Last tested: Jul 1, 2019
--concurrent-render-jobs startup flag determines how many Chromium processes can be creating PNGs or PDFs at any given time. The default is 2, and each render thread means another Chromium process would be starting up outside of Looker, which can be process intensive.
Going over 2 should only be done in unique circumstances, but for really large instances a general rough formula is
minimum(cpu core count - 2, (total memory - java heap reservation ) / 2gb )
As an example, a m5.4xlarge instance has 16 cores and 64gb of RAM. The default Looker startup script allocates . 60% of the total ram to the Looker process, so the formula would be
minimum((16 cores - 2), (64gb - 38.4gb)/2gb) = minimum(14, 12.8) = 12
It is always safer to go a little lower.