Improve prefetch (#483)

* Add prefetch-cache-size option

* Update and clarify documentation for prefetch including new prefetch-cache-size option

cmd/routedns/config.go

@@ -194,6 +194,7 @@ type group struct {
 	// Prefetch options
 	PrefetchWindow    time.Duration `toml:"prefetch-window"`    // Time period to track queries for prefetching, default 1h
 	PrefetchThreshold uint64        `toml:"prefetch-threshold"` // Minimum number of queries per window to enable prefetching, default 5
+	PrefetchCacheSize int			`toml:"prefetch-cache-size"` // Maximum number of items to cache
 	PrefetchMaxItems  int           `toml:"prefetch-max-items"` // Maximum number of items to track prefetch, default (0) unlimited
 }
 

cmd/routedns/example-config/prefetch.toml

@@ -10,8 +10,9 @@ resolvers = ["cloudflare-dot"]
 type = "prefetch"
 resolvers = ["cloudflare-cached"]
 prefetch-window = "15m"  # Minimum time between queries to remain eligible for prefetch
-prefetch-threshold = 3   # Min number of queries to enable prefetch
-prefetch-max-items = 100 # Max number of items to track for prefetch
+prefetch-threshold = 3   # Min number of queries required to enable prefetch
+prefetch-cache-size = 150 # Max number of queries to track for prefetch
+prefetch-max-items = 50  # Max number of items to prefetch
 
 [listeners.local-udp]
 address = "127.0.0.1:53"

cmd/routedns/main.go

@@ -699,6 +699,7 @@ func instantiateGroup(id string, g group, resolvers map[string]rdns.Resolver) er
 		opt := rdns.PrefetchOptions{
 			PrefetchWindow:    g.PrefetchWindow,
 			PrefetchThreshold: g.PrefetchThreshold,
+			PrefetchCacheSize: g.PrefetchCacheSize,
 			PrefetchMaxItems:  g.PrefetchMaxItems,
 		}
 		resolvers[id] = rdns.NewPrefetch(id, gr[0], opt)

doc/configuration.md

@@ -442,7 +442,7 @@ Example config files: [cache.toml](../cmd/routedns/example-config/cache.toml), [
 
 ### Prefetch
 
-While [Cache](#cache) has built-in prefetch capabilities, a the dedicated `prefetch` group may be more appropriate for some use cases. Its tracks the number of queries made and actively prefetches frequenty requested records. While it actively sends queries in order to refresh a cache, it does not cache responses itself and relies on a cache upstream from it.
+While [Cache](#cache) has built-in prefetch capabilities, a the dedicated `prefetch` group may be more appropriate for some use cases. Its tracks the number of queries made within a time window and actively prefetches frequenty requested records. While it actively sends queries in order to refresh a cache, it does not cache responses itself and relies on a cache upstream from it.
 
 #### Configuration
 
@@ -452,8 +452,9 @@ Options:
 
 - `resolvers` - Array of upstream resolvers, only one is supported.
 - `prefetch-window` - Minimum time between queries to remain eligible for prefetching. Supports time units `s`, `m`, and `h`. Default: 1h.
-- `prefetch-threshold` - Minimum number of queries for a name to enable prefetch. Default: 5.
-- `prefetch-max-items` - Maximum number of items to track for prefetch. Values that are too large can cause memory issues. Prefetch is disabled if 0 or not set. 
+- `prefetch-threshold` - Minimum number of queries required for a name to enable prefetch. Default: 5.
+- `prefetch-cache-size` - Maximum number of queries to track for prefetch. Values that are too large can cause memory issues. Prefetch is disabled if 0. Default: `prefetch-max-items * 3`.
+- `prefetch-max-items` - Maximum number of items to prefetch. Values that are too large can cause memory issues. Prefetch is disabled if 0 or not set.
 
 #### Examples
 
@@ -469,7 +470,8 @@ type = "prefetch"
 resolvers = ["cloudflare-cached"]
 prefetch-window = "15m"
 prefetch-threshold = 3
-prefetch-max-items = 100
+prefetch-cache-size = 150
+prefetch-max-items = 50
 ```
 
 Example config files: [prefetch.toml](../cmd/routedns/example-config/prefetch.toml)

prefetch.go

@@ -25,6 +25,7 @@ var _ Resolver = &RoundRobin{}
 type PrefetchOptions struct {
 	PrefetchWindow    time.Duration
 	PrefetchThreshold uint64
+	PrefetchCacheSize int
 	PrefetchMaxItems  int
 }
 
@@ -35,6 +36,9 @@ func NewPrefetch(id string, resolver Resolver, opt PrefetchOptions) *Prefetch {
 	if opt.PrefetchThreshold == 0 {
 		opt.PrefetchThreshold = 5
 	}
+	if opt.PrefetchCacheSize == 0 {
+		opt.PrefetchCacheSize = opt.PrefetchMaxItems * 3
+	}
 
 	r := &Prefetch{
 		id:       id,
@@ -42,7 +46,7 @@ func NewPrefetch(id string, resolver Resolver, opt PrefetchOptions) *Prefetch {
 		options:  opt,
 	}
 	r.nameCache = expirationcache.NewCache[atomic.Uint64](context.Background(), expirationcache.Options{
-		MaxSize: uint(opt.PrefetchMaxItems),
+		MaxSize: uint(opt.PrefetchCacheSize),
 	})
 	r.queryCache = expirationcache.NewCacheWithOnExpired(context.Background(), expirationcache.Options{
 		MaxSize: uint(opt.PrefetchMaxItems),