Your system information

• Steam Runtime Version:

  Steam Beta Branch:  Stable Client
  Steam Version:  1769025840
  Steam Client Build Date:  Wed, Jan 21 5:12 PM UTC -08:00
  Steam Web Build Date:  Wed, Jan 21 7:41 PM UTC -08:00
  Steam API Version:  SteamClient023
  

• Distribution: Ubuntu 24.04
• Link to your full system information: Gist link
• Have you checked for system updates?: Yes
• What compatibility tool are you using?: Proton 10.0-4 and Proton 9.0-4
• What versions are listed in steamapps/common/SteamLinuxRuntime/VERSIONS.txt? None, folder doesn't exist
• What versions are listed in steamapps/common/SteamLinuxRuntime_soldier/VERSIONS.txt? None, folder doesn't exist
• What versions are listed in steamapps/common/SteamLinuxRuntime_sniper/VERSIONS.txt?

  $ cat VERSIONS.txt 
    #Name   Version         Runtime Runtime_Version Comment
    depot   3.0.20260119.200241                     # Overall version number
    pressure-vessel 0.20260115.0    scout           # pressure-vessel-bin.tar.gz
    scripts 0.20260115.0                    # from steam-runtime-tools
    sniper  3.0.20260119.200241     sniper  3.0.20260119.200241     # sniper_platform_3.0.20260119.200241/
  

Please describe your issue in as much detail as possible:

Hello, I have an issue with Steam Linux runtime CPU affinity on hyperthreaded VMs (for instance an AWS 4gdn instance with 2 Core and 4 logical CPUs, also reproduce on similar GCP instance)

On startup Steam set its own CPU affinity to {0,2}, but on the machine this corresponds to both logical cores of the same physical core. Sibling topology on machine is actually:

• Core 1: {0,2}
• Core 2: {1,3}

So Steam runs on a single Core (runs on both thread of the same CPU core). Steam then launch games on a single CPU with huge impact performance. Games are running with the same CPU affinity as Steam.

Notes:

• This CPU affinity does not come from parent process which has [0-3], strace shows Steam set its own affinity (see detailed strace logs in Gist below)
• It does not affect similar setup on machine without hyperthreading (eg. Scaleway and Linode instances with similar specs do not have this problem)

I suspect Steam tried to avoid having multiple threads on the same CPU, but should have used {0,1} to use both CPU Cores instead of {0,2}.

Can we have Steam (and runned games) use proper CPU affinity, or force CPU affinity globally somehow ? Any workaround will be appreciated in the meantime :)

Technical details and context:

• Technical environment:
  • OS: Linux Ubuntu 24.04
  • Machine: AWS g4dn.xlarge with NVIDIA T4 GPU (4 vCPUs, 2 Core)
  • Steam runs under an Ubuntu 24.04 Docker container (along with X server, etc.)

For context: this machine is deployed with Cloudy Pad (I'm the creator/maintainer). It's a bit specific:

• Run an AWS instance (or any other Cloud VM) with Ubuntu 24.04
• Run Steam in a Docker Container based on Ubuntu 24.04

Steps to reproduce:

• Install Cloudy Pad
• Create an AWS instance with: cloudypad create aws
• Connect, run Steam, install a game
• Connect on instance via SSH and in container, check CPU affinity:

  ssh ubuntu@<instance_ip>
  $ docker exec -it cloudy bash
  $ ps -ef | grep steam     # find Steam PID
  $ taskset -pc <steam-pid> # see CPU affinity
  

I understand this is not a standard setup nor easy to reproduce. However please consider this completely breaks Steam on our system used by a fair mount of people.

I can provide direct access to an AWS instance to help dig further if someone from the team provide a public SSH key.

System CPU cibling data

System CPU sibling as appearing both on host and in container:

$ cat /sys/devices/system/cpu/cpu*/topology/thread_siblings_list
0,2
1,3
0,2
1,3
$ cat /sys/devices/system/cpu/cpu*/topology/thread_siblings
5
a
5
a

strace analysis showing sched_setaffinity calls

• strace analysis Gist showing Steam and Wineserver setting the game CPU affinity via sched_setaffinity() syscalls
  • Steam PID(s): 5563 > 5566 > 5685
  • Wineserver: 6842
  • Game thread (Expedition 33): 6988
  • Wineserver sets game CPU affinity: 6842 15:24:57.172008 sched_setaffinity(6988, 128, [0 2]) = 0
  • Steam set ts own CPU affinity: 5685 15:22:51.187160 sched_setaffinity(5685, 128, [0 2]) = 0

Steam and Proton logs

Runned Steam with:

STEAM_LINUX_RUNTIME_LOG=1 STEAM_LINUX_RUNTIME_VERBOSE=1 PRESSURE_VESSEL_VERBOSE=1 PROTON_LOG=1 PROTON_LOG_DIR=/tmp steam

Gist link with

• Steam log
• Proton logs

Workaround attempts

Using WINE_CPU_TOPOLOGY

I tried running Steam with WINE_CPU_TOPOLOGY such as:

• export WINE_CPU_TOPOLOGY=4:0,1,2,3: no effect, CPU affinity remained {0,2}
• export WINE_CPU_TOPOLOGY=2:0,1: resulted in CPU affinity {0} - it seems this further restrict CPU affinity: as "by default" we have {0,2}, trying to force use of {0,1} result in only {0} being used...

Launch option to use all CPU on start

Forcing game game to start with more CPU with custom launch option such as:

# Use all available CPU
taskset -c 0-$(($(nproc --all)-1)) %command%

# Or more specifically
taskset -c 0-3 %command%

This do help: our game ends-up using all available CPU and performance are drastically better. Though it does not seem to solve base issue and must be done for each game... Maybe there's a way to set this globally ?

Thanks in advance for your help ! Let me know if I can provide further details.