Optimize real-time updates in downloads service with throttling to reduce CPU usage

ui/src/app/app.ts

@@ -1,7 +1,7 @@
 import { AsyncPipe, DatePipe, KeyValuePipe } from '@angular/common';
 import { HttpClient } from '@angular/common/http';
 import { AfterViewInit, ChangeDetectionStrategy, ChangeDetectorRef, Component, DestroyRef, ElementRef, viewChild, inject, OnDestroy, OnInit } from '@angular/core';
-import { Observable, map, distinctUntilChanged } from 'rxjs';
+import { Observable, map, distinctUntilChanged, auditTime } from 'rxjs';
 import { FormsModule } from '@angular/forms';
 import { takeUntilDestroyed } from '@angular/core/rxjs-interop';
 import { FontAwesomeModule } from '@fortawesome/angular-fontawesome';
@@ -250,8 +250,13 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
       this.rebuildSortedDone();
       this.cdr.markForCheck();
     });
-    // Subscribe to real-time updates
-    this.downloads.updated.pipe(takeUntilDestroyed(this.destroyRef)).subscribe(() => {
+    // Subscribe to real-time updates (throttled to reduce CPU on large queues).
+    this.downloads.updated
+    .pipe(
+      auditTime(200),
+      takeUntilDestroyed(this.destroyRef)
+    )
+    .subscribe(() => {
       this.updateMetrics();
       this.cdr.markForCheck();
     });

ui/src/app/services/downloads.service.ts

@@ -36,6 +36,9 @@ export class DownloadsService {
   ytdlOptionsChanged = new Subject<Record<string, unknown>>();
   configurationChanged = new Subject<Record<string, unknown>>();
   updated = new Subject<void>();
+  private updateRefreshScheduled = false;
+  private readonly foregroundRefreshMs = 250;
+  private readonly backgroundRefreshMs = 30000;
 
   configuration: Record<string, unknown> = {};
   customDirs: Record<string, string[]> = {};
@@ -68,7 +71,7 @@ export class DownloadsService {
       data.checked = !!dl?.checked;
       data.deleting = !!dl?.deleting;
       this.queue.set(data.url, data);
-      this.updated.next();
+      this.scheduleUpdatedRefresh();
     });
     this.socket.fromEvent('completed')
     .pipe(takeUntilDestroyed())
@@ -117,6 +120,28 @@ export class DownloadsService {
     });
   }
 
+  private scheduleUpdatedRefresh() {
+    // Coalesce high-frequency download progress events into a capped refresh rate.
+    // requestAnimationFrame (~60fps) is smooth but expensive on large queues.
+    // We trade a little smoothness for much lower CPU:
+    // - foreground: ~8fps
+    // - background: ~1fps
+    if (this.updateRefreshScheduled) {
+      return;
+    }
+    this.updateRefreshScheduled = true;
+
+    const flush = () => {
+      this.updateRefreshScheduled = false;
+      this.updated.next();
+    };
+
+    const delay = document.hidden
+      ? this.backgroundRefreshMs
+      : this.foregroundRefreshMs;
+    setTimeout(() => flush(), delay);
+  }
+
   handleHTTPError(error: HttpErrorResponse) {
     const msg = error.error instanceof ErrorEvent
       ? error.error.message