loginwindow on Mac: what it is and why it uses CPU

What loginwindow does

After the kernel and launchd, loginwindow is the next thing a Mac needs to become usable. It draws the login screen, checks your password or Touch ID against the directory service, and then becomes the owner of your session. In that role it launches the Dock, Finder, SystemUIServer and your login items, restores the windows from your last session if you asked for that, tracks which apps are running so it can relaunch the ones that should always be there, and handles the lock screen, the screen saver, Fast User Switching, and the logout, restart and shut down sequence. The Force Quit window you get with Command-Option-Escape is loginwindow's too.

Howard Oakley's write-up at The Eclectic Light Company puts it well: its obvious task is the login window, but it persists until the Mac shuts down, because the session is its job, not the window. There is one loginwindow per signed-in user and another one, running as root, that waits behind the login screen.

What normal looks like

Why loginwindow is using CPU right now

loginwindow rarely does heavy work of its own. High CPU under its name usually means it is being made to do something repeatedly.

1. You just logged in. The first minute or two after login is loginwindow launching the Dock, Finder, login items and any apps restored from last time. On a Mac with many login items this is a real burst. It ends on its own.
2. An app keeps crashing and being relaunched. loginwindow is responsible for relaunching some session apps when they die. If one of them crashes on launch, it goes round in a loop, and the loop shows up as loginwindow CPU. The Console app's Crash Reports list tells you which app.
3. A broken login item or helper. An old utility that fails to start, asks for permission, fails again. System Settings, General, Login Items & Extensions is the list to check.
4. The screen saver or lock screen. Some screen savers are expensive to draw, and loginwindow hosts them. If CPU is high only while the Mac is locked, that is the saver.
5. A stuck logout or user switch. If you started a logout and an app refused to quit, loginwindow may be waiting and retrying. Finish or cancel the logout.
6. A bad state after a macOS update. The Apple Community thread quoted below was a Catalina user with 99 percent loginwindow and constant fans. A restart and an update were the answer then, and still are.

Is it safe to quit loginwindow?

No. This is the one process in this list where Force Quit has the biggest immediate consequence. loginwindow owns your session. End it, from Activity Monitor or any other way, and macOS ends the session with it: every app closes without saving, every window disappears, and you are back at the login screen. It is equivalent to a forced logout. Apple knows this, which is why Activity Monitor asks for confirmation and why Terminal needs administrator rights to touch it. There is no troubleshooting value in it either. If you want a clean session, use the Apple menu, Log Out, which gives apps a chance to save.

Use the checker above for the processes that loginwindow starts, because they are usually the real source of a busy session: Dock, Finder, SystemUIServer, and whatever sits in your Login Items.

What to do, in order

1. Wait two minutes after login

   If you just signed in, the burst is normal. Make a coffee. If loginwindow is back at zero when you return, there was never a problem.

2. Look for an app that keeps relaunching

   Watch the Dock for an icon that appears, disappears and appears again. Open Console, click Crash Reports in the sidebar, and look for the same app name repeating with recent timestamps. Quit it, or remove it from Login Items.

3. Trim Login Items

   System Settings, General, Login Items & Extensions. Remove anything you do not recognize or no longer use, and switch off background items from apps you have uninstalled. Then log out and back in to see the difference.

4. Change the screen saver

   If CPU is high only while locked, System Settings, Screen Saver: pick a simple one, or set the display to turn off instead of running a saver.

5. Log out and back in

   Apple menu, Log Out. This ends the session cleanly and gives you a fresh loginwindow with your login items started from scratch. Most stuck sessions are fixed here.

6. Restart, then update macOS

   A restart resets the root loginwindow as well as yours. If the problem began after an update, Software Update is where the fix will arrive. Safe mode (hold Shift at startup on Intel, or choose it from the startup options on Apple silicon) starts a session with no login items, which confirms whether one of them is the cause.

When to worry

Worry when loginwindow stays above 90 percent after a fresh login with no login items running, and safe mode shows the same thing. That is rare, and it points to macOS itself, which a Software Update or a reinstall over the top (which keeps your files) addresses. Worry a little if loginwindow's memory grows past a gigabyte and keeps growing; an old MacRumors report described five gigabytes, and a logout cleared it. Do not worry about the login burst, the lock screen, or a spike when you switch users. Those are loginwindow doing what it exists for, and the number falls as soon as the task ends.

Safe to Quit, the Mac app behind this guide, marks loginwindow as one to leave alone and tells you what the processes under it are doing instead, so you do not force quit the one thing that would end your session.