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Exploring .NET memory
management
A trip down memory lane
Maarten Balliauw
@maartenballiauw
Who am I?
Maarten Balliauw
Antwerp, Belgium
Developer Advocate, JetBrains
Founder, MyGet
AZUG
Focus on web
ASP.NET MVC, Az...
Agenda
Garbage Collector
Helping the GC
Allocations & hidden allocations
Strings
Exploring the heap
Garbage Collector
.NET runtime
Manages execution of programs
Just-in-time compilation: Intermediate Language (IL) ->machine code
Type safety...
Memory management and GC
Virtually unlimited memory for our applications
Big chunk of memory pre-allocated
Runtime manages...
.NET memory management 101
Memory allocation
“Managed heap” - region of address space for every new process
Objects alloca...
.NET memory management 101
Memory allocation
Memory release or “Garbage Collection” (GC)
GC releases objects no longer in ...
.NET memory management 101
Memory allocation
Memory release or “Garbage Collection” (GC)
Generations – be fast and efficie...
.NET memory management 101
Memory allocation
Memory release or “Garbage Collection” (GC)
Generations
Large Object Heap
Gen...
.NET memory management 101
Memory allocation
Memory release or “Garbage Collection” (GC)
Generations
Large Object Heap (LO...
The .NET garbage collector
When does it run? Vague… But usually:
Out of memory condition – when the system fails to alloca...
The .NET garbage collector
Runs very often for gen0
Short-lived objects, few references, fast to clean
Local variable
Web ...
Helping the GC, avoid pauses
Optimize allocations (but don’t do premature optimization – measure!)
Don’t allocate at all
M...
Helping the GC
DEMO
Allocations
When is memory allocated?
Not for value types (int, bool, struct, decimal, enum, float, byte, long, …)
Allocated on stack,...
Hidden allocations!
Boxing!
Put and int in a box
Take an int out of a box
Lambda’s/closures
Allocate compiler-generated
Di...
How to find them?
Experience
Intermediate Language (IL)
Profiler
“Heap allocations viewer”
ReSharper Heap Allocations View...
Hidden allocations
DEMO
ReSharper Heap Allocations Viewer plugin
Roslyn’s Heap Allocation Analyzer
Don’t optimize what
should not be
optimized.
Measure!
We know when allocations are done...
...but perhaps these don’t matter.
Measure!
How frequently are we allocating...
Always Be Measuring
DEMO
Object pools / object re-use
If it make sense, re-use objects
Fewer allocations, fewer objects for the GC to scan
Fewer me...
Garbage Collector & Allocations
GC is optimized for high memory traffic in short-lived objects
Use that knowledge! Don’t f...
Strings
Strings are objects
.NET tries to make them look like a value type, but they are a reference type
Read-only collection of ...
Measuring string
allocations
DEMO
String duplicates
Any .NET application has them (System.Globalization duplicates quite a few)
Are they bad?
.NET GC is fas...
String literals
Are all strings on the heap? Are all strings duplicated?
var a = "Hello, World!";
var b = "Hello, World!";...
Portable Executable (PE)
#UserStrings
DEMO
String literals in #US
Compile-time optimization
Store literals only once in PE header metadata stream ECMA-335 standard, ...
String interning
Store (and read) strings from the intern pool
Simply call String.Intern when “allocating” or reading the ...
String interning caveats
Why are not all strings interned by default?
CPU vs. memory
Not on the heap but on intern pool
No...
Exploring the heap
for fun and profit
How would you do it...
Build a managed type system, store in memory, CPU/memory friendly
Probably:
Store type info (what’s...
Stuff on the Stack
Stuff on the Managed Heap
(scroll down for more...)
IT is just mapping mappings.
Pointer to an “instance”
Instance
Pointer to Runtime Type Information (RTTI)
Field values (wh...
Theory is nice...
Microsoft.Diagnostics.Runtime (ClrMD)
“ClrMD is a set of advanced APIs for programmatically inspecting a...
ClrMD introduction
DEMO
String duplicates
DEMO
“Path to root” (why is
my object held in
memory)
DEMO
Conclusion
Conclusion
Garbage Collector (GC) optimized for high memory traffic + short-lived objects
Don’t fear allocations! But bewa...
Thank you!
Need training, coaching, mentoring, performance analysis?
Hire me! https://blog.maartenballiauw.be/hire-me.html...
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Exploring .NET memory management - A trip down memory lane - Copenhagen .NET User Group

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The .NET Garbage Collector (GC) is really cool. It helps providing 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? 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!

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Exploring .NET memory management - A trip down memory lane - Copenhagen .NET User Group

  1. 1. Exploring .NET memory management A trip down memory lane Maarten Balliauw @maartenballiauw
  2. 2. Who am I? Maarten Balliauw Antwerp, Belgium Developer Advocate, JetBrains Founder, MyGet AZUG Focus on web ASP.NET MVC, Azure, SignalR, ... Former MVP Azure & ASPInsider Big passion: Azure http://blog.maartenballiauw.be @maartenballiauw
  3. 3. Agenda Garbage Collector Helping the GC Allocations & hidden allocations Strings Exploring the heap
  4. 4. Garbage Collector
  5. 5. .NET runtime Manages execution of programs Just-in-time compilation: Intermediate Language (IL) ->machine code Type safety Exception handling Security Thread management Memory management Garbage collection (GC)
  6. 6. Memory management and GC Virtually unlimited memory for our applications Big chunk of memory pre-allocated Runtime manages allocation in that chunk Garbage Collector (GC) reclaims unused memory, making it available again
  7. 7. .NET memory management 101 Memory allocation “Managed heap” - region of address space for every new process Objects allocated in the heap Allocating memory is fast, it’s just adding a pointer Some unmanaged memory is also consumed (not GC-ed) .NET CLR, Dynamic libraries, Graphics buffer, … Memory release or “Garbage Collection” (GC) Generations Large Object Heap
  8. 8. .NET memory management 101 Memory allocation Memory release or “Garbage Collection” (GC) GC releases objects no longer in use by examining application roots GC builds a graph of all the objects that are reachable from these roots Object unreachable? Remove object, release memory, compact heap Takes time to scan all objects! Generations Large Object Heap
  9. 9. .NET memory management 101 Memory allocation Memory release or “Garbage Collection” (GC) Generations – be fast and efficient! Managed heap divided in segments: generation 0, 1 and 2 Gen0 – run GC more often, object in use -> move to gen1 Gen 1 – run GC less often, object in use -> move to gen2 Gen 2 – run GC even less often, object in use stays in gen2 Reduce time needed to perform GC Large Object Heap
  10. 10. .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)
  11. 11. .NET memory management 101 Memory allocation Memory release or “Garbage Collection” (GC) Generations Large Object Heap (LOH) Special segment for large objects (>85KB) Collected only during full garbage collection Not compacted (by default) Fragmentation can cause OutOfMemoryException
  12. 12. The .NET garbage collector 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 Failure of allocating some native resources – internal to .NET Profiler – when triggered from profiler API Forced – when calling methods on System.GC Application moves to background GC is not guaranteed to run http://blogs.msdn.com/b/oldnewthing/archive/2010/08/09/10047586.aspx http://blogs.msdn.com/b/abhinaba/archive/2008/04/29/when-does-the-net-compact-framework-garbage-collector-run.aspx
  13. 13. The .NET garbage collector Runs very often for gen0 Short-lived objects, few references, fast to clean Local variable Web request/response Higher generation Usually more references, slower to clean GC pauses the running application to do its thing Usually short, except when not… Background GC (enabled by default) Concurrent with application threads May still introduce short locks/pauses, usually just for one thread
  14. 14. Helping the GC, avoid pauses Optimize allocations (but don’t do premature optimization – measure!) Don’t allocate at all Make use of IDisposable / using statement Clean up references, giving the GC an easy job Finalizers Beware! Moved to finalizer queue -> always gen++ Weak references Allow the GC to collect these objects, no need for checks
  15. 15. Helping the GC DEMO
  16. 16. Allocations
  17. 17. When is memory allocated? Not for value types (int, bool, struct, decimal, enum, float, byte, long, …) Allocated on stack, not on heap Not managed by garbage collector For reference types When you new When you ""
  18. 18. Hidden allocations! Boxing! Put and int in a box Take an int out of a box Lambda’s/closures Allocate compiler-generated DisplayClass to capture state Params arrays And more! int i = 42; // boxing - wraps the value type in an "object box" // (allocating a System.Object) object o = i; // unboxing - unpacking the "object box" into an int again // (CPU effort to unwrap) int j = (int)o;
  19. 19. How to find them? Experience Intermediate Language (IL) Profiler “Heap allocations viewer” ReSharper Heap Allocations Viewer plugin Roslyn’s Heap Allocation Analyzer Don’t do premature optimization – measure!
  20. 20. Hidden allocations DEMO ReSharper Heap Allocations Viewer plugin Roslyn’s Heap Allocation Analyzer
  21. 21. Don’t optimize what should not be optimized.
  22. 22. Measure! We know when allocations are done... ...but perhaps these don’t matter. Measure! How frequently are we allocating? How frequently are we collecting? What generation do we end up on? Are our allocations introducing pauses? www.jetbrains.com/dotmemory (and www.jetbrains.com/dottrace)
  23. 23. Always Be Measuring DEMO
  24. 24. Object pools / object re-use If it make sense, re-use objects Fewer allocations, fewer objects for the GC to scan Fewer memory traffic that can trigger a full GC Object pooling - object pool pattern Create a pool of objects that can be cleaned and re-used https://www.codeproject.com/articles/20848/c-object-pooling “Optimize ASP.NET Core” - https://github.com/aspnet/AspLabs/issues/3
  25. 25. Garbage Collector & Allocations GC is optimized for high memory traffic in short-lived objects Use that knowledge! Don’t fear allocations! Don’t optimize what should not be optimized… GC is the concept that makes .NET / C# tick – use it! Know when allocations happen GC is awesome Gen2 collection that stop the world not so much… Measure!
  26. 26. Strings
  27. 27. 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 = "Hello, World!"; var c = httpClient.GetStringAsync("http://blog.maartenballiauw.be");
  28. 28. Measuring string allocations DEMO
  29. 29. 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 (it’s okay to have strings there)
  30. 30. 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?
  31. 31. Portable Executable (PE) #UserStrings DEMO
  32. 32. 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)); String interning to the rescue!
  33. 33. 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 = string.Intern("http://blog.maartenballiauw.be"); var stringList = new List<string>(); for (int i = 0; i < 1000000; i++) { stringList.Add(url); }
  34. 34. 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!
  35. 35. Exploring the heap for fun and profit
  36. 36. How would you do it... 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 (“who you gonna call?”) Inheritance information
  37. 37. Stuff on the Stack
  38. 38. Stuff on the Managed Heap (scroll down for more...)
  39. 39. IT is just mapping mappings. Pointer to an “instance” Instance Pointer to Runtime Type Information (RTTI) Field values (which can be pointers in turn) RTTI Interface addresses Instance method addresses Static method addresses …
  40. 40. 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.” Maarten’s definition: “LINQ-to-heap”
  41. 41. ClrMD introduction DEMO
  42. 42. String duplicates DEMO
  43. 43. “Path to root” (why is my object held in memory) DEMO
  44. 44. Conclusion
  45. 45. Conclusion Garbage Collector (GC) optimized for high memory traffic + short-lived objects Don’t fear allocations! But beware of gen2 “stop the world” String interning when lot of long-lived, few unique Don’t optimize what should not be optimized… Measure! Using a profiler/memory analysis tool ClrMD to automate inspections
  46. 46. Thank you! Need training, coaching, mentoring, performance analysis? Hire me! https://blog.maartenballiauw.be/hire-me.html http://blog.maartenballiauw.be @maartenballiauw

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