fix: decrease default routing table size (#3023)

Reduces the default routing table size to 1280.

This improves CPU/memory/network usage as we don't need to sort as
many peers when finding the closest ones to a given key, nor do we
need to contact as many to ensure only live peers are still in the
table.

Author: achingbrain

packages/kad-dht/src/index.ts

@@ -428,16 +428,37 @@ export interface KadDHTInit {
    * can be stored.
    *
    * Storing more peers means fewer lookups (and network operations) are needed
-   * to locate a certain peer, but also that more memory is consumed.
+   * to locate a certain peer, but also that more memory is consumed and more
+   * CPU while responding to queries (e.g. with more peers in the table sorting
+   * the closest peers becomes more expensive) and CPU/network during table
+   * maintenance (e.g. checking peers are still online).
    *
-   * @default 32
+   * The larger this value, the more prefix bits must be the same for a peer to
+   * be stored in a KAD bucket, so the fewer nodes that bucket is likely to
+   * contain.
+   *
+   * The total number of peers in the table is a factor of `prefixLength` and
+   * `kBucketSize`:
+   *
+   * ```
+   * (2 ^ prefixLength) * kBucketSize
+   * ```
+   *
+   * @default 6
    */
   prefixLength?: number
 
   /**
    * If true, only ever be a DHT client. If false, be a DHT client until told
    * to be a DHT server via `setMode`.
    *
+   * In general this should be left as the default because server mode will be
+   * selected automatically when libp2p establishes that the current node has
+   * a publicly dialable address.
+   *
+   * The exception to this is LAN-only DHT (e.g. for testing purposes) where it
+   * is safe to assume that the current node is dialable.
+   *
    * @default false
    */
   clientMode?: boolean

packages/kad-dht/src/routing-table/index.ts

@@ -14,7 +14,7 @@ import type { AbortOptions, ComponentLogger, CounterGroup, Logger, Metric, Metri
 import type { AdaptiveTimeoutInit } from '@libp2p/utils/adaptive-timeout'
 
 export const KBUCKET_SIZE = 20
-export const PREFIX_LENGTH = 8
+export const PREFIX_LENGTH = 6
 export const PING_NEW_CONTACT_TIMEOUT = 2000
 export const PING_NEW_CONTACT_CONCURRENCY = 20
 export const PING_NEW_CONTACT_MAX_QUEUE_SIZE = 100

packages/kad-dht/test/routing-table.spec.ts

@@ -484,4 +484,40 @@ describe('Routing Table', () => {
 
     await expect(table.find(peer.id)).to.eventually.be.ok()
   })
+
+  describe('max size', () => {
+    it('should constrain size to 10', async () => {
+      const prefixLength = 8
+      const kBucketSize = 20
+      const maxSize = Math.pow(2, prefixLength) * kBucketSize
+
+      table = new RoutingTable(components, {
+        logPrefix: '',
+        metricsPrefix: '',
+        protocol: PROTOCOL,
+        network,
+        prefixLength,
+        kBucketSize
+      })
+      await start(table)
+
+      // reset network stub so we can have specific behavior
+      table.network = network = stubInterface()
+
+      // all old peers answer pings, no peers should be evicted
+      network.sendRequest.callsFake(async function * (from: PeerId) {
+        yield peerResponseEvent({
+          from,
+          messageType: MessageType.PING
+        })
+      })
+
+      for (let i = 0; i < 2 * maxSize; i++) {
+        const remotePeer = await createPeerId()
+        await table.add(remotePeer)
+      }
+
+      expect(table.size).to.be.lessThanOrEqual(maxSize)
+    })
+  })
 })