Bug 1327082

Based on customer's request, copy-past of an email addressed to jheves, April 13

Hi Jana,

I'm sending this to you since you have made the latest update...

I was using the latest "Resource Management Guide" 0.0-1.6, and at the
end of section 2.3.2 about "Modifying Unit Files" it gives an example
for "ControlGroupAttribute=". Unfortunately this has been removed from
systemd some time ago and does not work in RHEL 7.2.

It seems like it might have never worked in RHEL7 at all!

https://bugzilla.redhat.com/show_bug.cgi?id=1172890

It would be worth fixing the documentation for RHEL 7.3 :-)

Cheers,
Matthias

Dear Lukáš,

Based on your comment in bug 1172890 [1], I've gone ahead and commented out the reference to "ControlGroupAttribute=" and the example on "Changing Low-level Cgroup Attributes" [2] in the Resource Management Guide [3].

May I ask you what the substitute directive that can be used in the unit file to facilitate resource management is?
Is it the swappiness parameter? 

After a short investigation, the best idea to me seems to provide an example similar to this [4], instructing the user to set vm.swappiness = 10.
Please let me know what you think.
 
Thank you very much for your help!
jana 


[1] https://bugzilla.redhat.com/show_bug.cgi?id=1172890#c2
[2] commit https://gitlab.cee.redhat.com/red-hat-enterprise-linux-documentation/doc-Red_Hat_Enterprise_Linux-7-Resource_Management_Guide/commit/46f2e47cf0e20ef7e838cd3eaa7bf9ae6e7fa9b0
[3] https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html-single/Resource_Management_Guide/index.html#sec-Modifying_Unit_Files 
[4] https://en.wikipedia.org/wiki/Swappiness

Every supported cgroup attribute that could be set directly through unit-files is documented in man page systemd.resource-control.

For the rest (like the swappiness) there is currently no way how to do it directly through systemd. The reason behind this is the new (still unfinished) cgroup interface in kernel. We don't want to support anything, that would be removed when we move systemd to the new cgroup hierarchy.

As a workaround you should be able to do something like:
ExecStartPost=/bin/bash -c "echo 70 > sys/fs/cgroup/memory/system.slice/testdd.service/memory.swappiness"

I'm the initial reporter (thanks, Jana, for having filed this properly!). Indeed as Lukáš has pointed out, there is no immediate replacement. In my particular case I'm stuck since I wanted to set the memory.memsw.limit_in_bytes to the same value as the MemoryLimit= in order to prevent any swap usage, but for a user slice, not a service. And the ExecStartPost= isn't valid for a slice.

In any case, I would be fine with the documentation just mentioning an ExecStartPost= for now, as services are the most common units on which people want to set cgroup attributes.

Created attachment 1147632 [details]
a documentation excerpt; section 2.3.2. Modifying Unit Files

Dear Lukáš and Matthias,

Thank you very much for your explanations.
I've added one new paragraph and an example to the guide based on your recommendations [1].

May I kindly ask you for a review? 
As Matthias does not have the access to our internal preview tool [2], I've attached an excerpt of the section in comment 5.

Thank you very much for your cooperation,
jana

[1] https://gitlab.cee.redhat.com/red-hat-enterprise-linux-documentation/doc-Red_Hat_Enterprise_Linux-7-Resource_Management_Guide/commit/54bfa4858cef2c9f2508327f4d3ed404cf08b858

[2] http://jenkinscat.gsslab.pnq.redhat.com:8080/view/RHEL7/job/doc-Red_Hat_Enterprise_Linux-7-Resource_Management_Guide%20%28html-single%29/lastSuccessfulBuild/artifact/tmp/en-US/html-single/index.html#sec-Modifying_Unit_Files

I'm not authorized to see the attachment, even when logged in, weird...

Anyway, I trust you to get a proper update/fix since the issue is clearly understood :-)

From Lukáš's email feedback :
---
Maybe I would explictly mention that systemd currently support only
subset of cgroup features and that the ExecStartPost is a workaround
for that.

Also there is a missing slash 
sys/fs/cgroup/memory/system.slice/testdd.service/memory.swappiness
should be 
/sys/fs/cgroup/memory/system.slice/testdd.service/memory.swappiness
---

Created attachment 1165676 [details]
screenshot for the final docs version

 ⁠2.3.2. Modifying Unit Files: ⁠Managing Other System Resources

Thank you both, Lukas and Matthias, for your assistance!

The final changes are provided in commits [1] and [2] and in the screenshot above.

jana

[1] https://gitlab.cee.redhat.com/red-hat-enterprise-linux-documentation/doc-Red_Hat_Enterprise_Linux-7-Resource_Management_Guide/commit/81ee7a5dc77766490f5f1db50be0bbd290b980be

[2] https://gitlab.cee.redhat.com/red-hat-enterprise-linux-documentation/doc-Red_Hat_Enterprise_Linux-7-Resource_Management_Guide/commit/d52b92a8fba1901575283c5903a5a357709aac58

With Matthias Saou's final testing and suggestions, I've made final changes to section 2.3.2. Modifying Unit Files:

https://gitlab.cee.redhat.com/red-hat-enterprise-linux-documentation/doc-Red_Hat_Enterprise_Linux-7-Resource_Management_Guide/commit/f3d0b13f1caf4ab884d98b9a276f318e16defc52

The change will appear on the RH Customer Portal with the 7.3 Beta release of the book. 
Closing this bug as verified.