chore(build): limit concurrency based on available memory (#25480)

Our automatic build concurrency calculation only uses number of available CPUs as a limiting heuristic. Also add a dimension to limit based on available memory, because my machine runs out of memory every time it builds.


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

build.sh

@@ -8,6 +8,7 @@ check_prereqs="true"
 check_compat="true"
 ci="false"
 scope=""
+concurrency=""
 while [[ "${1:-}" != "" ]]; do
     case $1 in
         -h|--help)
@@ -32,6 +33,10 @@ while [[ "${1:-}" != "" ]]; do
         --ci)
           ci=true
           ;;
+        -c|--concurrency)
+            concurrency="$2"
+            shift
+            ;;
         *)
             echo "Unrecognized parameter: $1"
             exit 1
@@ -93,8 +98,14 @@ if [ "$run_tests" == "true" ]; then
     runtarget="$runtarget,test"
 fi
 
-# Limit top-level concurrency to available CPUs - 1 to limit CPU load.
-concurrency=$(node -p 'Math.max(1, require("os").cpus().length - 1)')
+if [[ "$concurrency" == "" ]]; then
+    # Auto-limit top-level concurrency to:
+    # - available CPUs - 1 to limit CPU load
+    # - total memory / 4GB  (N.B: constant here may need to be tweaked, configurable with $CDKBUILD_MEM_PER_PROCESS)
+    mem_per_process=${CDKBUILD_MEM_PER_PROCESS:-4_000_000_000}
+    concurrency=$(node -p "Math.max(1, Math.min(require('os').cpus().length - 1, Math.round(require('os').totalmem() / $mem_per_process)))")
+    echo "Concurrency: $concurrency"
+fi
 
 flags=""
 if [ "$ci" == "true" ]; then