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chore(build-image): Simplify wrapper build & remove layer grouping

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Simplifies the wrapper script used to invoke Nix builds from Nixery to
just contain the essentials, since the layer grouping logic is moving
into the server itself.
This commit is contained in:
Vincent Ambo 2019-09-29 22:58:52 +01:00 committed by Vincent Ambo
parent 712b38cbbc
commit 0898d8a961
5 changed files with 19 additions and 450 deletions
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4
tools/nixery/build-image/build-image.nix
View file

@ -26,6 +26,8 @@
srcType ? "nixpkgs",
srcArgs ? "nixos-19.03",
importArgs ? { },
# Path to load-pkgs.nix
loadPkgs ? ./load-pkgs.nix,
# Packages to install by name (which must refer to top-level attributes of
# nixpkgs). This is passed in as a JSON-array in string form.
packages ? "[]"
@ -43,7 +45,7 @@ let
inherit (pkgs) lib runCommand writeText;
pkgs = import ./load-pkgs.nix { inherit srcType srcArgs importArgs; };
pkgs = import loadPkgs { inherit srcType srcArgs importArgs; };
# deepFetch traverses the top-level Nix package set to retrieve an item via a
# path specified in string form.

92
tools/nixery/build-image/default.nix
View file

@ -12,84 +12,18 @@
# See the License for the specific language governing permissions and
# limitations under the License.
# This file builds the tool used to calculate layer distribution and
# moves the files needed to call the Nix builds at runtime in the
# correct locations.
# This file builds a wrapper script called by Nixery to ask for the
# content information for a given image.
#
# The purpose of using a wrapper script is to ensure that the paths to
# all required Nix files are set correctly at runtime.
{ pkgs ? null, self ? ./.
{ pkgs ? import <nixpkgs> {} }:
# Because of the insanity occuring below, this function must mirror
# all arguments of build-image.nix.
, srcType ? "nixpkgs"
, srcArgs ? "nixos-19.03"
, tag ? null, name ? null, packages ? null, maxLayers ? null, popularityUrl ? null
}@args:
let pkgs = import ./load-pkgs.nix { inherit srcType srcArgs; };
in with pkgs; rec {
groupLayers = buildGoPackage {
name = "group-layers";
goDeps = ./go-deps.nix;
goPackagePath = "github.com/google/nixery/group-layers";
# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
# WARNING: HERE BE DRAGONS! #
# #
# The hack below is used to break evaluation purity. The issue is #
# that Nixery's build instructions (the default.nix in the folder #
# above this one) must build a program that can invoke Nix at #
# runtime, with a derivation that needs a program tracked in this #
# source tree (`group-layers`). #
# #
# Simply installing that program in the $PATH of Nixery does not #
# work, because the runtime Nix builds use their own isolated #
# environment. #
# #
# I first attempted to naively copy the sources into the Nix #
# store, so that Nixery could build `group-layers` when it starts #
# up - however those sources are not available to a nested Nix #
# build because they're not part of the context of the nested #
# invocation. #
# #
# Nix has several primitives under `builtins.` that can break #
# evaluation purity, these (namely readDir and readFile) are used #
# below to break out of the isolated environment and reconstruct #
# the source tree for `group-layers`. #
# #
# There might be a better way to do this, but I don't know what #
# it is. #
# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
src = runCommand "group-layers-srcs" { } ''
mkdir -p $out
${with builtins;
let
files =
(attrNames (lib.filterAttrs (_: t: t != "symlink") (readDir self)));
commands =
map (f: "cp ${toFile f (readFile "${self}/${f}")} $out/${f}") files;
in lib.concatStringsSep "\n" commands}
'';
meta = {
description =
"Tool to group a set of packages into container image layers";
license = lib.licenses.asl20;
maintainers = [ lib.maintainers.tazjin ];
};
};
buildImage = import ./build-image.nix
({ inherit pkgs groupLayers; } // (lib.filterAttrs (_: v: v != null) args));
# Wrapper script which is called by the Nixery server to trigger an
# actual image build. This exists to avoid having to specify the
# location of build-image.nix at runtime.
wrapper = writeShellScriptBin "nixery-build-image" ''
exec ${nix}/bin/nix-build \
--show-trace \
--no-out-link "$@" \
--argstr self "${./.}" \
-A buildImage ${./.}
'';
}
pkgs.writeShellScriptBin "nixery-build-image" ''
exec ${pkgs.nix}/bin/nix-build \
--show-trace \
--no-out-link "$@" \
--argstr loadPkgs ${./load-pkgs.nix} \
${./build-image.nix}
''

10
tools/nixery/build-image/go-deps.nix
View file

@ -1,10 +0,0 @@
# This file was generated by https://github.com/kamilchm/go2nix v1.3.0
[{
goPackagePath = "gonum.org/v1/gonum";
fetch = {
type = "git";
url = "https://github.com/gonum/gonum";
rev = "ced62fe5104b907b6c16cb7e575c17b2e62ceddd";
sha256 = "1b7q6haabnp53igpmvr6a2414yralhbrldixx4kbxxg1apy8jdjg";
};
}]

352
tools/nixery/build-image/group-layers.go
View file

@ -1,352 +0,0 @@
// 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, 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 main
import (
"encoding/json"
"flag"
"io/ioutil"
"log"
"regexp"
"sort"
"gonum.org/v1/gonum/graph/flow"
"gonum.org/v1/gonum/graph/simple"
)
// 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"`
}
// 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 pkgsMetadata = 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
}
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]+-`)
func (c *closure) DOTID() string {
return nixRegexp.ReplaceAllString(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
}
// The threshold value used here is currently roughly the
// minimum number of references that only 1% of packages in
// the entire package set have.
//
// TODO(tazjin): Do this more elegantly by calculating
// percentiles for each package and using those instead.
if c.Popularity >= 1000 {
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 *exportReferences, pop *pkgsMetadata) *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,
}
if p, ok := (*pop)[node.DOTID()]; 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())...)
}
return layer{
Contents: contents,
// TODO(tazjin): The point of this is to factor in
// both the size and the popularity when making merge
// decisions, but there might be a smarter way to do
// it than a plain multiplication.
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.Printf("Ideal image has %v layers, but budget is %v\n", len(layers), budget)
}
for len(layers) > budget {
merged := layers[0].merge(layers[1])
layers[1] = merged
layers = layers[1:]
}
return layers
}
func main() {
graphFile := flag.String("graph", ".attrs.json", "Input file containing graph")
popFile := flag.String("pop", "popularity.json", "Package popularity data")
outFile := flag.String("out", "layers.json", "File to write layers to")
layerBudget := flag.Int("budget", 94, "Total layer budget available")
flag.Parse()
// Parse graph data
file, err := ioutil.ReadFile(*graphFile)
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)
}
// Parse popularity data
popBytes, err := ioutil.ReadFile(*popFile)
if err != nil {
log.Fatalf("Failed to load input: %s\n", err)
}
var pop pkgsMetadata
err = json.Unmarshal(popBytes, &pop)
if err != nil {
log.Fatalf("Failed to deserialise input: %s\n", err)
}
graph := buildGraph(&refs, &pop)
layers := dominate(*layerBudget, graph)
j, _ := json.Marshal(layers)
ioutil.WriteFile(*outFile, j, 0644)
}

11
tools/nixery/default.nix
View file

@ -17,11 +17,7 @@
with pkgs;
let buildImage = import ./build-image {
srcType = "path";
srcArgs = <nixpkgs>;
};
in rec {
rec {
# Go implementation of the Nixery server which implements the
# container registry interface.
#
@ -30,9 +26,8 @@ in rec {
# data dependencies.
nixery-server = callPackage ./server { };
# Implementation of the image building & layering logic
nixery-build-image = buildImage.wrapper;
nixery-group-layers = buildImage.groupLayers;
# Implementation of the Nix image building logic
nixery-build-image = import ./build-image { inherit pkgs; };
# Use mdBook to build a static asset page which Nixery can then
# serve. This is primarily used for the public instance at