(cross-posting to devel and desktop lists, ideally reply to both)
Hello,
this is a request for feedback regarding adjusting process limits to make Steam/Wine work better on Fedora.
A quick background:
In August Valve announced [1] Proton [2], their own fork of Wine, to get included in their Steam Play functionality, and allowing Linux players to run Windows-only games transparently from Steam without any complex configuration, just with a press of a button. Not everything works of course, but the success rate is more than decent [3]. As far as I can tell, the reception by Linux gamers has been very enthusiastic.
The technical details:
In order to boost performance, Valve included DXVK [4] which converts DirectX calls to Vulkan (instead of OpenGL in vanilla Wine), and esync [5], an existing wine patchset that performs process synchronization in a more efficient manner than vanilla Wine, using file descriptors. You can read the linked readme for full explanation.
The esync patchset is the main subject of this email. It uses a lot of file descriptors and the default kernel limits are not sufficient for many games. Valve notes that in their requirements document (under "FD LIMIT REQUIREMENTS") [6] and it is further described in the esync readme [5]. The documents also say that Debian and its derivatives like Ubuntu have already raised the limit on open file descriptors, so those distributions work out of the box with esync. Fedora is one of those that doesn't. I wonder if we can consider changing that.
Debian and Ubuntu:
I've installed both Debian (Sid) and Ubuntu (18.10) to verify this, and can confirm it. The default soft limit stays the same (1024), but the hard limit is increased from 4096 to 1048576 (2^20). However, this only applies to the systemd's system instance (systemd --system, PID 1), and not to systemd user instances (systemd --user). Most of the apps you start in your session are children of gnome-shell, which doesn't run under the systemd user instance, so the higher limits apply for them as well (including Steam). However some apps (probably started via dbus, I'm not really sure, but importantly this includes also gnome-terminal) are started as children of the systemd user instance and therefore have the original low limits applied. I don't know whether this is intentional or just an omission. I tried really hard to find the place where Debian/Ubuntu patches the upstream limits, so that I could read some justification/explanation of the change, but I wasn't able to find it (I searched the available patches for kernel, systemd and pam).
Configuring the limits:
You can display the soft+hard limits of your current terminal using ulimit:
$ ulimit -Sn
1024
$ ulimit -Hn
4096
1024
$ ulimit -Hn
4096
However, note that gnome-terminal runs under the systemd user session, so at least in Debian/Ubuntu this will not see higher limits (i.e. neither will Steam started from the terminal).
You can also use prlimit to see the limits of any running process. You can use this to check limits of the systemd system instance, systemd user instance, running Steam, etc.
$ sudo prlimit --nofile --pid 1
RESOURCE DESCRIPTION SOFT HARD UNITS
NOFILE max number of open files 1048576 1048576 files
RESOURCE DESCRIPTION SOFT HARD UNITS
NOFILE max number of open files 1048576 1048576 files
You can modify the limits on the fly like this:
$ sudo prlimit --nofile=1024:1048576 --pid PID
You can increase the default limits by editing /etc/systemd/system.conf (and /etc/systemd/user.conf, if you want to edit systemd user session as well) and setting:
DefaultLimitNOFILE=1024:1048576
Alternatively, you can drop a file like this:
[Manager]
DefaultLimitNOFILE=1024:1048576
into /etc/systemd/system.conf.d/ (and /etc/systemd/user.conf.d/).
Default limits in Fedora:
From a technical point of view I'm not able to judge whether raising the fileno limits by default is a trivial change or something with important security implications. That's why I'm writing this email to hopefully get replies from more knowledgeable people. The fact that Debian raised the limits gives me hope we could do the same in Fedora (perhaps just in Workstation, if the change in not welcome in the whole distribution). If somebody can find the justification of Debian devs, that would be great. I'd very much like to see Fedora (Workstation) being a good choice for Linux gamers (we already packaged gamemode), and this might be an important step to make sure Steam games don't work worse than on Ubuntu (but hopefully even better, due to our more recent drivers).
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