54 lines
1.7 KiB
Go
54 lines
1.7 KiB
Go
// Package gc experiments with providing central gc tooling to ensure
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// deterministic resource removal within containerd.
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//
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// For now, we just have a single exported implementation that can be used
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// under certain use cases.
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package gc
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// Tricolor implements basic, single-thread tri-color GC. Given the roots, the
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// complete set and a refs function, this returns the unreachable objects.
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//
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// Correct usage requires that the caller not allow the arguments to change
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// until the result is used to delete objects in the system.
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//
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// It will allocate memory proportional to the size of the reachable set.
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//
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// We can probably use this to inform a design for incremental GC by injecting
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// callbacks to the set modification algorithms.
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func Tricolor(roots []string, all []string, refs func(ref string) []string) []string {
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var (
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grays []string // maintain a gray "stack"
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seen = map[string]struct{}{} // or not "white", basically "seen"
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reachable = map[string]struct{}{} // or "block", in tri-color parlance
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)
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grays = append(grays, roots...)
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for len(grays) > 0 {
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// Pick any gray object
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id := grays[len(grays)-1] // effectively "depth first" because first element
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grays = grays[:len(grays)-1]
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seen[id] = struct{}{} // post-mark this as not-white
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// mark all the referenced objects as gray
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for _, target := range refs(id) {
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if _, ok := seen[target]; !ok {
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grays = append(grays, target)
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}
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}
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// mark as black when done
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reachable[id] = struct{}{}
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}
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// All black objects are now reachable, and all white objects are
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// unreachable. Free those that are white!
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var whites []string
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for _, obj := range all {
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if _, ok := reachable[obj]; !ok {
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whites = append(whites, obj)
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}
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}
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return whites
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}
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