Add configuration for LRU Cache Size (vercel#21535)

This allows the user to adjust the LRU cache size according to the
application need:

- optimize the cache size based on average page HTML & JSON size
- remove the cache limit and let it grow to cache all pages
- make it so small to practically disable it

The hard-coded default of 50MB has been moved from the code to the
default configuration object.

Fixes vercel#21535
See also vercel#27325

docs/api-reference/next.config.js/lrucachesize.md

@@ -0,0 +1,30 @@
+---
+description: Control the size of the LRU cache in Next.js
+---
+
+# LRU Cache Size
+
+<details>
+  <summary><b>Version History</b></summary>
+
+| Version   | Changes               |
+| --------- | --------------------- |
+| `v11.0.2` | LRU Cache Size added. |
+
+</details>
+
+Next.js keeps the data for frequently accessed paths in a least recently used (LRU) cache in memory.
+`lruCacheSize` allows you to adjust the cache size for the application.
+
+Setting it to 0 will remove the size restriction and the cache will only grow.
+
+Setting it to 1 will basically disable the cache as it will only ever contain the data for the last
+accessed path.
+
+The default is 50MB:
+
+```js
+module.exports = {
+  lruCacheSize: 50 * 1024 * 1024,
+}
+```

packages/next/server/config-shared.ts

@@ -138,6 +138,8 @@ export const defaultConfig: NextConfig = {
   serverRuntimeConfig: {},
   publicRuntimeConfig: {},
   reactStrictMode: false,
+  // default to 50MB limit
+  lruCacheSize: 50 * 1024 * 1024,
   experimental: {
     cpus: Math.max(
       1,

packages/next/server/incremental-cache.ts

@@ -51,7 +51,7 @@ export class IncrementalCache {
     locales,
   }: {
     dev: boolean
-    max?: number
+    max: number
     distDir: string
     pagesDir: string
     flushToDisk?: boolean
@@ -81,8 +81,7 @@ export class IncrementalCache {
     }
 
     this.cache = new LRUCache({
-      // default to 50MB limit
-      max: max || 50 * 1024 * 1024,
+      max,
       length({ value }) {
         if (!value || value.kind === 'REDIRECT') return 25
         // rough estimate of size of cache value

packages/next/server/next-server.ts

@@ -270,6 +270,7 @@ export default class Server {
         'pages'
       ),
       locales: this.nextConfig.i18n?.locales,
+      max: this.nextConfig.lruCacheSize,
       flushToDisk: !minimalMode && this.nextConfig.experimental.sprFlushToDisk,
     })
     this.responseCache = new ResponseCache(this.incrementalCache)