Finer concurrency control than concurrentTasks

[brainrake]

concurrentTasks is not an adequate control for concurrency because evaluation, download, and build are all tasks. If concurrentTasks is too low, which depends on current load (lots of downloads that don't use cpu, eval waiting for IFD), the server is underutilised. However if it is too high, it runs out of memory easily.
To utilise a server adequately, we need to set limits CPU-intensive, network-intensive, and other (eg. eval waiting for IFD) tasks separately. Limiting concurrent jobs is also useful, to avoid slowdown caused by spreading resources too thin.
Eg.
concurrentJobs = 4
concurrentBuilds = 16
concurrentDownloads = 32
Number of evaluation tasks should be just enough to allow an eval for each job and IFD.

[L-as]

FWIW I think downloads and builds are considered the same by Nix