MacBooks without batteries severely throttled #

MacBook and MacBook Pro: Mac reduces processor speed when battery is removed while operating from an A/C adaptor - "If the battery is removed from a MacBook or MacBook Pro, the computer will automatically reduce the processor speed. This prevents the computer from shutting down if it demands more power than the A/C adaptor alone can provide."
Apple Notebooks Take Huge Performance Hit without Battery - "Apple's reason for forcing a drop in processor speed doesn't make a whole lot of sense."
MacBooks Run Slow Without Battery - "Gearlog's own results show a decrease of 36 percent in processor speeds without a battery attached on their unibody MacBook Pro. Our own tests show a decrease of 50 percent when rendering with one CPU, a decrease of 52 percent in multiprocessor rendering, while the GPU suffered a 40 percent decrease in performance."
"Fixing" kernel_task CPU Problems in MacOS 10.7/10.8 - Tweak IOPlatformPluginFamily.kext to disable throttling on faulty battery.
Disable OS X kernel_task throttling - Updated instructions for removing IOPlatformPluginFamily.kext.

Update

Resolved the throttling issue on a Mid-2012 unibody MacBook Pro running Ubuntu Linux 24.04.3:

Caution: This bypasses a platform protection signal. Without a working battery, peak power demand may exceed the adapter’s capacity and cause shutdowns. Replacing the battery is the proper fix.

Check

Confirm throttling by sampling CPU frequencies under load:

python3 - <<'PY'
import glob
import subprocess
import time

p = subprocess.Popen([
    'python3', '-c',
    'import time\n'
    'end = time.monotonic() + 4\n'
    'while time.monotonic() < end: pass'
])

for _ in range(4):
    time.sleep(0.8)
    paths = sorted(glob.glob(
        '/sys/devices/system/cpu/cpu[0-9]*/cpufreq/scaling_cur_freq'
    ))
    print('MHz:', [int(open(f).read()) // 1000 for f in paths])

p.wait()
PY

Before the workaround, all four samples showed:

MHz: [1197, 1197, 1197, 1197]

Test

Clear bit 0 of MSR_POWER_CTL (0x1fc), leaving other bits intact:

sudo apt install msr-tools
sudo modprobe msr

original=$(sudo rdmsr -p 0 0x1fc)
printf 'Original: %s\n' "$original"

sudo wrmsr -p 0 0x1fc \
  "$(printf '%x' "$((16#$original & ~1))")"

A set bit doesn't prove the signal is active, it just enables the CPU's response to it; see Intel's MSR documentation.

The CPU check now shows:

MHz: [3007, 2893, 2914, 2893]
MHz: [3044, 2893, 1200, 1200]
MHz: [3092, 1600, 1200, 1200]
MHz: [3092, 1200, 3100, 2893]

To undo the temporary change from the same shell:

sudo wrmsr -p 0 0x1fc "$original"

Persist

To persist across boot and resume, run as root:

#!/bin/bash
set -euo pipefail

[[ $EUID -eq 0 ]] || {
    echo "Run with sudo." >&2
    exit 1
}
[[ $(cat /sys/class/dmi/id/product_name) == MacBookPro9,2 ]]
command -v rdmsr >/dev/null
command -v wrmsr >/dev/null

install -d /usr/local/sbin /etc/systemd/system \
    /usr/lib/systemd/system-sleep

cat > /usr/local/sbin/disable-bd-prochot <<'SCRIPT'
#!/bin/bash
set -euo pipefail
export PATH=/usr/sbin:/usr/bin:/sbin:/bin

[[ $(cat /sys/class/dmi/id/product_name) == MacBookPro9,2 ]]
modprobe msr

original=$(rdmsr -p 0 0x1fc)
updated=$(printf '%x' "$((16#$original & ~1))")
wrmsr -p 0 0x1fc "$updated"

verified=$(rdmsr -p 0 0x1fc)
(( (16#$verified & 1) == 0 )) || {
    echo "BD PROCHOT remained enabled" >&2
    exit 1
}
echo "BD PROCHOT disabled: MSR 0x1fc $original -> $verified"
SCRIPT

chmod 755 /usr/local/sbin/disable-bd-prochot

cat > /etc/systemd/system/disable-bd-prochot.service <<'UNIT'
[Unit]
Description=Disable external BD PROCHOT on MacBookPro9,2
After=systemd-modules-load.service

[Service]
Type=oneshot
ExecStart=/usr/local/sbin/disable-bd-prochot
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target
UNIT

cat > /usr/lib/systemd/system-sleep/disable-bd-prochot <<'HOOK'
#!/bin/sh
if [ "$1" = post ]; then
    /usr/local/sbin/disable-bd-prochot
fi
HOOK

chmod 755 /usr/lib/systemd/system-sleep/disable-bd-prochot
systemctl daemon-reload
systemctl enable --now disable-bd-prochot.service
systemctl --no-pager status disable-bd-prochot.service

Revert

After replacing the battery, run as root and reboot to allow firmware to reinitialize the register:

#!/bin/bash
set -euo pipefail

systemctl disable --now disable-bd-prochot.service

rm -f /etc/systemd/system/disable-bd-prochot.service \
      /usr/lib/systemd/system-sleep/disable-bd-prochot \
      /usr/local/sbin/disable-bd-prochot

systemctl daemon-reload
echo "Workaround removed. Reboot now."

/mac | Nov 15, 2018

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