ASP.NET Performance: Dynamically Loaded Assemblies Cause Application Recycles (Problem and Solution)


 Alik Levin    In my speak - dynamically loaded assemblies are those assemblies that were compiled during run time dynamically via CodeProvider like CSharpCodeProvider directly or by using types that use this class internally. Assemblies that are loaded dynamically using reflection via Load/LoadFrom method are out of the scope of this post.

Customer Case Study

The customer complained on periodic restarts/recycles of the ASP.NET application. We observed relative entries in the Event Log that showed up systematically on timely basis. We also observed memory utilization growth in Task Manager. After reaching 500 MB of memory utilization the application would recycle spawning another w3wp.exe to accept new incoming requests while draining the old w3wp.exe.

Analysis

After short discussion with the dev team about the design of the application we thought that the memory leak might be caused by improper usage of XmlSerializer that generate dynamic assemblies. Tess published fantastic walk through specifically dedicated to this case. Using either perfmon (.NET CLR Loading\Current Assemblies) or Process Explorer (see pic below)we observed unusual number (thousands) of loaded assemblies (notice Assemblies column). Also, notice the csc.exe in red - this is CSharp compiler that is invoked on each request:

Process Explorer ASP.NET Performance Dynamically Loaded Assemblies

We decided to take a memory dump to deeply investigate the case. Following are the steps that we took while analyzing the dump using WinDBG to identify the root cause:

Step 1 - Dumping memory heap to identify object allocated on heap

This is the fragment of the long list of objects. Our attention was caught by unusually large number of reflected assemblies.

!dumpheap –stat
9,019       216,456 System.Reflection.Assembly
112         4,032 System.Xml.Serialization.TempAssembly
104         5,408 System.Xml.Serialization.TypeDesc

After 3 minutes the number of dynamic assemblies is larger by more 350 assemblies (from subsequent dump):

!dumpheap –stat
9,379       225,096 System.Reflection.Assembly
114         4,104 System.Xml.Serialization.TempAssembly
102         5,304 System.Xml.Serialization.TypeDesc

Step 2 - Dumping appdomains to identify loaded assemblies

Another cross check to make sure we are dealing with tons of loaded assemblies.

!dumpdomain -stat 
    Domain              Num Assemblies   Size Assemblies    Name
0x793f15d8                       1                       2,142,208        System Domain
0x793f2aa8                       56                    16,012,288      Shared Domain
0x000ab7d8                      2                       2,498,560        DefaultDomain
0x000d3368                  9,018                  55,447,040      /LM/W3SV......

Total 4 Domains, Total Size 76,100,096

Step 3 - Dumping all dynamic assemblies

How many of the assemblies are dynamic? (dda stands for dumpdynamicassemblies)

!dda

Domain: 0x793f15d8
-------------------
Domain: .
-------------------
Domain: DefaultDomain
-------------------
Domain: /LM/W3SVC/1/ROOT/......
-------------------
Assembly: 0x19058818 [RegexAssembly133_0] Dynamic Module: 0x16f4c220 loaded at: 0x0 Size: 0x0((null))
Assembly: 0x19058818 [RegexAssembly133_0] Dynamic Module: 0x190696a0 loaded at: 0x0 Size: 0x0((null))
Assembly: 0x19103ee8 [-0g5u8-v] Dynamic Module: 0x1920d6f8 loaded at: 0x19911000 Size: 0xc000((null))
Assembly: 0x190c9a40 [cvmmynwf] Dynamic Module: 0x190dc0d0 loaded at: 0x19a71000 Size: 0x4000((null))
Assembly: 0x1911bad8 [0ikhy_lx] Dynamic Module: 0x1911aa98 loaded at: 0x19f21000 Size: 0xc00((null))
.......
Assembly: 0x43199720 [nv1lvdiy] Dynamic Module: 0x431b3190 loaded at: 0x4cf61000 Size: 0xc00((null))
Assembly: 0x2d2bf008 [rk6dabem] Dynamic Module: 0x2d2bf258 loaded at: 0x4cf71000 Size: 0xc00((null))
--------------------------------------

Total 8,911 Dynamic Assemblies, Total size: 0x1d5b600(30,782,976) bytes.

Step 4 - Saving dynamic assembly to physical DLL

Save assemblies to the filesystem

!savemodule 0x2d2bc4c8 C: \0x1c344438.dll

clip_image002

To save all the assemblies to the file system use the following command:

!dda -save C:\MODULES

Step 5 - Using Reflector to reverse engineer the DLL:

Use Reflector to inspect the implementation/source code of the dynamic assemblies.

clip_image001

Step 6 - Using Reflector to find  ExpressionEvaluator class

Try to locate the class in the static assemblies hopefully hitting the code that generates it:

image

Step 7 - Bingo! Each constructor for ExpressionEvaluator invokes compiler

ICodeCompiler compiler = new CSharpCodeProvider().CreateCompiler();
...
CompilerResults results = compiler.CompileAssemblyFromSource(options, builder.ToString());
...
this._Compiled = results.CompiledAssembly.CreateInstance("NavServices._ExpressionEvaluator");

This is actually the code that causes Process Explorer to show csc.exe under w3wp.exe (see red line in the first pic). And this is the code that caused number of loaded assemblies to grow. And this is the code that caused the application restarts.

Acknowledgements

During this investigation the following resources were used. Big THANK-YOU goes to:

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