Under the Hood of .NET Memory Management - Simple Talk

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Chapter 2: The Simple Heap Model GC operation also depends on whether the application is server- or workstation-based, and on its latency mode. To avoid confusion at this stage, let's go through the general principles of generational garbage collection first, and then we will cover these more advanced topics in the next chapter. So now let's take a look at what happens when a GC is triggered from each of the generations. Gen 0 collection Gen 0 objects have never been looked at by the GC before, so they have two possible options when they are finally inspected: • move to Gen 1 (if they have a root reference in their hierarchy) • die and be compacted (if they are rootless and therefore no longer in use). Either way, the result of a Gen 0 GC is an empty Gen 0, with all rooted objects in Gen 0 being copied to (and reclassified as) Gen 1, joining the other Gen 1 objects. Remember, in a Gen 0 collection, existing Gen 1 objects stay where they are. Figure 2.5 shows the state of the SOH after a GC on the heap (as previously displayed in Figure 2.4 ). Notice how Object Y has now joined Object X in Gen 1, leaving Gen 0 empty. Object Z didn't have a root reference, and so will be effectively overwritten by the next allocation, because the NOP is simply repositioned below its heap location. After a GC, Gen 0 will always be empty. 46
Chapter 2: The Simple Heap Model Figure 2.5: SOH after a Gen 0 collection. Gen 1 collection Gen 1 collections collect both Gen 1 and Gen 0 objects. Again, to illustrate, let's see the outcome of a Gen 1 collection on the heap state in Figure 2.4. Figure 2.6: SOH after Gen 1 collection. 47
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