builder_worker: make number of workers configureable

Author: Mic92

buildbot_nix/buildbot_nix/worker.py

@@ -25,9 +25,11 @@ class WorkerConfig:
     worker_name: str = field(
         default_factory=lambda: os.environ.get("WORKER_NAME", socket.gethostname())
     )
-    worker_count: int = int(
-        os.environ.get("WORKER_COUNT", str(multiprocessing.cpu_count())),
-    )
+    worker_count_str: str | None = os.environ.get("WORKER_COUNT")
+    if worker_count_str is not None:
+        worker_count = int(worker_count_str)
+    else:
+        worker_count = multiprocessing.cpu_count()
     buildbot_dir: Path = field(
         default_factory=lambda: Path(require_env("BUILDBOT_DIR"))
     )

examples/worker.nix

@@ -3,5 +3,8 @@
   services.buildbot-nix.worker = {
     enable = true;
     workerPasswordFile = pkgs.writeText "worker-password-file" "XXXXXXXXXXXXXXXXXXXX";
+    # The number of workers to start (default: 0 == the number of CPU cores).
+    # If you experience flaky builds under high load, try to reduce this value.
+    # workers = 0;
   };
 }

nix/worker.nix

@@ -56,6 +56,14 @@ in
         default = pkgs.nix-eval-jobs;
         description = "nix-eval-jobs to use for evaluation";
       };
+      workers = lib.mkOption {
+        type = lib.types.int;
+        default = 0;
+        description = ''
+          The number of workers to start (default: 0 == to the number of CPU cores).
+          If you experience flaky builds under high load, try to reduce this value.
+        '';
+      };
       masterUrl = lib.mkOption {
         type = lib.types.str;
         default = "tcp:host=localhost:port=9989";
@@ -114,6 +122,7 @@ in
       }/${packages.python.sitePackages}";
       environment.MASTER_URL = cfg.masterUrl;
       environment.BUILDBOT_DIR = buildbotDir;
+      environment.WORKER_COUNT = builtins.toString cfg.workers;
 
       serviceConfig = {
         # We rather want the CI job to fail on OOM than to have a broken buildbot worker.