Add .envrc file to set maximum number of test threads

We noticed that total test run time was much longer (30+ minutes vs. 10
minutes) on our new 16 logical core machines than our older 8 logical
core machines.

The tests seem to spend a long time starting parallel threads, either
because the (many) `require` statements in the Rakefile take a long time
when running many cores, or because they're waiting for a lockfile used
by the `parallel` gem.

Co-authored-by: Nat Bennett <[email protected]>
Co-authored-by: Matt Royal <[email protected]>

.envrc

@@ -0,0 +1,10 @@
+cores=$(/usr/sbin/sysctl -n hw.logicalcpu)
+
+if (( cores > 8 )); then
+  # This environment variable overrides the `parallel_test` gem's default behavior
+  # which is to use all available logical cores.
+  # Running with 16 cores is very slow for reasons we don't fully understand
+  # but are probably due to lock file or disk resource contention
+
+  export PARALLEL_TEST_PROCESSORS=8
+fi