SteamOS 3.5 is almost here, featuring a fix for Steam Deck's sneakiest performance bug

SteamOS 3.5 is almost here, featuring a fix for Steam Deck's sneakiest performance bug

SteamOS 3.5 includes a new linux kernel, graphics driver, and a fix for a bad SMT performance bug.


We tend to groan when a new Windows Update arrives. Really, right now, Windows? What will this break? do i really need it "Remind me later." On the other hand, a new version of the Linux kernel always seems like a minor event in the open source community, promising exciting new features and performance improvements for many devices running Linux. The kernel is the basic building block of Linux, and many distributions such as Ubuntu and Arch (which Valve's SteamOS is based on) have evolved into full-fledged operating systems.


The Steam Deck will soon join the excitement of this "new kernel day" as Valve's next major update will be the first to upgrade the Linux kernel since the launch of SteamOS 3.5. While this means a lot of good things to Steam Deck owners, most of them you probably won't even notice.


Valve's Pierre-Loup Griffais told PC Gamer that updating the core means "going closer to the latest and greatest with lots of good additions, performance fixes, and functional fixes that will improve every aspect of the system." "In terms of core functionality, running games, and perfection - at this point such fixes mostly go away, so you wouldn't expect anything transformative here."


However, this summary contains a caveat: Valve has a major performance fix for the Steam Deck coming in SteamOS 3.5. Steam Deck's processor supports a common modern feature called SMT or simultaneous multi-threading; for this you may be more familiar with Intel's name which is Hyper-Threading. Simply put, SMT is the ability to run two threads on a single CPU core. This should be a general performance improvement, but an obscure issue in the Linux kernel shipped with Steam Deck meant that this actually caused a performance degradation in some cases.


"There was a behavior hardcoded in the Linux kernel on the AMD side, where if a kernel goes to sleep, it invalidates its cache, because when it wakes, the cache may not be valid," Griffais said. "When there are two threads in the same core and one thread is still working on something and the second SMT thread goes to sleep, it uses the same logic to trash its cache. So the worker thread suddenly loses its L3 cache. In terms of CPU, that's really bad. Everything from memory It needs to re-fetch - too much latency, basically a 'bubble' in computing. And your frame time is going to shit. That's what it meant."


AMD found and fixed the issue before Valve identified it; actually, that's how the SteamOS developers figured out what was going on. With the kernel update, they will finally be able to make it available to users.


The problem occurred mostly in older games and affected emulators not built for modern multi-threading; If you've seen big stutters or oddly low framerates in older games, SMT was probably the culprit. Depending on what you're playing in the deck, you may not even notice a change.


However, some Steam Deck owners noticed the problem and started using a plugin called PowerTools to disable SMT. Griffais mentioned that there were calls for Valve to integrate this feature into SteamOS, but that wasn't the path they wanted to take.


“Instead of adding an illogical setting that people shouldn't have to disable, we needed to fix the bug that was making people have to deal with it,” he said. "It needed to improve Perf extensively. So we wanted to look at the root cause of the problem, rather than adding options that are difficult for people to use and possibly counterintuitive, especially if there's already a mod to do it in the meantime."

Griffais praised some of the plugins that Steam Deck tinkerers had developed, but the way Valve explained its approach to the SMT issue highlighted the tricky balance Deck had to strike, unlike other game consoles.


"We're always going, 'Are we such a complex power user product? Or are we just a device-like product, is this actually a really easy way to get into PC gaming?'' We know that Steam Deck is nothing. Represents computer games. But there is a lot of value in having this device-like experience. to make sure everyone has access to both. We didn't want to be in a situation where people had to turn off some settings for things to perform well. We just wanted to eliminate that."


Based on internal testing, Griffais believes that the SteamOS 3.5 update should do this so that there is no longer a need to disable SMT, but they will need data from the public release to make sure the bug goes away completely.


SteamOS 3.5 will also include a new graphics driver, but Valve took care to fast-track part of this update to 3.4.6, released March 13, supporting the Resident Evil 4 Remake. When 3.5 arrives, it will be tested on the Steam Deck preview channel until it's ready for stable release.