gopls/internal/lsp/filecache: touch only files older than 1h

This change implements an optimization similar to one
in the go command's cache: we skip the chtimes(2) call
to update the file access times unless it would change
by at least one hour. This avoids turning every read
into a write.

It makes quite a difference to the Get benchmark:
330MB/s before, 430MB/s after.

Change-Id: I5b5b6e99a1968c73bf1032b410bec989961c4a90
Reviewed-on: https://go-review.googlesource.com/c/tools/+/496438
Reviewed-by: Robert Findley <[email protected]>
Run-TryBot: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
gopls-CI: kokoro <[email protected]>

Author: adonovan

gopls/internal/lsp/filecache/filecache.go

@@ -110,24 +110,24 @@ func Get(kind string, key [32]byte) ([]byte, error) {
 
 	// Update file times used by LRU eviction.
 	//
-	// This turns every read into a write operation.
-	// If this is a performance problem, we should
-	// touch the files asynchronously, or, follow
-	// the approach used in the go command's cache
-	// and update only if the existing timestamp is
-	// older than, say, one hour.
+	// Because this turns a read into a write operation,
+	// we follow the approach used in the go command's
+	// cache and update the access time only if the
+	// existing timestamp is older than one hour.
 	//
 	// (Traditionally the access time would be updated
 	// automatically, but for efficiency most POSIX systems have
 	// for many years set the noatime mount option to avoid every
 	// open or read operation entailing a metadata write.)
 	now := time.Now()
-	if err := os.Chtimes(indexName, now, now); err != nil {
-		return nil, fmt.Errorf("failed to update access time of index file: %w", err)
-	}
-	if err := os.Chtimes(casName, now, now); err != nil {
-		return nil, fmt.Errorf("failed to update access time of CAS file: %w", err)
+	touch := func(filename string) {
+		st, err := os.Stat(filename)
+		if err == nil && now.Sub(st.ModTime()) > time.Hour {
+			os.Chtimes(filename, now, now) // ignore error
+		}
 	}
+	touch(indexName)
+	touch(casName)
 
 	if useMemCache {
 		memCache.Set(memKey{kind, key}, value, len(value))