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hacking-howto: document thought-experiment (why cgroups don’t solve the window starts on wrong workspace problem)

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Michael Stapelberg 2011-08-27 19:36:07 +02:00
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Just send us the generated file via email.
== Thought experiments
In this section, we collect thought experiments, so that we don’t forget our
thoughts about specific topics. They are not necessary to get into hacking i3,
but if you are interested in one of the topics they cover, you should read them
before asking us why things are the way they are or why we don’t implement
things.
=== Using cgroups per workspace
cgroups (control groups) are a linux-only feature which provides the ability to
group multiple processes. For each group, you can individually set resource
limits, like allowed memory usage. Furthermore, and more importantly for our
purposes, they serve as a namespace, a label which you can attach to processes
and their children.
One interesting use for cgroups is having one cgroup per workspace (or
container, doesn’t really matter). That way, you could set different priorities
and have a workspace for important stuff (say, writing a LaTeX document or
programming) and a workspace for unimportant background stuff (say,
JDownloader). Both tasks can obviously consume a lot of I/O resources, but in
this example it doesn’t really matter if JDownloader unpacks the download a
minute earlier or not. However, your compiler should work as fast as possible.
Having one cgroup per workspace, you would assign more resources to the
programming workspace.
Another interesting feature is that an inherent problem of the workspace
concept could be solved by using cgroups: When starting an application on
workspace 1, then switching to workspace 2, you will get the application’s
window(s) on workspace 2 instead of the one you started it on. This is because
the window manager does not have any mapping between the process it starts (or
gets started in any way) and the window(s) which appear.
Imagine for example using dmenu: The user starts dmenu by pressing Mod+d, dmenu
gets started with PID 3390. The user then decides to launch Firefox, which
takes a long time. So he enters firefox into dmenu and presses enter. Firefox
gets started with PID 4001. When it finally finishes loading, it creates an X11
window and uses MapWindow to make it visible. This is the first time i3
actually gets in touch with Firefox. It decides to map the window, but it has
no way of knowing that this window (even though it has the _NET_WM_PID property
set to 4001) belongs to the dmenu the user started before.
How do cgroups help with this? Well, when pressing Mod+d to launch dmenu, i3
would create a new cgroup, let’s call it i3-3390-1. It launches dmenu in that
cgroup, which gets PID 3390. As before, the user enters firefox and Firefox
gets launched with PID 4001. This time, though, the Firefox process with PID
4001 is *also* member of the cgroup i3-3390-1 (because fork()ing in a cgroup
retains the cgroup property). Therefore, when mapping the window, i3 can look
up in which cgroup the process is and can establish a mapping between the
workspace and the window.
There are multiple problems with this approach:
. Every application has to properly set +_NET_WM_PID+. This is acceptable and
patches can be written for the few applications which don’t set the hint yet.
. It does only work on Linux, since cgroups are a Linux-only feature. Again,
this is acceptable.
. The main problem is that some applications create X11 windows completely
independent of UNIX processes. An example for this is Chromium (or
gnome-terminal), which, when being started a second time, communicates with
the first process and lets the first process open a new window. Therefore, if
you have a Chromium window on workspace 2 and you are currently working on
workspace 3, starting +chromium+ does not lead to the desired result (the
window will open on workspace 2).
Therefore, my conclusion is that the only proper way of fixing the "window gets
opened on the wrong workspace" problem is in the application itself. Most
modern applications support freedesktop startup-notifications which can be
used for this.