Under the Hood of .NET Memory Management - Simple Talk

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Chapter 6: A Few More Advanced Topics In most cases, you will probably want to use Server GC whenever you are running server processes on a server OS but, on the high end of scaling, this may not always be the case. Essentially, while Server GC is faster than Workstation GC on the same size heap, there can be problems if you're running hundreds of instances of an app because of excessive context switching between threads Added to this mix, we have Concurrent GC, which splits the garbage collection process in two and handles the two cases differently. Concurrent GC is available only under Workstation mode. Generation 0 and 1 collections are handled normally, since they will typically finish quickly anyway, but Generation 2 GCs are handled a little differently. This is where the concurrent part comes into play because, to a certain extent, your program can continue running while the Generation 2 garbage is collected. Without Concurrent GC, all threads in your application are blocked until the garbage collection is complete. The concurrent part comes into play by allowing your code to continue executing during the Generation 2 GC, albeit within certain strict limitations. Your program can continue running until the active thread's ephemeral segment is full (the ephemeral segment is the segment that was most recently allocated by the GC). When you run out of room on this segment, you will be back to being completely blocked until garbage collection is finished. While the total time to run garbage collection will be longer in Concurrent mode, the amount of lag time or halted processing will be reduced, from a user's perspective. In fact, for a large portion of the time, you may not be blocked at all. The default is to enable Concurrent GC whenever you are running under Workstation mode. However, if you explicitly switch from Server mode to Workstation mode as mentioned earlier (in a shared hosting environment), you should also explicitly disable Concurrent GC. With that in mind, you can easily disable/enable Concurrent GC in the config file, as in Listing 6.1. 184
Chapter 6: A Few More Advanced Topics Listing 6.1: Configuration settings to enable/disable Concurrent GC. Concurrent GC is intended to minimize screen freezes during garbage collection and is best suited to application scenarios where a responsive UI is the most critical design goal. Unless you are running a server process on a server OS, you will want to keep Concurrent GC enabled. .NET 4.0 introduced a new flavor to the table: Background GC. Technically this isn't really a new option, as it replaces Concurrent GC, although there have been improvements. In particular, Background GC works just like Concurrent GC except that your application isn't blocked when the ephemeral segment is full. Instead we can run a foreground Generation 0 and Generation 1 collection against the ephemeral segment. While this happens, instead of blocking the foreground threads because we are garbage collecting, the background thread running the garbage collection is suspended until the foreground is finished. Because this is strictly a Generation 0 and Generation 1 collection against a single segment, this collection will be very fast. This further reduces the latency on the foreground threads. There are no new configuration settings for Background GC, so when you target the 4.0 version of the framework and enable Concurrent GC, you will get Background GC. 185
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