make the number of workers configurable

config/config.example.js

@@ -89,6 +89,14 @@ module.exports = {
  */
     //httpSafePort: 3001,
 
+/*  CryptPad will launch a child process for every core available
+ *  in order to perform CPU-intensive tasks in parallel.
+ *  Some host environments may have a very large number of cores available
+ *  or you may want to limit how much computing power CryptPad can take.
+ *  If so, set 'maxWorkers' to a positive integer.
+ */
+    // maxWorkers: 4,
+
     /* =====================
      *         Admin
      * ===================== */

lib/historyKeeper.js

@@ -253,6 +253,8 @@ module.exports.create = function (config, cb) {
             channelExpirationMs: config.channelExpirationMs,
             verbose: config.verbose,
             openFileLimit: config.openFileLimit,
+
+            maxWorkers: config.maxWorkers,
         }, w(function (err) {
             if (err) {
                 throw new Error(err);

lib/workers/index.js

@@ -193,7 +193,32 @@ Workers.initialize = function (Env, config, _cb) {
     };
 
     nThen(function (w) {
-        OS.cpus().forEach(function () {
+        const max = config.maxWorkers;
+
+        var limit;
+        if (typeof(max) !== 'undefined') {
+            // the admin provided a limit on the number of workers
+            if (typeof(max) === 'number' && !isNaN(max)) {
+                if (max < 1) {
+                    Log.info("INSUFFICIENT_MAX_WORKERS", max);
+                    limit = 1;
+                }
+            } else {
+                Log.error("INVALID_MAX_WORKERS", '[' + max + ']');
+            }
+        }
+
+        var logged;
+
+        OS.cpus().forEach(function (cpu, index) {
+            if (limit && index >= limit) {
+                if (!logged) {
+                    logged = true;
+                    Log.info('WORKER_LIMIT', "(Opting not to use available CPUs beyond " + index + ')');
+                }
+                return;
+            }
+
             initWorker(fork(DB_PATH), w(function (err) {
                 if (!err) { return; }
                 w.abort();