During Jitsi video streams the CPU on my ThinkPad P14s was throttling badly — frame drops, lag, the works. The root cause turned out to be the Intel RAPL power limits being reset to a very low value (35W burst) by the firmware, starving the CPU of the headroom it needs for sustained load.

What is RAPL

Intel RAPL (Running Average Power Limit) lets the OS control CPU power envelopes via the powercap sysfs interface. There are two constraints:

Constraint	Name	Purpose
constraint_0	PL1	Sustained (long-term) power limit
constraint_1	PL2	Burst (short-term) power limit

The limits are exposed at /sys/class/powercap/intel-rapl:0/ and mirrored at intel-rapl-mmio:0. The firmware resets them on boot and on resume from sleep.

Diagnosing the problem

cat /sys/class/powercap/intel-rapl:0/constraint_0_power_limit_uw
# 35000000  → 35W — too low

With PL1 at 35W the CPU cannot sustain the load of encoding and transmitting video, and the kernel’s power capping kicks in to enforce the limit via frequency scaling.

The fix

Two pieces are needed: apply the limits at boot, and re-apply them after every resume (the firmware resets them on wake).

Boot service

Create /etc/systemd/system/rapl-fix.service:

[Unit]
Description=Fix ThinkPad P14s MMIO RAPL PL1 power limit
DefaultDependencies=no
After=sysinit.target

[Service]
Type=oneshot
ExecStart=/bin/bash -c 'echo 55000000 > /sys/class/powercap/intel-rapl-mmio:0/constraint_0_power_limit_uw'
ExecStart=/bin/bash -c 'echo 64000000 > /sys/class/powercap/intel-rapl-mmio:0/constraint_1_power_limit_uw'
ExecStart=/bin/bash -c 'echo 55000000 > /sys/class/powercap/intel-rapl:0/constraint_0_power_limit_uw'
ExecStart=/bin/bash -c 'echo 64000000 > /sys/class/powercap/intel-rapl:0/constraint_1_power_limit_uw'
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Enable it:

systemctl enable --now rapl-fix.service

Sleep hook

The firmware resets RAPL limits on resume. Systemd runs scripts in /usr/lib/systemd/system-sleep/ on suspend and resume — create /usr/lib/systemd/system-sleep/rapl-fix.sh:

#!/bin/bash
# Re-apply RAPL power limits after resume from sleep (firmware resets them)
if [ "$1" = "post" ]; then
    sleep 2
    echo 55000000 > /sys/class/powercap/intel-rapl-mmio:0/constraint_0_power_limit_uw
    echo 64000000 > /sys/class/powercap/intel-rapl-mmio:0/constraint_1_power_limit_uw
    echo 55000000 > /sys/class/powercap/intel-rapl:0/constraint_0_power_limit_uw
    echo 64000000 > /sys/class/powercap/intel-rapl:0/constraint_1_power_limit_uw
fi

Make it executable:

chmod +x /usr/lib/systemd/system-sleep/rapl-fix.sh

The sleep 2 gives the hardware a moment to settle before the limits are written — writing them immediately after wake occasionally fails silently.

Verifying

# Check service status
systemctl status rapl-fix.service

# Check current limits
cat /sys/class/powercap/intel-rapl:0/constraint_0_power_limit_uw
# 55000000
cat /sys/class/powercap/intel-rapl:0/constraint_1_power_limit_uw
# 64000000

After applying this, Jitsi streams run without throttling.