Use process.env.PARCEL_MAX_CONCURRENT_CALLS environment variable (#1595)

CONTRIBUTING.md

@@ -61,6 +61,8 @@ You can set `PARCEL_WORKERS` to the number of worker processes to spawn.
 
 **NOTE:** When developing plugins or new Asset types, run with `--no-cache` (or pass `cache: false` to `Bundler` options). Parcel uses caching by default, but during development you'll normally pass incomplete results into the cache. This can leave you wondering why you're constantly seeing old results.
 
+You can set `PARCEL_MAX_CONCURRENT_CALLS` to change the limit of concurrent calls per worker.
+
 ## Link to local parcel build
 
 While contributing to parcel, you may need to run a local version of parcel and set it as the global cli option in your command line, so that when you run `parcel ...`, it will find your new local variant of parcel instead of the global package installed before (if you had it installed globally before).

src/workerfarm/WorkerFarm.js

@@ -10,7 +10,7 @@ class WorkerFarm extends EventEmitter {
     this.options = Object.assign(
       {
         maxConcurrentWorkers: WorkerFarm.getNumWorkers(),
-        maxConcurrentCallsPerWorker: 10,
+        maxConcurrentCallsPerWorker: WorkerFarm.getConcurrentCallsPerWorker(),
         forcedKillTime: 100,
         warmWorkers: true,
         useLocalWorker: true,
@@ -276,6 +276,10 @@ class WorkerFarm extends EventEmitter {
     }
     return cores || 1;
   }
+
+  static getConcurrentCallsPerWorker() {
+    return parseInt(process.env.PARCEL_MAX_CONCURRENT_CALLS) || 10;
+  }
 }
 
 module.exports = WorkerFarm;