Take into account RAM usage for multiproduct_kati

Apparently we have some instances of Soong taking 16GB of RAM. While
that's a problem to solve on it's own, make multiproduct_kati's
auto-parallelism detection smarter by taking into account the total RAM
on the machine instead of just the number of CPUs.

Bug: 146925549
Test: check soong.log for autodetected parallelism values
Change-Id: Ice34c2cf73ea4f8f235521cbefc8654a79a04eef

cmd/multiproduct_kati/main.go

@@ -37,18 +37,7 @@ import (
 	"android/soong/zip"
 )
 
-// We default to number of cpus / 4, which seems to be the sweet spot for my
-// system. I suspect this is mostly due to memory or disk bandwidth though, and
-// may depend on the size ofthe source tree, so this probably isn't a great
-// default.
-func detectNumJobs() int {
-	if runtime.NumCPU() < 4 {
-		return 1
-	}
-	return runtime.NumCPU() / 4
-}
-
-var numJobs = flag.Int("j", detectNumJobs(), "number of parallel kati jobs")
+var numJobs = flag.Int("j", 0, "number of parallel jobs [0=autodetect]")
 
 var keepArtifacts = flag.Bool("keep", false, "keep archives of artifacts")
 var incremental = flag.Bool("incremental", false, "run in incremental mode (saving intermediates)")
@@ -233,6 +222,21 @@ func main() {
 		trace.SetOutput(filepath.Join(config.OutDir(), "build.trace"))
 	}
 
+	var jobs = *numJobs
+	if jobs < 1 {
+		jobs = runtime.NumCPU() / 4
+
+		ramGb := int(config.TotalRAM() / 1024 / 1024 / 1024)
+		if ramJobs := ramGb / 20; ramGb > 0 && jobs > ramJobs {
+			jobs = ramJobs
+		}
+
+		if jobs < 1 {
+			jobs = 1
+		}
+	}
+	log.Verbosef("Using %d parallel jobs", jobs)
+
 	setMaxFiles(log)
 
 	finder := build.NewSourceFinder(buildCtx, config)
@@ -318,7 +322,7 @@ func main() {
 	}()
 
 	var wg sync.WaitGroup
-	for i := 0; i < *numJobs; i++ {
+	for i := 0; i < jobs; i++ {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()