In /proc/stat, guest is already inside user (and guest_nice inside nice). Peardata was adding guest into the total again, which shrinks every other slice — including idle.

readFileCached capped at 64KB. On 56-core hosts /proc/interrupts (and similar) often exceeds that — reads were silently truncated. Buffer now grows (up to 1MB). That didn’t break system.cpu idle (aggregate line is first), but it was a real high-core regression.

server/utils/fd-cache.js

@@ -1,6 +1,8 @@
 import fs from 'fs'
 
-const READ_BUF = Buffer.alloc(65536)
+/** Shared buffer — grows if a read fills it (e.g. /proc/interrupts on 56-core hosts). */
+let readBuf = Buffer.alloc(65536)
+const MAX_READ_BUF = 1024 * 1024
 const SMALL_BUF = Buffer.alloc(8192)
 const fdCache = new Map()
 
@@ -18,8 +20,21 @@ export function readFileCached(path) {
   }
   if (!entry) return null
   try {
-    const bytesRead = fs.readSync(entry.fd, READ_BUF, 0, READ_BUF.length, 0)
-    return READ_BUF.toString('utf8', 0, bytesRead)
+    let bytesRead = fs.readSync(entry.fd, readBuf, 0, readBuf.length, 0)
+    // Filled the buffer → likely truncated; grow and retry (common on high-core /proc files)
+    while (bytesRead === readBuf.length && readBuf.length < MAX_READ_BUF) {
+      readBuf = Buffer.alloc(Math.min(readBuf.length * 4, MAX_READ_BUF))
+      bytesRead = fs.readSync(entry.fd, readBuf, 0, readBuf.length, 0)
+    }
+    if (bytesRead === readBuf.length) {
+      // Still truncated at cap — full read for correctness
+      try {
+        return fs.readFileSync(path, 'utf8')
+      } catch {
+        /* fall through with truncated buffer */
+      }
+    }
+    return readBuf.toString('utf8', 0, bytesRead)
   } catch {
     return null
   }