project/backend: optimize ProcessStats with singleton pattern

- Add getInstance() static method to ProcessStats class
- Convert all ProcessStats instantiations to use singleton
- Eliminates duplicate system calls (getconf CLK_TCK, PAGESIZE)
- Provides shared process state for better CPU percentage calculations
- Reduces memory footprint by reusing single /proc scanner
- Remove redundant ProcessStats instance in exec-stream.ts

Affected files:
- packages/backend/process-stats.ts: Add singleton pattern
- packages/project/exec-stream.ts: Use singleton, remove unused instance
- packages/backend/execute-code.ts: Use singleton
- packages/project/project-info/server.ts: Use singleton

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: haraldschilly

src/packages/backend/execute-code.ts

@@ -345,7 +345,7 @@ function doSpawn(
     const pid = child.pid;
     const { job_id, job_config } = opts;
     if (job_id == null || pid == null || job_config == null) return;
-    const monitor = new ProcessStats();
+    const monitor = ProcessStats.getInstance();
     await monitor.init();
     await delay(1000);
     if (callback_done) return;

src/packages/backend/process-stats.ts

@@ -38,6 +38,8 @@ interface ProcessStatsOpts {
 }
 
 export class ProcessStats {
+  private static instance: ProcessStats;
+
   private readonly testing: boolean;
   private readonly procLimit: number;
   private readonly dbg: Function;
@@ -51,6 +53,13 @@ export class ProcessStats {
     this.init();
   }
 
+  public static getInstance(opts?: ProcessStatsOpts): ProcessStats {
+    if (!ProcessStats.instance) {
+      ProcessStats.instance = new ProcessStats(opts);
+    }
+    return ProcessStats.instance;
+  }
+
   // this grabs some kernel configuration values we need. they won't change
   public init = reuseInFlight(async () => {
     if (this.ticks == null) {

src/packages/project/exec-stream.ts

@@ -156,13 +156,10 @@ async function executeStream(options) {
     let stats: ExecuteCodeStats = [];
 
     // Start process stats monitoring if we have a PID
-    let statsMonitor: ProcessStats | undefined;
     if (job.pid != null) {
       logger.debug(
         `executeStream: starting stats monitoring for PID ${job.pid}`,
       );
-      statsMonitor = new ProcessStats();
-      await statsMonitor.init();
       startStatsMonitoring(job.pid, job.start, stats, stream, () => done);
     } else {
       logger.debug(`executeStream: no PID available for stats monitoring`);
@@ -268,7 +265,7 @@ async function startStatsMonitoring(
   logger.debug(
     `startStatsMonitoring: beginning stats monitoring for PID ${pid}`,
   );
-  const monitor = new ProcessStats();
+  const monitor = ProcessStats.getInstance();
   await monitor.init();
   await delay(1000); // Initial delay
   if (isDone()) {

src/packages/project/project-info/server.ts

@@ -567,7 +567,7 @@ export class ProjectInfoServer extends EventEmitter {
     // Initialize tmpfs detection once at startup
     this.tmpIsMemoryBased = await isTmpMemoryBased();
     this.running = true;
-    this.processStats = new ProcessStats({
+    this.processStats = ProcessStats.getInstance({
       testing: this.testing,
       dbg: this.dbg,
     });