lib/storage: keep (date, metricID) entries only for the last two dates

Entries for the previous dates is usually not used, so there is little sense in keeping them in memory.

This should reduce the size of storage/date_metricID cache, which can be monitored
via vm_cache_entries{type="storage/date_metricID"} metric.

lib/storage/storage.go

@@ -2352,6 +2352,8 @@ func (dmc *dateMetricIDCache) syncLocked() {
 	// Merge data from byDate into byDateMutable and then atomically replace byDate with the merged data.
 	byDate := dmc.byDate.Load()
 	byDateMutable := dmc.byDateMutable
+	byDateMutable.hotEntry.Store(&byDateMetricIDEntry{})
+
 	for k, e := range byDateMutable.m {
 		v := byDate.get(k.generation, k.date)
 		if v == nil {
@@ -2365,11 +2367,8 @@ func (dmc *dateMetricIDCache) syncLocked() {
 			v: *v,
 		}
 		byDateMutable.m[k] = dme
-		he := byDateMutable.hotEntry.Load()
-		if he.k == k {
-			byDateMutable.hotEntry.Store(dme)
-		}
 	}
+
 	// Copy entries from byDate, which are missing in byDateMutable
 	for k, e := range byDate.m {
 		v := byDateMutable.get(k.generation, k.date)
@@ -2379,6 +2378,24 @@ func (dmc *dateMetricIDCache) syncLocked() {
 		byDateMutable.m[k] = e
 	}
 
+	if len(byDateMutable.m) > 2 {
+		// Keep only entries for the last two dates - these are usually
+		// the current date and the next date.
+		dates := make([]uint64, 0, len(byDateMutable.m))
+		for k := range byDateMutable.m {
+			dates = append(dates, k.date)
+		}
+		sort.Slice(dates, func(i, j int) bool {
+			return dates[i] < dates[j]
+		})
+		maxDate := dates[len(dates)-2]
+		for k := range byDateMutable.m {
+			if k.date < maxDate {
+				delete(byDateMutable.m, k)
+			}
+		}
+	}
+
 	// Atomically replace byDate with byDateMutable
 	dmc.byDate.Store(dmc.byDateMutable)
 	dmc.byDateMutable = newByDateMetricIDMap()

lib/storage/storage_test.go

@@ -98,7 +98,7 @@ func testDateMetricIDCache(c *dateMetricIDCache, concurrent bool) error {
 	m := make(map[dmk]bool)
 	for i := 0; i < 1e5; i++ {
 		generation := uint64(i) % 2
-		date := uint64(i) % 3
+		date := uint64(i) % 2
 		metricID := uint64(i) % 1237
 		if !concurrent && c.Has(generation, date, metricID) {
 			if !m[dmk{generation, date, metricID}] {
@@ -127,7 +127,7 @@ func testDateMetricIDCache(c *dateMetricIDCache, concurrent bool) error {
 	// Verify fast path after sync.
 	for i := 0; i < 1e5; i++ {
 		generation := uint64(i) % 2
-		date := uint64(i) % 3
+		date := uint64(i) % 2
 		metricID := uint64(i) % 123
 		c.Set(generation, date, metricID)
 	}
@@ -136,7 +136,7 @@ func testDateMetricIDCache(c *dateMetricIDCache, concurrent bool) error {
 	c.mu.Unlock()
 	for i := 0; i < 1e5; i++ {
 		generation := uint64(i) % 2
-		date := uint64(i) % 3
+		date := uint64(i) % 2
 		metricID := uint64(i) % 123
 		if !concurrent && !c.Has(generation, date, metricID) {
 			return fmt.Errorf("c.Has(%d, %d, %d) must return true after sync", generation, date, metricID)