear allocations! But beware of gen2 “stop the world” Don’t optimize what should not be optimized… Measure! Using a profiler/memory analysis tool ClrMD to automate inspections dotMemory Unit, Benchmark.NET, … to profile unit tests Blog series:
  41. 41. Thank you! Maarten Balliauw @maartenballiauw —