From 92f90d552a49e48d737bb5b2635d0e0e05007d77 Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Sun, 13 Feb 2022 21:35:53 +0100
Subject: [PATCH] cli: set default parallelism to cpus/2

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
---
 packages/cli/src/lib/parallel.test.ts | 16 ++++++++++------
 packages/cli/src/lib/parallel.ts      |  8 ++++----
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/packages/cli/src/lib/parallel.test.ts b/packages/cli/src/lib/parallel.test.ts
index 4fbcae2191..032d66afac 100644
--- a/packages/cli/src/lib/parallel.test.ts
+++ b/packages/cli/src/lib/parallel.test.ts
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import os from 'os';
 import {
   parseParallelismOption,
   getEnvironmentParallelism,
@@ -22,6 +23,8 @@ import {
   runWorkerThreads,
 } from './parallel';
 
+const defaultParallelism = Math.ceil(os.cpus().length / 2);
+
 describe('parseParallelismOption', () => {
   it('coerces false no parallelism', () => {
     expect(parseParallelismOption(false)).toBe(1);
@@ -29,10 +32,10 @@ describe('parseParallelismOption', () => {
   });
 
   it('coerces true or undefined to default parallelism', () => {
-    expect(parseParallelismOption(true)).toBe(4);
-    expect(parseParallelismOption('true')).toBe(4);
-    expect(parseParallelismOption(undefined)).toBe(4);
-    expect(parseParallelismOption(null)).toBe(4);
+    expect(parseParallelismOption(true)).toBe(defaultParallelism);
+    expect(parseParallelismOption('true')).toBe(defaultParallelism);
+    expect(parseParallelismOption(undefined)).toBe(defaultParallelism);
+    expect(parseParallelismOption(null)).toBe(defaultParallelism);
   });
 
   it('coerces number string to number', () => {
@@ -52,13 +55,13 @@ describe('getEnvironmentParallelism', () => {
     expect(getEnvironmentParallelism()).toBe(2);
 
     process.env.BACKSTAGE_CLI_BUILD_PARALLEL = 'true';
-    expect(getEnvironmentParallelism()).toBe(4);
+    expect(getEnvironmentParallelism()).toBe(defaultParallelism);
 
     process.env.BACKSTAGE_CLI_BUILD_PARALLEL = 'false';
     expect(getEnvironmentParallelism()).toBe(1);
 
     delete process.env.BACKSTAGE_CLI_BUILD_PARALLEL;
-    expect(getEnvironmentParallelism()).toBe(4);
+    expect(getEnvironmentParallelism()).toBe(defaultParallelism);
   });
 });
 
@@ -70,6 +73,7 @@ describe('runParallelWorkers', () => {
 
     const work = runParallelWorkers({
       items: [0, 1, 2, 3, 4],
+      parallelismSetting: 4,
       parallelismFactor: 0.5, // 2 at a time
       worker: async item => {
         started.push(item);
diff --git a/packages/cli/src/lib/parallel.ts b/packages/cli/src/lib/parallel.ts
index d51e9b3b2d..7b4ae4609e 100644
--- a/packages/cli/src/lib/parallel.ts
+++ b/packages/cli/src/lib/parallel.ts
@@ -18,17 +18,17 @@ import os from 'os';
 import { ErrorLike } from '@backstage/errors';
 import { Worker } from 'worker_threads';
 
-export const DEFAULT_PARALLELISM = 4;
+const defaultParallelism = Math.ceil(os.cpus().length / 2);
 
-export const PARALLEL_ENV_VAR = 'BACKSTAGE_CLI_BUILD_PARALLEL';
+const PARALLEL_ENV_VAR = 'BACKSTAGE_CLI_BUILD_PARALLEL';
 
 export type ParallelismOption = boolean | string | number | null | undefined;
 
 export function parseParallelismOption(parallel: ParallelismOption): number {
   if (parallel === undefined || parallel === null) {
-    return DEFAULT_PARALLELISM;
+    return defaultParallelism;
   } else if (typeof parallel === 'boolean') {
-    return parallel ? DEFAULT_PARALLELISM : 1;
+    return parallel ? defaultParallelism : 1;
   } else if (typeof parallel === 'number' && Number.isInteger(parallel)) {
     if (parallel < 1) {
       return 1;