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Number of tasks to perform simultaneously.

A task is a single derivation build, an evaluation or an effect run. At minimum, you need 2 concurrent tasks for x86_64-linux in your cluster, to allow for import from derivation.

concurrentTasks 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.

    positive integer, meaning >0 or value "auto" (singular enum)
      _type = "literalMD";
      text = "`\"auto\"`, meaning equal to the number of CPU cores.\n";