Better handling of low memory situations

Drop down to a single high-mem task when we've got <=16GB, as the system
probably isn't completely free RAM, ninja will be using a couple gigs,
along with whatever "normal" actions will be running concurrently. So
it's unlikely that we can handle two 6-8GB actions along with everything
else.

Also print warnings when we detect <=16GB total RAM, or when we're
running more parallel jobs than than we have GB RAM. These both notify
the user and suggest lowering the `-j` value if they run into problems.

Bug: 141066538
Test: fake totalRAM to [0.5,8,16]GB, checking warning
Test: fake totalRAM to 17GB, `m -j4 nothing` has no warning
Test: `m -j187 nothing` on a 188GB system
Test: `m -j188 nothing` on a 188GB system
Change-Id: Ieb008e9f462d5f40fb65781d94cf116b1caf8446
Merged-In: Ieb008e9f462d5f40fb65781d94cf116b1caf8446
(cherry picked from commit 570a292bab)

ui/build/build.go

@@ -142,7 +142,26 @@ func help(ctx Context, config Config, what int) {
 func Build(ctx Context, config Config, what int) {
 	ctx.Verboseln("Starting build with args:", config.Arguments())
 	ctx.Verboseln("Environment:", config.Environment().Environ())
-	ctx.Verbosef("Total RAM: %dGB", config.TotalRAM()/1024/1024/1024)
+
+	if totalRAM := config.TotalRAM(); totalRAM != 0 {
+		ram := float32(totalRAM) / (1024 * 1024 * 1024)
+		ctx.Verbosef("Total RAM: %.3vGB", ram)
+
+		if ram <= 16 {
+			ctx.Println("************************************************************")
+			ctx.Printf("You are building on a machine with %.3vGB of RAM\n", ram)
+			ctx.Println("")
+			ctx.Println("The minimum required amount of free memory is around 16GB,")
+			ctx.Println("and even with that, some configurations may not work.")
+			ctx.Println("")
+			ctx.Println("If you run into segfaults or other errors, try reducing your")
+			ctx.Println("-j value.")
+			ctx.Println("************************************************************")
+		} else if ram <= float32(config.Parallel()) {
+			ctx.Printf("Warning: high -j%d count compared to %.3vGB of RAM", config.Parallel(), ram)
+			ctx.Println("If you run into segfaults or other errors, try a lower -j value")
+		}
+	}
 
 	ctx.BeginTrace(metrics.Total, "total")
 	defer ctx.EndTrace()

ui/build/config.go

@@ -738,6 +738,9 @@ func (c *configImpl) HighmemParallel() int {
 	} else if c.totalRAM == 0 {
 		// Couldn't detect the total RAM, don't restrict highmem processes.
 		return parallel
+	} else if c.totalRAM <= 16*1024*1024*1024 {
+		// Less than 16GB of ram, restrict to 1 highmem processes
+		return 1
 	} else if c.totalRAM <= 32*1024*1024*1024 {
 		// Less than 32GB of ram, restrict to 2 highmem processes
 		return 2