Improve calculation for used system memory

The memory information reported by the supervisor currently
estimates the value of used memory as `MemTotal - MemFree`.
However, linux systems will try to cache and buffer as much
memory as possible, which will affect the output of `MemFree`
(from /proc/meminfo) and in consequence the memory usage seen
by the user on the dashboard, which will appear much greater than
it is.

The correct calculation should be `MemTotal - MemFree - Buffers - Cached`,
which the calculation performed by `htop` and the `free` commands.

Change-type: patch
Signed-off-by: Felipe Lalanne <felipe@balena.io>
Connects-to: #1471

src/lib/system-info.ts

@@ -54,7 +54,7 @@ export async function getStorageInfo() {
 export async function getMemoryInformation() {
 	const mem = await systeminformation.mem();
 	return {
-		used: bytesToMb(mem.used),
+		used: bytesToMb(mem.used - mem.cached - mem.buffers),
 		total: bytesToMb(mem.total),
 	};
 }

test/35-system-info.spec.ts

@@ -0,0 +1,38 @@
+import { SinonStub, stub } from 'sinon';
+import * as systeminformation from 'systeminformation';
+
+import { getMemoryInformation } from '../src/lib/system-info';
+import { expect } from './lib/chai-config';
+
+function kbToMb(kilobytes: number) {
+	return Math.floor(kilobytes / 1024);
+}
+
+describe('System information', function () {
+	it('Returns the correct memory values', async () => {
+		// Stub the output of the systeminformation module
+		stub(systeminformation, 'mem').resolves({
+			total: 763472 * 1024,
+			free: 143896 * 1024,
+			used: (763472 - 143896) * 1024,
+			cached: 368360 * 1024,
+			buffers: 16724 * 1024,
+			slab: 86736 * 1024,
+			buffcache: (368360 + 16724 + 86736) * 1024,
+			available: 558376 * 1024,
+			active: (763472 - 558376) * 1024,
+			swaptotal: 190864 * 1024,
+			swapfree: 190864 * 1024,
+			swapused: 0,
+		});
+
+		const { used, total } = await getMemoryInformation();
+		expect(total).to.be.equal(kbToMb(763472));
+
+		// used memory = total - free - (cached + buffers)
+		// this is how `htop` and `free` calculate it
+		expect(used).to.be.equal(kbToMb(763472 - 143896 - (368360 + 16724)));
+
+		(systeminformation.mem as SinonStub).restore();
+	});
+});