diff options
author | Vincent Ambo <tazjin@google.com> | 2019-08-11T15·44+0100 |
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committer | Vincent Ambo <github@tazj.in> | 2019-08-13T23·02+0100 |
commit | ce31598f4264a3c8af749b3fd533a905dcd69edc (patch) | |
tree | d37c58fa44831879c9791f60690f089234c179dd /tools/nixery | |
parent | 3e385dc379de4a1e8dd619a7a47714925e99490a (diff) |
feat(group-layers): Implement first half of new layering strategy
The strategy is described in-depth in the comment at the top of the implementation file, as well as in the design document: https://storage.googleapis.com/nixdoc/nixery-layers.html
Diffstat (limited to 'tools/nixery')
-rw-r--r-- | tools/nixery/group-layers/group-layers.go | 267 |
1 files changed, 267 insertions, 0 deletions
diff --git a/tools/nixery/group-layers/group-layers.go b/tools/nixery/group-layers/group-layers.go new file mode 100644 index 000000000000..1ee67433deeb --- /dev/null +++ b/tools/nixery/group-layers/group-layers.go @@ -0,0 +1,267 @@ +// This program reads an export reference graph (i.e. a graph representing the +// runtime dependencies of a set of derivations) created by Nix and groups them +// in a way that is likely to match the grouping for other derivation sets with +// overlapping dependencies. +// +// This is used to determine which derivations to include in which layers of a +// container image. +// +// # Inputs +// +// * a graph of Nix runtime dependencies, generated via exportReferenceGraph +// * a file containing absolute popularity values of packages in the +// Nix package set (in the form of a direct reference count) +// * a maximum number of layers to allocate for the image (the "layer budget") +// +// # Algorithm +// +// It works by first creating a (directed) dependency tree: +// +// img (root node) +// │ +// ├───> A ─────┐ +// │ v +// ├───> B ───> E +// │ ^ +// ├───> C ─────┘ +// │ │ +// │ v +// └───> D ───> F +// │ +// └────> G +// +// Each node (i.e. package) is then visited to determine how important +// it is to separate this node into its own layer, specifically: +// +// 1. Is the node within a certain threshold percentile of absolute +// popularity within all of nixpkgs? (e.g. `glibc`, `openssl`) +// +// 2. Is the node's runtime closure above a threshold size? (e.g. 100MB) +// +// In either case, a bit is flipped for this node representing each +// condition and an edge to it is inserted directly from the image +// root, if it does not already exist. +// +// For the rest of the example we assume 'G' is above the threshold +// size and 'E' is popular. +// +// This tree is then transformed into a dominator tree: +// +// img +// │ +// ├───> A +// ├───> B +// ├───> C +// ├───> E +// ├───> D ───> F +// └───> G +// +// Specifically this means that the paths to A, B, C, E, G, and D +// always pass through the root (i.e. are dominated by it), whilst F +// is dominated by D (all paths go through it). +// +// The top-level subtrees are considered as the initially selected +// layers. +// +// If the list of layers fits within the layer budget, it is returned. +// +// Otherwise layers are merged together in this order: +// +// * layers whose root meets neither condition above +// * layers whose root is popular +// * layers whose root is big +// * layers whose root meets both conditions +// +// # Threshold values +// +// Threshold values for the partitioning conditions mentioned above +// have not yet been determined, but we will make a good first guess +// based on gut feeling and proceed to measure their impact on cache +// hits/misses. +// +// # Example +// +// Using the logic described above as well as the example presented in +// the introduction, this program would create the following layer +// groupings (assuming no additional partitioning): +// +// Layer budget: 1 +// Layers: { A, B, C, D, E, F, G } +// +// Layer budget: 2 +// Layers: { G }, { A, B, C, D, E, F } +// +// Layer budget: 3 +// Layers: { G }, { E }, { A, B, C, D, F } +// +// Layer budget: 4 +// Layers: { G }, { E }, { D, F }, { A, B, C } +// +// ... +// +// Layer budget: 10 +// Layers: { E }, { D, F }, { A }, { B }, { C } +package main + +import ( + "encoding/json" + "flag" + "io/ioutil" + "log" + "fmt" + "regexp" + "os" + + "gonum.org/v1/gonum/graph/simple" + "gonum.org/v1/gonum/graph/flow" + "gonum.org/v1/gonum/graph/encoding/dot" +) + +// closureGraph represents the structured attributes Nix outputs when asking it +// for the exportReferencesGraph of a list of derivations. +type exportReferences struct { + References struct { + Graph []string `json:"graph"` + } `json:"exportReferencesGraph"` + + Graph []struct { + Size uint64 `json:"closureSize` + Path string `json:"path"` + Refs []string `json:"references"` + } `json:"graph"` +} + +// closure as pointed to by the graph nodes. +type closure struct { + GraphID int64 + Path string + Size uint64 + Refs []string + // TODO(tazjin): popularity and other funny business +} + +func (c *closure) ID() int64 { + return c.GraphID +} + +var nixRegexp = regexp.MustCompile(`^/nix/store/[a-z0-9]+-`) +func (c *closure) DOTID() string { + return nixRegexp.ReplaceAllString(c.Path, "") +} + +func insertEdges(graph *simple.DirectedGraph, cmap *map[string]*closure, node *closure) { + for _, c := range node.Refs { + // Nix adds a self reference to each node, which + // should not be inserted. + if c != node.Path { + edge := graph.NewEdge(node, (*cmap)[c]) + graph.SetEdge(edge) + } + } +} + +// Create a graph structure from the references supplied by Nix. +func buildGraph(refs *exportReferences) *simple.DirectedGraph { + cmap := make(map[string]*closure) + graph := simple.NewDirectedGraph() + + // Insert all closures into the graph, as well as a fake root + // closure which serves as the top of the tree. + // + // A map from store paths to IDs is kept to actually insert + // edges below. + root := &closure { + GraphID: 0, + Path: "image_root", + } + graph.AddNode(root) + + for idx, c := range refs.Graph { + node := &closure { + GraphID: int64(idx + 1), // inc because of root node + Path: c.Path, + Size: c.Size, + Refs: c.Refs, + } + + graph.AddNode(node) + cmap[c.Path] = node + } + + // Insert the top-level closures with edges from the root + // node, then insert all edges for each closure. + for _, p := range refs.References.Graph { + edge := graph.NewEdge(root, cmap[p]) + graph.SetEdge(edge) + } + + for _, c := range cmap { + insertEdges(graph, &cmap, c) + } + + gv, err := dot.Marshal(graph, "deps", "", "") + if err != nil { + log.Fatalf("Could not encode graph: %s\n", err) + } + fmt.Print(string(gv)) + os.Exit(0) + + return graph +} + +// Calculate the dominator tree of the entire package set and group +// each top-level subtree into a layer. +func dominate(graph *simple.DirectedGraph) { + dt := flow.Dominators(graph.Node(0), graph) + + // convert dominator tree back into encodable graph + dg := simple.NewDirectedGraph() + + for nodes := graph.Nodes(); nodes.Next(); { + dg.AddNode(nodes.Node()) + } + + for nodes := dg.Nodes(); nodes.Next(); { + node := nodes.Node() + for _, child := range dt.DominatedBy(node.ID()) { + edge := dg.NewEdge(node, child) + dg.SetEdge(edge) + } + } + + gv, err := dot.Marshal(dg, "deps", "", "") + if err != nil { + log.Fatalf("Could not encode graph: %s\n", err) + } + fmt.Print(string(gv)) + + // fmt.Printf("%v edges in the graph\n", graph.Edges().Len()) + // top := 0 + // for _, n := range dt.DominatedBy(0) { + // fmt.Printf("%q is top-level\n", n.(*closure).Path) + // top++ + // } + // fmt.Printf("%v total top-level nodes\n", top) + // root := dt.Root().(*closure) + // fmt.Printf("dominator tree root is %q\n", root.Path) + // fmt.Printf("%v nodes can reach to 1\n", nodes.Len()) +} + +func main() { + inputFile := flag.String("input", ".attrs.json", "Input file containing graph") + flag.Parse() + + file, err := ioutil.ReadFile(*inputFile) + if err != nil { + log.Fatalf("Failed to load input: %s\n", err) + } + + var refs exportReferences + err = json.Unmarshal(file, &refs) + if err != nil { + log.Fatalf("Failed to deserialise input: %s\n", err) + } + + graph := buildGraph(&refs) + dominate(graph) +} |