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Diffstat (limited to 'tools/nixery/builder/layers.go')
| -rw-r--r-- | tools/nixery/builder/layers.go | 353 |
1 files changed, 353 insertions, 0 deletions
diff --git a/tools/nixery/builder/layers.go b/tools/nixery/builder/layers.go new file mode 100644 index 000000000000..5e37e626810f --- /dev/null +++ b/tools/nixery/builder/layers.go @@ -0,0 +1,353 @@ +// Copyright 2022 The TVL Contributors +// SPDX-License-Identifier: Apache-2.0 + +// This package reads an export reference graph (i.e. a graph representing the +// runtime dependencies of a set of derivations) created by Nix and groups it 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 +// * popularity values of each package 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, a merge rating is calculated for each layer. This is the +// product of the layer's total size and its root node's popularity. +// +// Layers are then merged in ascending order of merge ratings until +// they fit into the layer budget. +// +// # 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 builder + +import ( + "crypto/sha1" + "fmt" + "regexp" + "sort" + "strings" + + log "github.com/sirupsen/logrus" + "gonum.org/v1/gonum/graph/flow" + "gonum.org/v1/gonum/graph/simple" +) + +// runtimeGraph represents structured information from Nix about the runtime +// dependencies of a derivation. +// +// This is generated in Nix by using the exportReferencesGraph feature. +type runtimeGraph 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"` +} + +// Popularity data for each Nix package that was calculated in advance. +// +// Popularity is a number from 1-100 that represents the +// popularity percentile in which this package resides inside +// of the nixpkgs tree. +type Popularity = map[string]int + +// Layer represents the data returned for each layer that Nix should +// build for the container image. +type layer struct { + Contents []string `json:"contents"` + MergeRating uint64 +} + +// Hash the contents of a layer to create a deterministic identifier that can be +// used for caching. +func (l *layer) Hash() string { + sum := sha1.Sum([]byte(strings.Join(l.Contents, ":"))) + return fmt.Sprintf("%x", sum) +} + +func (a layer) merge(b layer) layer { + a.Contents = append(a.Contents, b.Contents...) + a.MergeRating += b.MergeRating + return a +} + +// closure as pointed to by the graph nodes. +type closure struct { + GraphID int64 + Path string + Size uint64 + Refs []string + Popularity int +} + +func (c *closure) ID() int64 { + return c.GraphID +} + +var nixRegexp = regexp.MustCompile(`^/nix/store/[a-z0-9]+-`) + +// PackageFromPath returns the name of a Nix package based on its +// output store path. +func packageFromPath(path string) string { + return nixRegexp.ReplaceAllString(path, "") +} + +// DOTID provides a human-readable package name. The name stems from +// the dot format used by GraphViz, into which the dependency graph +// can be rendered. +func (c *closure) DOTID() string { + return packageFromPath(c.Path) +} + +// bigOrPopular checks whether this closure should be considered for +// separation into its own layer, even if it would otherwise only +// appear in a subtree of the dominator tree. +func (c *closure) bigOrPopular() bool { + const sizeThreshold = 100 * 1000000 // 100MB + + if c.Size > sizeThreshold { + return true + } + + // Threshold value is picked arbitrarily right now. The reason + // for this is that some packages (such as `cacert`) have very + // few direct dependencies, but are required by pretty much + // everything. + if c.Popularity >= 100 { + return true + } + + return false +} + +func insertEdges(graph *simple.DirectedGraph, cmap *map[string]*closure, node *closure) { + // Big or popular nodes get a separate edge from the top to + // flag them for their own layer. + if node.bigOrPopular() && !graph.HasEdgeFromTo(0, node.ID()) { + edge := graph.NewEdge(graph.Node(0), node) + graph.SetEdge(edge) + } + + 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 *runtimeGraph, pop *Popularity) *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, + } + + // The packages `nss-cacert` and `iana-etc` are added + // by Nixery to *every single image* and should have a + // very high popularity. + // + // Other popularity values are populated from the data + // set assembled by Nixery's popcount. + id := node.DOTID() + if strings.HasPrefix(id, "nss-cacert") || strings.HasPrefix(id, "iana-etc") { + // glibc has ~300k references, these packages need *more* + node.Popularity = 500000 + } else if p, ok := (*pop)[id]; ok { + node.Popularity = p + } else { + node.Popularity = 1 + } + + 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) + } + + return graph +} + +// Extracts a subgraph starting at the specified root from the +// dominator tree. The subgraph is converted into a flat list of +// layers, each containing the store paths and merge rating. +func groupLayer(dt *flow.DominatorTree, root *closure) layer { + size := root.Size + contents := []string{root.Path} + children := dt.DominatedBy(root.ID()) + + // This iteration does not use 'range' because the list being + // iterated is modified during the iteration (yes, I'm sorry). + for i := 0; i < len(children); i++ { + child := children[i].(*closure) + size += child.Size + contents = append(contents, child.Path) + children = append(children, dt.DominatedBy(child.ID())...) + } + + // Contents are sorted to ensure that hashing is consistent + sort.Strings(contents) + + return layer{ + Contents: contents, + MergeRating: uint64(root.Popularity) * size, + } +} + +// Calculate the dominator tree of the entire package set and group +// each top-level subtree into a layer. +// +// Layers are merged together until they fit into the layer budget, +// based on their merge rating. +func dominate(budget int, graph *simple.DirectedGraph) []layer { + dt := flow.Dominators(graph.Node(0), graph) + + var layers []layer + for _, n := range dt.DominatedBy(dt.Root().ID()) { + layers = append(layers, groupLayer(&dt, n.(*closure))) + } + + sort.Slice(layers, func(i, j int) bool { + return layers[i].MergeRating < layers[j].MergeRating + }) + + if len(layers) > budget { + log.WithFields(log.Fields{ + "layers": len(layers), + "budget": budget, + }).Info("ideal image exceeds layer budget") + } + + for len(layers) > budget { + merged := layers[0].merge(layers[1]) + layers[1] = merged + layers = layers[1:] + } + + return layers +} + +// groupLayers applies the algorithm described above the its input and returns a +// list of layers, each consisting of a list of Nix store paths that it should +// contain. +func groupLayers(refs *runtimeGraph, pop *Popularity, budget int) []layer { + graph := buildGraph(refs, pop) + return dominate(budget, graph) +} |