* (global): added --no-mem-optimization flag

This commit adds a new command-line argument that disables memory
optimizations AGH is using. These memory optimizations might be
necessary on low-memory devices, but they aren't free and there's a
performance hit (see #2044). Now they can be disabled - just pass
--no-mem-optimization when you run AGH or when you install the service
-- ./AdGuardHome -s install --no-mem-optimization

Closes: #2044

home/memory.go

@@ -0,0 +1,44 @@
+package home
+
+import (
+	"os"
+	"runtime/debug"
+	"time"
+
+	"github.com/AdguardTeam/golibs/log"
+)
+
+// memoryUsage implements a couple of not really beautiful hacks which purpose is to
+// make OS reclaim the memory freed by AdGuard Home as soon as possible.
+// See this for the details on the performance hits & gains:
+// https://github.com/AdguardTeam/AdGuardHome/issues/2044#issuecomment-687042211
+func memoryUsage(args options) {
+	if args.disableMemoryOptimization {
+		log.Info("Memory optimization is disabled")
+		return
+	}
+
+	// Makes Go allocate heap at a slower pace
+	// By default we keep it at 50%
+	debug.SetGCPercent(50)
+
+	// madvdontneed: setting madvdontneed=1 will use MADV_DONTNEED
+	// instead of MADV_FREE on Linux when returning memory to the
+	// kernel. This is less efficient, but causes RSS numbers to drop
+	// more quickly.
+	_ = os.Setenv("GODEBUG", "madvdontneed=1")
+
+	// periodically call "debug.FreeOSMemory" so
+	// that the OS could reclaim the free memory
+	go func() {
+		ticker := time.NewTicker(5 * time.Minute)
+		for {
+			select {
+			case t := <-ticker.C:
+				t.Second()
+				log.Debug("Free OS memory")
+				debug.FreeOSMemory()
+			}
+		}
+	}()
+}