Override the default VRAM cap on Apple Silicon to allocate more unified memory to GPU tasks like running local LLMs.

Apple Silicon’s unified memory architecture shares the same high-speed RAM between CPU and GPU — no discrete GPU required. This is why you can run large language models locally on a MacBook without a separate graphics card.

The Default VRAM Cap

macOS doesn’t expose 100% of your RAM as VRAM. On a 64GB machine, the Metal GPU driver typically caps VRAM at ~75–80% of total RAM — roughly 51GB out of 64GB:

That ~12GB gap is reserved for macOS and background processes. Reasonable for general use — but if your machine is dedicated to LLM inference, that’s 12GB of wasted context window or a smaller quantization than necessary.

Override the VRAM Limit

One sysctl command adjusts the GPU wired memory limit at runtime:

sudo sysctl iogpu.wired_limit_mb=61440

The value is in megabytes. Calculate it as desired_GB * 1024:

# 60GB → 60 * 1024 = 61440
sudo sysctl iogpu.wired_limit_mb=61440

Verify the change

sysctl iogpu.wired_limit_mb

iogpu.wired_limit_mb: 61440
```

No reboot required — takes effect immediately. Restart LM Studio (or whichever app reads VRAM) to pick up the new limit:

How Much to Allocate

Leave at least 4–8GB for macOS. Allocating 100% will cause system lockups, beachballs, or a hard reset.

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Persist Across Reboots

The sysctl change resets on every reboot. Create a Launch Daemon to apply it automatically.

Create the plist file:

sudo tee /Library/LaunchDaemons/com.local.iogpu-wired-limit.plist

Label
com.local.iogpu-wired-limit
ProgramArguments

/usr/sbin/sysctl
iogpu.wired_limit_mb=61440

RunAtLoad

EOF
```

Replace 61440 with your desired value from the table above.

Load the daemon:

sudo launchctl load /Library/LaunchDaemons/com.local.iogpu-wired-limit.plist

Verify it’s registered:

sudo launchctl list | grep iogpu
> - 0 com.local.iogpu-wired-limit

The VRAM override now applies on every boot — no manual intervention.

Remove the Override

To revert to default macOS behavior:

# Unload the daemon
sudo launchctl unload /Library/LaunchDaemons/com.local.iogpu-wired-limit.plist
# Delete the plist
sudo rm /Library/LaunchDaemons/com.local.iogpu-wired-limit.plist
# Reset to default (0 = system default)
sudo sysctl iogpu.wired_limit_mb=0

Important Notes

• This works on macOS Sonoma (14.x) and later with Apple Silicon
• The iogpu.wired_limit_mb parameter replaced the older debug.iogpu.wired_limit key
• No SIP disabling required — this is a runtime kernel parameter, not a protected config
• Setting the value to 0 restores default behavior
• Monitor Activity Monitor’s Memory tab after increasing — if memory pressure stays red, reduce the allocation