Image Generator (Ngen.exe)
The Ngen.exe (Native Image Generator) allows you to run the JIT compiler on your assembly's MSIL and generate native machine code which is cached to disk. When the image is created, .NET runtime will use the image to run the code rather than from the hard disk. The Running of Ngen.exe on an assembly potentially allows the assembly to load & execute faster, since it restores the code and data structures from the native image cache rather than generating them dynamically.
Following are some points to keep on mind for Native Image Generator:-
1) The Native images load faster as compared to MSIL as JIT compilation and type-safety verification is eliminated.
2) If you are sharing code between process the Ngen.exe increase the performance significantly. As the Native image generated Windows PE file so a single DLL file is shared across applications. On contrast JIT produced code are private to an assembly and can not be shared.
3) Start up time performance improves a lot. We get considerable gains when applications share component assemblies because after the first application has been started the shared components are already loaded for subsequent applications. If assemblies in the application must be loaded from hard disk, it does not benefit as much from native images as the hard disk access time shadows everything.
4)The Assemblies in the GAC do not benefit from Native image generator as the loader performs extra validation on the strong named assemblies thus shadowing the benefits of Native Image Generator.
5) If any one of the assemblies changes then Native image should also be updated.
6)You should have the administrative privilege for running the Ngen.exe.
7)While this may fasten your application start up time as the code is statically compiled but it can be somewhat slower than the code generated dynamically through the JIT compiler.Thus you need to compare how the whole application performance with Ngen.exe and how iot goes without it.
To run the Ngen.exe, we can use the following command line.
ngen.exe install
This will synchronously precompile the specified assembly and all of its dependencies. And the generated native images are stored in the native image cache.
In the .NET Framework 2.0 there is a service (.NET Runtime Optimization Service) which can precompile managed assemblies at the background. Assemblies can be schedule to be precompiled asynchronously by queueing them up with the NGEN Service. We use the following command line.
ngen.exe install /queue:
Assemblies that are critical to your application's start up time should either be precompiled synchronously or by asynchronously with 1. Priority 1 & 2 assemblies are precompiled aggressively while the Priority 3 assemblies are only precompiled during the machine idle-time. Synchronously precompiling your critical assemblies assure you that the native images will be available on prior to the first time your end user launches the application but increases the time taken to run your application's set up program.
You can uninstall an assembly and its all dependencies (if no other assemblies are dependent on them) from the native image cache by running the following command.
ngen.exe uninstall
Native images created using Ngen.exe cannot be deployed; instead they need to be created on the end user's machine. These commands therefore need to be issued as the part of the application's setup program. The .NET Visual Studio can be used to implement this behavior by defining custom actions in a Microsoft Installer (MSI) package.