diff --git a/changelogs/unreleased/zj-linguist-ruby-sidecar.yml b/changelogs/unreleased/zj-linguist-ruby-sidecar.yml
new file mode 100644
index 0000000000000000000000000000000000000000..bae12a105998e1d8066d81dc95f04b766c062c8c
--- /dev/null
+++ b/changelogs/unreleased/zj-linguist-ruby-sidecar.yml
@@ -0,0 +1,5 @@
+---
+title: Use caching for linguist results
+merge_request: 751
+author:
+type: performance
diff --git a/cmd/gitaly/main.go b/cmd/gitaly/main.go
index 591387a590abf3c28c76105167612e073601e8e7..22764f14cab5891dbb11e17c113e7039a5c1b80d 100644
--- a/cmd/gitaly/main.go
+++ b/cmd/gitaly/main.go
@@ -14,7 +14,6 @@ import (
 	"gitlab.com/gitlab-org/gitaly/internal/config"
 	"gitlab.com/gitlab-org/gitaly/internal/connectioncounter"
 	"gitlab.com/gitlab-org/gitaly/internal/git"
-	"gitlab.com/gitlab-org/gitaly/internal/linguist"
 	"gitlab.com/gitlab-org/gitaly/internal/rubyserver"
 	"gitlab.com/gitlab-org/gitaly/internal/server"
 	"gitlab.com/gitlab-org/gitaly/internal/tempdir"
@@ -43,10 +42,6 @@ func loadConfig(configPath string) error {
 		return err
 	}
 
-	if err := linguist.LoadColors(); err != nil {
-		return fmt.Errorf("load linguist colors: %v", err)
-	}
-
 	return nil
 }
 
diff --git a/doc/configuration/README.md b/doc/configuration/README.md
index 91755e4f558c0a4e91dfe16972bcc7289bd0d401..eef0cf297b5fa0102d1a694bd3f5c3d578b05769 100644
--- a/doc/configuration/README.md
+++ b/doc/configuration/README.md
@@ -111,7 +111,6 @@ max\_rss limit.
 |graceful_restart_timeout|string|no|Grace period to allow a gitaly-ruby process to finish ongoing requests. Default 10 minutes ("10m").|
 |restart_delay|string|no|Time memory must be high before a restart is triggered, in seconds. Default 5 minutes ("5m").|
 |num_workers|integer|no|Number of gitaly-ruby worker processes. Try increasing this number in case of ResourceExhausted errors. Default 2, minimum 2.|
-|linguist_languages_path|string|no|Override for dynamic languages.json discovery. Default: "" (use dynamic discovery).|
 
 ### Logging
 
diff --git a/internal/linguist/linguist.go b/internal/linguist/linguist.go
deleted file mode 100644
index 9c7feeb2f78c9d5b9e47a5365a93bbf193646346..0000000000000000000000000000000000000000
--- a/internal/linguist/linguist.go
+++ /dev/null
@@ -1,98 +0,0 @@
-package linguist
-
-import (
-	"context"
-	"crypto/sha256"
-	"encoding/json"
-	"fmt"
-	"io"
-	"io/ioutil"
-	"os"
-	"os/exec"
-	"path"
-
-	"gitlab.com/gitlab-org/gitaly/internal/command"
-	"gitlab.com/gitlab-org/gitaly/internal/config"
-)
-
-var (
-	colorMap = make(map[string]Language)
-)
-
-// Language is used to parse Linguist's language.json file.
-type Language struct {
-	Color string `json:"color"`
-}
-
-// Stats returns the repository's language stats as reported by 'git-linguist'.
-func Stats(ctx context.Context, repoPath string, commitID string) (map[string]int, error) {
-	cmd := exec.Command("bundle", "exec", "bin/ruby-cd", repoPath, "git-linguist", "--commit="+commitID, "stats")
-	cmd.Dir = config.Config.Ruby.Dir
-	reader, err := command.New(ctx, cmd, nil, nil, nil, os.Environ()...)
-	if err != nil {
-		return nil, err
-	}
-
-	data, err := ioutil.ReadAll(reader)
-	if err != nil {
-		return nil, err
-	}
-
-	stats := make(map[string]int)
-	return stats, json.Unmarshal(data, &stats)
-}
-
-// Color returns the color Linguist has assigned to language.
-func Color(language string) string {
-	if color := colorMap[language].Color; color != "" {
-		return color
-	}
-
-	colorSha := sha256.Sum256([]byte(language))
-	return fmt.Sprintf("#%x", colorSha[0:3])
-}
-
-// LoadColors loads the name->color map from the Linguist gem.
-func LoadColors() error {
-	jsonReader, err := openLanguagesJSON()
-	if err != nil {
-		return err
-	}
-	defer jsonReader.Close()
-
-	return json.NewDecoder(jsonReader).Decode(&colorMap)
-}
-
-func openLanguagesJSON() (io.ReadCloser, error) {
-	if jsonPath := config.Config.Ruby.LinguistLanguagesPath; jsonPath != "" {
-		// This is a fallback for environments where dynamic discovery of the
-		// linguist path via Bundler is not working for some reason, for example
-		// https://gitlab.com/gitlab-org/gitaly/issues/1119.
-		return os.Open(jsonPath)
-	}
-
-	linguistPathSymlink, err := ioutil.TempFile("", "gitaly-linguist-path")
-	if err != nil {
-		return nil, err
-	}
-	defer os.Remove(linguistPathSymlink.Name())
-
-	if err := linguistPathSymlink.Close(); err != nil {
-		return nil, err
-	}
-
-	// We use a symlink because we cannot trust Bundler to not print garbage
-	// on its stdout.
-	rubyScript := `FileUtils.ln_sf(Bundler.rubygems.find_name('github-linguist').first.full_gem_path, ARGV.first)`
-	cmd := exec.Command("bundle", "exec", "ruby", "-rfileutils", "-e", rubyScript, linguistPathSymlink.Name())
-	cmd.Dir = config.Config.Ruby.Dir
-
-	if err := cmd.Run(); err != nil {
-		if exitError, ok := err.(*exec.ExitError); ok {
-			err = fmt.Errorf("%v; stderr: %q", exitError, exitError.Stderr)
-		}
-		return nil, err
-	}
-
-	return os.Open(path.Join(linguistPathSymlink.Name(), "lib/linguist/languages.json"))
-}
diff --git a/internal/linguist/linguist_test.go b/internal/linguist/linguist_test.go
deleted file mode 100644
index aac91d866d0991e85b0feeeffbc082628be307c1..0000000000000000000000000000000000000000
--- a/internal/linguist/linguist_test.go
+++ /dev/null
@@ -1,32 +0,0 @@
-package linguist
-
-import (
-	"path/filepath"
-	"testing"
-
-	"github.com/stretchr/testify/require"
-	"gitlab.com/gitlab-org/gitaly/internal/config"
-	"gitlab.com/gitlab-org/gitaly/internal/testhelper"
-)
-
-func TestLoadLanguages(t *testing.T) {
-	testhelper.ConfigureRuby()
-
-	colorMap = make(map[string]Language)
-	require.NoError(t, LoadColors(), "load colors")
-
-	require.Equal(t, "#701516", Color("Ruby"), "color value for 'Ruby'")
-}
-
-func TestLoadLanguagesCustomPath(t *testing.T) {
-	jsonPath, err := filepath.Abs("testdata/fake-languages.json")
-	require.NoError(t, err)
-
-	testhelper.ConfigureRuby()
-	config.Config.Ruby.LinguistLanguagesPath = jsonPath
-
-	colorMap = make(map[string]Language)
-	require.NoError(t, LoadColors(), "load colors")
-
-	require.Equal(t, "foo color", Color("FooBar"))
-}
diff --git a/internal/linguist/testdata/fake-languages.json b/internal/linguist/testdata/fake-languages.json
deleted file mode 100644
index 83f7687f8e62b1fd69efd562b5eb7319d236cd14..0000000000000000000000000000000000000000
--- a/internal/linguist/testdata/fake-languages.json
+++ /dev/null
@@ -1,3 +0,0 @@
-{
-"FooBar": { "color": "foo color" }
-}
diff --git a/internal/service/commit/languages.go b/internal/service/commit/languages.go
index 5d8227a6a321061ba7500ea876d8ba74c89fef08..5662d03c8bd1b92f3e22096387a9520a8529d2e2 100644
--- a/internal/service/commit/languages.go
+++ b/internal/service/commit/languages.go
@@ -1,93 +1,34 @@
 package commit
 
 import (
-	"io/ioutil"
-	"sort"
-	"strings"
-
-	"google.golang.org/grpc/codes"
-	"google.golang.org/grpc/status"
-
-	"gitlab.com/gitlab-org/gitaly/internal/git"
-	"gitlab.com/gitlab-org/gitaly/internal/helper"
-	"gitlab.com/gitlab-org/gitaly/internal/linguist"
-	"gitlab.com/gitlab-org/gitaly/internal/service/ref"
-
 	pb "gitlab.com/gitlab-org/gitaly-proto/go"
+	"gitlab.com/gitlab-org/gitaly/internal/rubyserver"
+	"gitlab.com/gitlab-org/gitaly/internal/service/ref"
 
 	"golang.org/x/net/context"
 )
 
-func (*server) CommitLanguages(ctx context.Context, req *pb.CommitLanguagesRequest) (*pb.CommitLanguagesResponse, error) {
+func (s *server) CommitLanguages(ctx context.Context, req *pb.CommitLanguagesRequest) (*pb.CommitLanguagesResponse, error) {
 	repo := req.Repository
 
-	revision := string(req.Revision)
-	if revision == "" {
+	if len(req.Revision) == 0 {
 		defaultBranch, err := ref.DefaultBranchName(ctx, req.Repository)
 		if err != nil {
 			return nil, err
 		}
-		revision = string(defaultBranch)
-	}
 
-	commitID, err := lookupRevision(ctx, repo, revision)
-	if err != nil {
-		return nil, err
+		req.Revision = defaultBranch
 	}
 
-	repoPath, err := helper.GetRepoPath(repo)
+	client, err := s.CommitServiceClient(ctx)
 	if err != nil {
 		return nil, err
 	}
-	stats, err := linguist.Stats(ctx, repoPath, commitID)
-	if err != nil {
-		return nil, err
-	}
-
-	resp := &pb.CommitLanguagesResponse{}
-	if len(stats) == 0 {
-		return resp, nil
-	}
-
-	total := 0
-	for _, count := range stats {
-		total += count
-	}
 
-	if total == 0 {
-		return nil, status.Errorf(codes.Internal, "linguist stats added up to zero: %v", stats)
-	}
-
-	for lang, count := range stats {
-		l := &pb.CommitLanguagesResponse_Language{
-			Name:  lang,
-			Share: float32(100*count) / float32(total),
-			Color: linguist.Color(lang),
-		}
-		resp.Languages = append(resp.Languages, l)
-	}
-
-	sort.Sort(languageSorter(resp.Languages))
-
-	return resp, nil
-}
-
-type languageSorter []*pb.CommitLanguagesResponse_Language
-
-func (ls languageSorter) Len() int           { return len(ls) }
-func (ls languageSorter) Swap(i, j int)      { ls[i], ls[j] = ls[j], ls[i] }
-func (ls languageSorter) Less(i, j int) bool { return ls[i].Share > ls[j].Share }
-
-func lookupRevision(ctx context.Context, repo *pb.Repository, revision string) (string, error) {
-	revParse, err := git.Command(ctx, repo, "rev-parse", revision)
+	clientCtx, err := rubyserver.SetHeaders(ctx, repo)
 	if err != nil {
-		return "", err
-	}
-
-	revParseBytes, err := ioutil.ReadAll(revParse)
-	if err != nil {
-		return "", err
+		return nil, err
 	}
 
-	return strings.TrimSpace(string(revParseBytes)), nil
+	return client.CommitLanguages(clientCtx, req)
 }
diff --git a/internal/service/commit/languages_test.go b/internal/service/commit/languages_test.go
index 0deeb7ca06170e608c7ad22270adce51cf0a57a4..b810108f1b252d5c1999d409137973aac1625027 100644
--- a/internal/service/commit/languages_test.go
+++ b/internal/service/commit/languages_test.go
@@ -33,9 +33,9 @@ func TestLanguages(t *testing.T) {
 	require.NotZero(t, len(resp.Languages), "number of languages in response")
 
 	expectedLanguages := []pb.CommitLanguagesResponse_Language{
-		{Name: "Ruby", Share: 66, Color: "#701516"},
+		{Name: "Ruby", Share: 65, Color: "#701516"},
 		{Name: "JavaScript", Share: 22, Color: "#f1e05a"},
-		{Name: "HTML", Share: 7, Color: "#e34c26"},
+		{Name: "HTML", Share: 8, Color: "#e34c26"},
 		{Name: "CoffeeScript", Share: 2, Color: "#244776"},
 		// Modula-2 is a special case because Linguist has no color for it. This
 		// test case asserts that we invent a color for it (SHA256 of the name).
diff --git a/internal/service/commit/testhelper_test.go b/internal/service/commit/testhelper_test.go
index fe939f3f9274f9445836a1de89dc6e0cb09b58c9..f7f4551430a58c2390d01ba202172968933d5306 100644
--- a/internal/service/commit/testhelper_test.go
+++ b/internal/service/commit/testhelper_test.go
@@ -6,7 +6,6 @@ import (
 	"testing"
 	"time"
 
-	"gitlab.com/gitlab-org/gitaly/internal/linguist"
 	"gitlab.com/gitlab-org/gitaly/internal/rubyserver"
 	"gitlab.com/gitlab-org/gitaly/internal/testhelper"
 
@@ -18,8 +17,6 @@ import (
 	pb "gitlab.com/gitlab-org/gitaly-proto/go"
 )
 
-var ()
-
 func TestMain(m *testing.M) {
 	os.Exit(testMain(m))
 }
@@ -30,9 +27,6 @@ func testMain(m *testing.M) int {
 	defer testhelper.MustHaveNoChildProcess()
 
 	testhelper.ConfigureRuby()
-	if err := linguist.LoadColors(); err != nil {
-		log.Fatal(err)
-	}
 
 	var err error
 	rubyServer, err = rubyserver.Start()
diff --git a/ruby/Gemfile b/ruby/Gemfile
index e06683b615d8fe3b1cc9b31daba4df8ebaf314e8..1a7b87dfb74153e46ae448dc3e94241e8d1f1ba9 100644
--- a/ruby/Gemfile
+++ b/ruby/Gemfile
@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 gem 'rugged', '~> 0.27.2'
-gem 'github-linguist', '~> 5.3.3', require: 'linguist'
+gem 'github-linguist', '~> 6.1', require: 'linguist'
 gem 'gitlab-markup', '~> 1.6.2'
 gem 'gitaly-proto', '~> 0.107.0', require: 'gitaly'
 gem 'activesupport', '~> 5.0.2'
diff --git a/ruby/Gemfile.lock b/ruby/Gemfile.lock
index ab61f1041381479795e3e4e90a0033440632cc6f..ee7caf8d1dcd12244075910e5f203cba575af987 100644
--- a/ruby/Gemfile.lock
+++ b/ruby/Gemfile.lock
@@ -12,7 +12,7 @@ GEM
     charlock_holmes (0.7.6)
     concurrent-ruby (1.0.5)
     diff-lcs (1.3)
-    escape_utils (1.1.1)
+    escape_utils (1.2.1)
     faraday (0.12.2)
       multipart-post (>= 1.2, < 3)
     gemojione (3.3.0)
@@ -20,9 +20,9 @@ GEM
     gitaly-proto (0.107.0)
       google-protobuf (~> 3.1)
       grpc (~> 1.10)
-    github-linguist (5.3.3)
-      charlock_holmes (~> 0.7.5)
-      escape_utils (~> 1.1.0)
+    github-linguist (6.2.0)
+      charlock_holmes (~> 0.7.6)
+      escape_utils (~> 1.2.0)
       mime-types (>= 1.19)
       rugged (>= 0.25.1)
     github-markup (1.7.0)
@@ -143,7 +143,7 @@ DEPENDENCIES
   activesupport (~> 5.0.2)
   faraday (~> 0.12)
   gitaly-proto (~> 0.107.0)
-  github-linguist (~> 5.3.3)
+  github-linguist (~> 6.1)
   gitlab-gollum-lib (~> 4.2)
   gitlab-gollum-rugged_adapter (~> 0.4.4)
   gitlab-markup (~> 1.6.2)
diff --git a/ruby/lib/gitaly_server.rb b/ruby/lib/gitaly_server.rb
index a03f0dab6bb595ff76a511b799ab3a3dbfeb855e..1866d183f9c22277f1574486921b9e7587ba67f5 100644
--- a/ruby/lib/gitaly_server.rb
+++ b/ruby/lib/gitaly_server.rb
@@ -1,6 +1,8 @@
 require 'gitaly'
 
 require_relative 'gitlab/git.rb'
+require_relative 'gitlab/linguist/repository_languages.rb'
+require_relative 'gitlab/linguist/cache.rb'
 
 require_relative 'gitaly_server/client.rb'
 require_relative 'gitaly_server/utils.rb'
diff --git a/ruby/lib/gitaly_server/commit_service.rb b/ruby/lib/gitaly_server/commit_service.rb
index 9e4ccd26177cf0533b7a859510d52ba8fdd30c55..dea4355810883069cd77d08ffd6bf73e22e1e907 100644
--- a/ruby/lib/gitaly_server/commit_service.rb
+++ b/ruby/lib/gitaly_server/commit_service.rb
@@ -1,3 +1,5 @@
+require 'linguist'
+
 module GitalyServer
   class CommitService < Gitaly::CommitService::Service
     include Utils
@@ -158,6 +160,31 @@ module GitalyServer
       end
     end
 
+    def commit_languages(request, call)
+      bridge_exceptions do
+        repo = Gitlab::Git::Repository.from_gitaly(request.repository, call)
+
+        commit = Gitlab::Git::Commit.find(repo, request.revision)
+        raise GRPC::InvalidArgument, 'revision could not be resolved' unless commit
+
+        languages =
+          Gitlab::Linguist::RepositoryLanguages.new(repo, commit)
+          .detect
+
+        total_bytes = languages.sum(&:last).to_f
+
+        languages.map! do |name, bytes|
+          Gitaly::CommitLanguagesResponse::Language.new(
+            name: name.to_s,
+            color: ::Linguist::Language.find_by_name(name)&.color || "##{Digest::SHA256.hexdigest(name)[0..5]}",
+            share: ((bytes / total_bytes) * 100).round,
+          )
+        end
+
+        Gitaly::CommitLanguagesResponse.new(languages: languages)
+      end
+    end
+
     private
 
     # yields either signature chunks or signed_text chunks to the passed block
diff --git a/ruby/lib/gitlab/linguist/cache.rb b/ruby/lib/gitlab/linguist/cache.rb
new file mode 100644
index 0000000000000000000000000000000000000000..292e281f77e4c2c784cb626f4253074eee7a8761
--- /dev/null
+++ b/ruby/lib/gitlab/linguist/cache.rb
@@ -0,0 +1,50 @@
+module Gitlab
+  module Linguist
+    class Cache
+      OLD_STATS_KEY = 'old_stats'.freeze
+      OLD_COMMIT_OID_KEY = 'old_commit_oid'.freeze
+
+      def initialize(repo_path)
+        @path = repo_path
+      end
+
+      def write(linguist, commit_oid)
+        return if old_commit_oid == commit_oid
+
+        FileUtils.mkdir_p(linguist_cache_directory) unless Dir.exist?(linguist_cache_directory)
+
+        new_cache = { OLD_STATS_KEY => linguist.cache, OLD_COMMIT_OID_KEY => commit_oid }
+
+        File.write(cache_path, Marshal.dump(new_cache))
+      end
+
+      def old_stats
+        cache[OLD_STATS_KEY]
+      end
+
+      def old_commit_oid
+        cache[OLD_COMMIT_OID_KEY]
+      end
+
+      private
+
+      def cache
+        @cache ||= if File.exist?(cache_path)
+                     Marshal.load(File.binread(cache_path))
+                   else
+                     {}
+                   end
+      rescue ArgumentError
+        @cache = {}
+      end
+
+      def cache_path
+        File.join(linguist_cache_directory, 'linguist-cache')
+      end
+
+      def linguist_cache_directory
+        File.join(@path, 'gitaly')
+      end
+    end
+  end
+end
diff --git a/ruby/lib/gitlab/linguist/repository_languages.rb b/ruby/lib/gitlab/linguist/repository_languages.rb
new file mode 100644
index 0000000000000000000000000000000000000000..8ddbb32a2e6d8b5b11afd688dd8e8b96ab7d521c
--- /dev/null
+++ b/ruby/lib/gitlab/linguist/repository_languages.rb
@@ -0,0 +1,24 @@
+module Gitlab
+  module Linguist
+    class RepositoryLanguages
+      def initialize(repo, commit)
+        @repo = repo
+        @commit = commit
+        @cache = Gitlab::Linguist::Cache.new(repo.path)
+      end
+
+      def detect
+        linguist = ::Linguist::Repository.incremental(@repo.rugged, @commit.id, @cache.old_commit_oid, @cache.old_stats)
+
+        languages = linguist
+          .languages
+          .sort_by { |_k, v| v }
+          .reverse
+
+        @cache.write(linguist, @commit.id)
+
+        languages
+      end
+    end
+  end
+end
diff --git a/ruby/spec/lib/gitlab/linguist/cache_spec.rb b/ruby/spec/lib/gitlab/linguist/cache_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..d5f0d997f4c70e63910d4d1c236d8d7ce0d8a4ce
--- /dev/null
+++ b/ruby/spec/lib/gitlab/linguist/cache_spec.rb
@@ -0,0 +1,37 @@
+require 'spec_helper'
+
+describe Gitlab::Linguist::Cache do
+  include TestRepo
+
+  let(:repository) { gitlab_git_from_gitaly(new_mutable_test_repo) }
+  let(:old_stats) { [{ 'foo.rb' => 'Ruby'}, { 'bar.go' => 'Go' }] }
+  let(:linguist) { double('linguist', cache: old_stats) }
+
+  subject { described_class.new(repository.path) }
+
+  describe '#write' do
+    it 'writes the cache in the Gitaly cache directory' do
+      subject.write(linguist, '0' * 40)
+
+      expect(File.exist?(File.join(repository.path, 'gitaly', 'linguist-cache'))).to be(true)
+    end
+  end
+
+  describe 'old_stats' do
+    context 'when there is no cache yet' do
+      it 'returns nil' do
+        expect(subject.old_stats).to be_nil
+      end
+
+    end
+
+    context 'when the cache has been written' do
+      before do
+        subject.write(linguist, '0' * 40)
+
+        expect(subject.old_stats).not_to be_nil
+        expect(subject.old_stats).to eq('0' * 40)
+      end
+    end
+  end
+end