Download speeds bottlenecked by E cores of a CPU

[Aemony]

Hi,

Don't know when it happened, but recently the Steam client seemingly started to limit its decompression speed by the number of available E cores on the CPU.

I have a 1000 Mbps fiber link backed by NVMe drives, and are used to downloading games at constant 85+ Mbps transfer seeds. However recently I noticed that my downloads were peaking out at ~55 Mbps for some unknown reason.

Looking at the CPU graphics in Task Manager showed that my 4 E cores were being maxed out, while barely any load were put on my 8 P cores. This surprised me as I am well aware that Steam's decompression process loves CPU power.

Through Task Manager I changed the CPU affinity for Steam.exe to prevent it from running on the E cores, and instantly I saw an increase of my download speeds, from 55 Mbps to 90 Mbps per second, and my 8 P cores maxing out as well.

So clearly something have changed recently in Steam, or possibly Windows 11, which seemingly limits the number of decompression threads that the Steam client uses and artificially caps the download speeds of games as a result of the number of E cores a CPU has available.

I could reproduce the same issue on both Stable and Beta update channels of Steam, and I'm on a fully up-to-date Windows 11 24H2 system.

[Aemony]

I've enabled all 8 E cores on my 12900K and verified that while the Steam client continues to perform all the downloading/decompression stuff on the E cores, at least it's somewhat maxing out my internet line now.

HOWEVER, experimenting a bit more with the CPU affinity in Task Manager in relation to the E cores shows that as long as a single E core or more is available, the Steam client will dedicate all download/decompression process to the available E cores and completely offload them from the P cores.

This means that if I restrict the Steam.exe process from all my E cores except for one (so it has 8 P cores available to it, and 1 E core), then my download speeds tanks down to a meager ~15 Mbps as all processing is exclusively handled by the single E core available to the client...

So to recap: If you have a CPU with E cores available to it, and a 1000 Mbps fiber connection or better, I really hope you have a high-end CPU with 8 or more E cores available as otherwise you'll probably be bottlenecked artificially by your CPU.

[THEWIZARDDK]

basically same issue in this report
https://steamcommunity.com/groups/SteamClientBeta/discussions/0/591771170072760250/?tscn=1747165475