Maoni Stephens
ABSTRACT
In this talk we will look at a few interesting optimizations done in the.NET GC. One of the.NET GC’s principles is to separate mechanisms from policies which allows for much more dynamic tuning decisions. We will also use these optimizations to illustrate this principle.
How does the.NET GC organize its objects on the heap when processing them during various GC phases? It groups adjacent live objects into what’s called plugs and uses dead objects inbetween them, called gaps, to store necessary info to facilitate relocation. Special handling is done for pinned objects. Dead spaces inbetween pinned plugs which are threaded onto a free list can get used sooner to satisfy allocation requests by a mechanism called demotion. And we can decide if we want to make them available sooner by separating mechanisms from policies.
The GC can also make a decision whether it’s more productive to do a compacting or sweeping GC after calculating the relocation information (another example of separating mechanisms from policies). If it turns out to be a sweeping GC, the allocations done in the free list of the older generation will need to be repaired. We repair this by an UNDO field on each free list item and keeping a count on the original free list item we took off for allocation, and we only need to repair that many when threading back the items whose UNDO field was touched.
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