diff --git a/changelogs/unreleased/zj-repo-languages-failed-precond.yml b/changelogs/unreleased/zj-repo-languages-failed-precond.yml
new file mode 100644
index 0000000000000000000000000000000000000000..d38c00a0bf4f459a08f22aac7d8647b907566e7d
--- /dev/null
+++ b/changelogs/unreleased/zj-repo-languages-failed-precond.yml
@@ -0,0 +1,5 @@
+---
+title: Return FailedPrecondition when commit languages are requested too often
+merge_request: 1690
+author:
+type: performance
diff --git a/internal/service/commit/languages.go b/internal/service/commit/languages.go
index 8d73542e66671e496da7b720596d954cb6d4a63a..a9c2c06cb1713b10e3a12d2123cd77f9a79fef09 100644
--- a/internal/service/commit/languages.go
+++ b/internal/service/commit/languages.go
@@ -3,14 +3,16 @@ package commit
 import (
 	"context"
 	"io/ioutil"
+	"os"
+	"path/filepath"
 	"sort"
+	"time"
 
 	"gitlab.com/gitlab-org/gitaly/internal/git"
 	"gitlab.com/gitlab-org/gitaly/internal/helper"
 	"gitlab.com/gitlab-org/gitaly/internal/helper/text"
 	"gitlab.com/gitlab-org/gitaly/internal/linguist"
 	"gitlab.com/gitlab-org/gitaly/internal/metadata/featureflag"
-	"gitlab.com/gitlab-org/gitaly/internal/service/ref"
 	"gitlab.com/gitlab-org/gitaly/proto/go/gitalypb"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
@@ -19,18 +21,11 @@ import (
 func (*server) CommitLanguages(ctx context.Context, req *gitalypb.CommitLanguagesRequest) (*gitalypb.CommitLanguagesResponse, error) {
 	repo := req.Repository
 
-	if err := git.ValidateRevisionAllowEmpty(req.Revision); err != nil {
+	if err := git.ValidateRevision(req.Revision); err != nil {
 		return nil, helper.ErrInvalidArgument(err)
 	}
 
 	revision := string(req.Revision)
-	if revision == "" {
-		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 {
@@ -41,6 +36,11 @@ func (*server) CommitLanguages(ctx context.Context, req *gitalypb.CommitLanguage
 	if err != nil {
 		return nil, err
 	}
+
+	if cacheRecentlyUpdated(repoPath) {
+		return nil, helper.ErrPreconditionFailedf("repository languages recently scanned")
+	}
+
 	stats, err := linguist.Stats(ctx, repoPath, commitID)
 	if err != nil {
 		return nil, err
@@ -104,3 +104,18 @@ func lookupRevision(ctx context.Context, repo *gitalypb.Repository, revision str
 
 	return text.ChompBytes(revParseBytes), nil
 }
+
+// Gitaly uses Linguist to detect the languages, and to improve performance, the
+// results will be cached. If the modified time is later than a trashold it's
+// considered recently updated enough that we do not scan again
+func cacheRecentlyUpdated(path string) bool {
+	fullPath := filepath.Join(path, "language-stats.cache")
+
+	fi, err := os.Stat(fullPath)
+	if err != nil {
+		return false
+	}
+
+	const sentinel = -time.Hour * 24 * 7
+	return fi.ModTime().After(time.Now().Add(sentinel))
+}
diff --git a/internal/service/commit/languages_test.go b/internal/service/commit/languages_test.go
index b456719a52b032e8d85b27f43e1cf95a98e21ddf..7218bcf519b5e5b484b6ffbf01d8c13f27727a22 100644
--- a/internal/service/commit/languages_test.go
+++ b/internal/service/commit/languages_test.go
@@ -53,6 +53,12 @@ func TestLanguages(t *testing.T) {
 		actualLanguage := resp.Languages[i]
 		requireLanguageEqual(t, &el, actualLanguage)
 	}
+
+	// if another request is made within short succession (1 week), the second
+	// one is rejected
+	_, err = client.CommitLanguages(ctx, request)
+	require.Error(t, err)
+	testhelper.RequireGrpcError(t, err, codes.FailedPrecondition)
 }
 
 func TestFileCountIsZeroWhenFeatureIsDisabled(t *testing.T) {
@@ -92,36 +98,6 @@ func requireLanguageEqual(t *testing.T, expected, actual *gitalypb.CommitLanguag
 	require.Equal(t, expected.Bytes, actual.Bytes)
 }
 
-func TestLanguagesEmptyRevision(t *testing.T) {
-	server, serverSocketPath := startTestServices(t)
-	defer server.Stop()
-
-	client, conn := newCommitServiceClient(t, serverSocketPath)
-	defer conn.Close()
-
-	testRepo, _, cleanupFn := testhelper.NewTestRepo(t)
-	defer cleanupFn()
-
-	request := &gitalypb.CommitLanguagesRequest{
-		Repository: testRepo,
-	}
-
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-	resp, err := client.CommitLanguages(ctx, request)
-	require.NoError(t, err)
-
-	require.NotZero(t, len(resp.Languages), "number of languages in response")
-
-	foundRuby := false
-	for _, l := range resp.Languages {
-		if l.Name == "Ruby" {
-			foundRuby = true
-		}
-	}
-	require.True(t, foundRuby, "expected to find Ruby as a language on HEAD")
-}
-
 func TestInvalidCommitLanguagesRequestRevision(t *testing.T) {
 	server, serverSocketPath := startTestServices(t)
 	defer server.Stop()