[metrics] add lru evict atime metric

cache/disk/disk.go

@@ -20,6 +20,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/buchgr/bazel-remote/cache"
 	"github.com/buchgr/bazel-remote/cache/disk/casblob"
@@ -31,6 +32,8 @@ import (
 	pb "github.com/buchgr/bazel-remote/genproto/build/bazel/remote/execution/v2"
 	"google.golang.org/protobuf/proto"
 
+	"github.com/prometheus/client_golang/prometheus"
+
 	"golang.org/x/sync/semaphore"
 )
 
@@ -83,6 +86,8 @@ type diskCache struct {
 
 	mu  sync.Mutex
 	lru SizedLRU
+
+	gaugeCacheAge prometheus.Gauge
 }
 
 type nameAndInfo struct {
@@ -135,6 +140,11 @@ func New(dir string, maxSizeBytes int64, opts ...Option) (Cache, error) {
 		// I suppose it's better to slow down processing than to crash
 		// when hitting the 10k limit or to run out of disk space.
 		fileRemovalSem: semaphore.NewWeighted(5000),
+
+		gaugeCacheAge: prometheus.NewGauge(prometheus.GaugeOpts{
+			Name: "bazel_remote_disk_cache_age_seconds",
+			Help: "The file `atime` of oldest item in the LRU cache. Depending on filemount opts (e.g. relatime), the resolution may be meausured in 'days' and not accurate to the second",
+		}),
 	}
 
 	cc := CacheConfig{diskCache: &c}
@@ -143,19 +153,7 @@ func New(dir string, maxSizeBytes int64, opts ...Option) (Cache, error) {
 	// This function is only called while the lock is held
 	// by the current goroutine.
 	onEvict := func(key Key, value lruItem) {
-		ks := key.(string)
-		hash := ks[len(ks)-sha256.Size*2:]
-		var kind cache.EntryKind = cache.AC
-		if strings.HasPrefix(ks, "cas") {
-			kind = cache.CAS
-		} else if strings.HasPrefix(ks, "ac") {
-			kind = cache.AC
-		} else if strings.HasPrefix(ks, "raw") {
-			kind = cache.RAW
-		}
-
-		f := filepath.Join(dir, c.FileLocation(kind, value.legacy, hash, value.size, value.random))
-
+		f := c.getElementPath(key, value)
 		// Run in a goroutine so we can release the lock sooner.
 		go c.removeFile(f)
 	}
@@ -211,6 +209,60 @@ func New(dir string, maxSizeBytes int64, opts ...Option) (Cache, error) {
 // Non-test users must call this to expose metrics.
 func (c *diskCache) RegisterMetrics() {
 	c.lru.RegisterMetrics()
+
+	prometheus.MustRegister(c.gaugeCacheAge)
+
+	// Update the cache age metric on a static interval
+	// Note: this could be modeled as a GuageFunc that updates as needed
+	// but since the updater func must lock the cache mu, it was deemed
+	// necessary to have greater control of when to get the cache age
+	go c.pollCacheAge()
+}
+
+// Update metric every minute with the current age of the cache
+func (c *diskCache) pollCacheAge() {
+	ticker := time.NewTicker(60 * time.Second)
+	for ; true; <-ticker.C {
+		c.updateCacheAgeMetric()
+	}
+}
+
+// Get the back item in the cache, and store its atime in a metric
+func (c *diskCache) updateCacheAgeMetric() {
+	c.mu.Lock()
+
+	key, value := c.lru.PeakOldestCacheItem()
+	age := 0.0
+
+	if key != nil {
+		f := c.getElementPath(key, value)
+		ts, err := atime.Stat(f)
+
+		if err != nil {
+			log.Printf("ERROR: failed to determine cache age: %v, unable to stat %s", err, f)
+		} else {
+			age = time.Now().Sub(ts).Seconds()
+		}
+	}
+
+	c.mu.Unlock()
+
+	c.gaugeCacheAge.Set(age)
+}
+
+func (c *diskCache) getElementPath(key Key, value lruItem) string {
+	ks := key.(string)
+	hash := ks[len(ks)-sha256.Size*2:]
+	var kind cache.EntryKind = cache.AC
+	if strings.HasPrefix(ks, "cas") {
+		kind = cache.CAS
+	} else if strings.HasPrefix(ks, "ac") {
+		kind = cache.AC
+	} else if strings.HasPrefix(ks, "raw") {
+		kind = cache.RAW
+	}
+
+	return filepath.Join(c.dir, c.FileLocation(kind, value.legacy, hash, value.size, value.random))
 }
 
 func (c *diskCache) removeFile(f string) {

cache/disk/lru.go

@@ -280,3 +280,13 @@ func (c *SizedLRU) removeElement(e *list.Element) {
 func roundUp4k(n int64) int64 {
 	return (n + BlockSize - 1) & -BlockSize
 }
+
+// Get the back item of the LRU cache.
+func (c *SizedLRU) PeakOldestCacheItem() (Key, lruItem) {
+	ele := c.ll.Back()
+	if ele != nil {
+		kv := ele.Value.(*entry)
+		return kv.key, kv.value
+	}
+	return nil, lruItem{}
+}