lib/workingsetcache: reset the cache mode when the cache is reset

This should reduce memory usage if the working set is reduced after the cache reset.

docs/CHANGELOG.md

@@ -6,6 +6,8 @@ sort: 15
 
 ## tip
 
+* FEATURE: reduce memory usage when performing heavy queries over high number of time series.
+
 * BUGFIX: remove `{ %space %}` typo in `/targets` output. The typo has been introduced in v1.62.0. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1408).
 
 

lib/workingsetcache/cache.go

@@ -218,6 +218,8 @@ func (c *Cache) Reset() {
 	prev.Reset()
 	curr := c.curr.Load().(*fastcache.Cache)
 	curr.Reset()
+	// Reset the mode to `split` in the hope the working set size becomes smaller after the reset.
+	atomic.StoreUint64(&c.mode, split)
 }
 
 // UpdateStats updates fcs with cache stats.