feat: Use env variable to configure number of workers (#1233)

* feat: Use env variable to configure number of workers

* feat: Use CPU size from /sys/fs

src/ancillary/multiprocess.ts

@@ -1,6 +1,43 @@
 import EventEmitter from 'node:events'
 import cluster from 'node:cluster'
 import { cpus } from 'node:os'
+import { readFileSync } from 'node:fs'
+
+/**
+ * Read a number from a file.
+ */
+function readNumber(filename: string | URL): number {
+  return Number(readFileSync(filename, 'utf-8'))
+}
+
+/**
+ * Read CPU limits from /sys/fs.
+ * See https://www.kernel.org/doc/Documentation/scheduler/sched-bwc.txt
+ * and https://github.com/xiaoxiaojx/get_cpus_length
+ */
+function readCPULimits(): number {
+  try {
+    return (
+      readNumber('/sys/fs/cgroup/cpu/cpu.cfs_quota_us') /
+      readNumber('/sys/fs/cgroup/cpu/cpu.cfs_period_us')
+    )
+  } catch (err) {
+    return -1
+  }
+}
+
+/**
+ * Return number of CPUs available.
+ */
+function cpuSize(): number {
+  const maybeResult = readCPULimits()
+
+  if (maybeResult > 0) {
+    return maybeResult
+  }
+
+  return cpus().length
+}
 
 export type MultiprocessOptions = {
   /**
@@ -64,7 +101,10 @@ export class Multiprocess extends EventEmitter {
       this.work()
       return
     }
-    let processes = options.workers || cpus().length // TODO workers = -1 or undef means no workers
+    const workersFromEnv = process.env['MULTIPROCESS_SIZE']
+      ? parseInt(process.env['MULTIPROCESS_SIZE'], 10)
+      : undefined
+    let processes = options.workers || workersFromEnv || cpuSize()
     process.on('SIGINT', this.stop).on('SIGTERM', this.stop)
     cluster.on('online', (wrk) => {
       this.emit('worker', wrk.process.pid)