nixos/hercules-ci-agent: Set default concurrency to auto

1 files changed, 10 insertions, 3 deletions

diff --git a/nixos/modules/services/continuous-integration/hercules-ci-agent/common.nix b/nixos/modules/services/continuous-integration/hercules-ci-agent/common.nix
index 9f9b86ee61c..c6f743a71d4 100644
--- a/nixos/modules/services/continuous-integration/hercules-ci-agent/common.nix
+++ b/nixos/modules/services/continuous-integration/hercules-ci-agent/common.nix
@@ -37,15 +37,22 @@ let
         description = ''
           Number of tasks to perform simultaneously.
 
-          A task is a single derivation build or an evaluation.
+          A task is a single derivation build, an evaluation or an effect run.
           At minimum, you need 2 concurrent tasks for <literal>x86_64-linux</literal>
           in your cluster, to allow for import from derivation.
 
           <literal>concurrentTasks</literal> can be around the CPU core count or lower if memory is
           the bottleneck.
+
+          The optimal value depends on the resource consumption characteristics of your workload,
+          including memory usage and in-task parallelism. This is typically determined empirically.
+
+          When scaling, it is generally better to have a double-size machine than two machines,
+          because each split of resources causes inefficiencies; particularly with regards
+          to build latency because of extra downloads.
         '';
-        type = types.int;
-        default = 4;
+        type = types.either types.ints.positive (types.enum [ "auto" ]);
+        default = "auto";
       };
       workDirectory = mkOption {
         description = ''