allocate swap size for containers based on system ratio

src/system.js

@@ -3,7 +3,8 @@
 exports = module.exports = {
     getDisks,
     checkDiskSpace,
-    getMemory
+    getMemory,
+    getMemoryAllocation
 };
 
 const apps = require('./apps.js'),
@@ -148,14 +149,24 @@ function checkDiskSpace(callback) {
     });
 }
 
-function getMemory(callback) {
-    assert.strictEqual(typeof callback, 'function');
-
+function getSwapSize() {
     const stdout = safe.child_process.execSync('swapon --noheadings --raw --bytes --show=SIZE', { encoding: 'utf8' });
     const swap = !stdout ? 0 : stdout.trim().split('\n').map(x => parseInt(x, 10) || 0).reduce((acc, cur) => acc + cur);
 
+    return swap;
+}
+
+function getMemory(callback) {
+    assert.strictEqual(typeof callback, 'function');
+
     callback(null, {
         memory: os.totalmem(),
-        swap: swap
+        swap: getSwapSize()
     });
 }
+
+function getMemoryAllocation(limit) {
+    const pc = os.totalmem() / (os.totalmem() + getSwapSize());
+    const ratio = Math.round(pc * 10) / 10; // a simple ratio
+    return Math.round(Math.round(limit * ratio) / 1048576) * 1048576; // nearest MB
+}