Another daemon for managing control groups [LWN.net]

Another daemon for managing control groups

Posted Dec 17, 2013 5:20 UTC (Tue) by Cyberax (✭ supporter ✭, #52523)
In reply to: Another daemon for managing control groups by rahulsundaram
Parent article: Another daemon for managing control groups

SHOULD is not MUST. An optional ability to use an external manager would do systemd only good.

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Another daemon for managing control groups

Posted Dec 17, 2013 5:27 UTC (Tue) by rahulsundaram (subscriber, #21946) [Link] (12 responses)

It is only optional if systemd has its own implementation of similar functionality and at that point, there is even less of a reason to depend on another daemon. external dependencies for a init system must be as minimal as possible. standardize on external interfaces if needed but not a specific implementation itself.

Another daemon for managing control groups

Posted Dec 17, 2013 6:17 UTC (Tue) by Cyberax (✭ supporter ✭, #52523) [Link] (11 responses)

Right now the whole systemd and single-tree cgroups API is an abomination that should be burned alive and then have its ashes scattered over an active volcano.

So another API is sorely needed. And if another library forces systemd to adopt something sane then it might be the only solution.

Another daemon for managing control groups

Posted Dec 17, 2013 6:57 UTC (Tue) by mchapman (subscriber, #66589) [Link] (10 responses)

> So another API is sorely needed.

Why? How is that going to help anything?

Maybe I've missed something in this whole argument, but I fail to see why there must be only one implementation of a cgroups manager. Yes, you may only be able to *run* one at any time due to kernel limitations, but that doesn't mean multiple projects can't choose to implement the functionality in different ways if they have different goals.

Isn't that the whole point of FOSS?

Another thing I don't really understand is why systemd should be "forced" to adapt to use some other manager. systemd doesn't need another manager; it does enough management of cgroups for itself already. What compelling reason is there for the systemd developers to throw all that out?

Another daemon for managing control groups

Posted Jan 3, 2014 18:23 UTC (Fri) by aigarius (guest, #7329) [Link] (9 responses)

Those are two competing points right there - currently systemd developers want to manage cgroups inside systemd and do not want to support delegating this management to any other outside daemon or library. For any other implementation of cgroups management to be able to work on a systemd system it would need systemd cooperation as above.

Therefore there are two options - either systemd must be relegated to be a tiny minority init version that can easily be replaced with something else on any distro, so that you can actually use a different cgroups manager or any other cgroups managers will be unusable without a very major hassle on many popular distros.

The refusal of systemd developers to cooperate in a flexible solution (combined with the marketing drive to make systemdbe default in major distros) is the very thing that is poisonous to alternative cgroup management implementations.

Another daemon for managing control groups

Posted Jan 3, 2014 18:57 UTC (Fri) by raven667 (guest, #5198) [Link] (8 responses)

> The refusal of systemd developers to cooperate in a flexible solution (combined with the marketing drive to make systemdbe default in major distros) is the very thing that is poisonous to alternative cgroup management implementations.

No, what is poisonous to other competing implementations is that systemd is technically advanced and comprehensive so that there is little need for other re-implementations because it works so well. That is the reason it has gained so much uptake, do you really think that all the people who build distributions are technically incompetent and are switching to systemd components because of some "marketing drive" or because these components solve the problems they are trying to solve better than the systems which came before?

It's true that the systemd developers refuse to re-design their system to have cgroup management handled by an external process, the "flexability" you talk about, but there are very rational technical reason for that which you have not addressed. Do you understand the kernel cgroups api and your workload and the systemd configuration knobs enough to be able to tell us where the systemd cgroup managment fails in your workload that another cgroup manager would be able to do better that couldn't be implemented in systemd? Do you have any examples or is this an entirely philosophical conversation with no real backing in reality?

Another daemon for managing control groups

Posted Jan 3, 2014 19:15 UTC (Fri) by dlang (guest, #313) [Link] (7 responses)

even the systemd developers admit that there are linux systems out there that should not run systemd (embedded type systems for example), but those systems may still want to use some of the features that systemd is trying to take over and kill off the competition in.

how would you use cgroups on a wireless access point where you don't want to run systemd but still want to limit some set of processes?

Another daemon for managing control groups

Posted Jan 3, 2014 20:03 UTC (Fri) by raven667 (guest, #5198) [Link] (6 responses)

It's a tautology really, if you don't want to run systemd for whatever reason, then don't run systemd.

> ... features that systemd is trying to take over and kill off the competition in.

I'm not sure what "take over" means in this context if you aren't running systemd on a system because you don't want to than what systemd does is not of relevance to you except as an example. The only way they are "killing the competition" is by making a high-quality, feature-rich offering which people want to use, they are out-competing the competition. I hope we can all agree that they shouldn't hobble the implementation and make it less useful just to make the competition look better...

> how would you use cgroups on a wireless access point where you don't want to run systemd but still want to limit some set of processes?

You do whatever you want to do, within the confines of the kernel interface, which in the future will want a single-writer (although compatibility with the existing interface will be maintained for some time as I understand it). So you can make your own cgroup manager daemon, being advised that there are fundamental problems with restarts, race conditions, etc. and that this can only manage a subset of processes (ones that it starts), or you write your custom manager into pid 1 either from scratch or patching one of the existing inits to have the logic you want.

There are probably going to be a handful of small cgroup managers that you can start from which are dedicated to specific tasks like HPC or Google, although if systemd incorporates whatever configuration toggles these workloads need then many will probably just use that rather than re-implement something themselves, but there is always someone out there who wants to do it themselves and is distrustful of any third party code 8-)

Really, how is this different from today where if you want to use this kernel feature you have to have your software talk to the kernel and twiddle it. How is it different than iproute2 or iptables?

Another daemon for managing control groups

Posted Jan 3, 2014 21:44 UTC (Fri) by dlang (guest, #313) [Link] (4 responses)

> The only way they are "killing the competition" is by making a high-quality, feature-rich offering which people want to use, they are out-competing the competition.

not quite, they also campaign to get the competition removed from the distro (or at least the default installation of the distro), rsyslog vs journald in Fedora is an example of this in practice.

Another daemon for managing control groups

Posted Jan 3, 2014 21:48 UTC (Fri) by mchapman (subscriber, #66589) [Link] (3 responses)

> they also campaign to get the competition removed from the distro (or at least the default installation of the distro)

Is that so surprising? Clearly they think they have a better solution. They *should* be campaigning for it, if that's what they really think.

I'm sure distro maintainers are smart enough to judge the various options available on their merits.

Another daemon for managing control groups

Posted Jan 3, 2014 21:54 UTC (Fri) by dlang (guest, #313) [Link] (2 responses)

they can campaign to have their system added, but what's the justification for removing the other option, especially when it can co-exist with theirs and provide capabilities that they don't offer?

Another daemon for managing control groups

Posted Jan 3, 2014 21:59 UTC (Fri) by mchapman (subscriber, #66589) [Link]

> they can campaign to have their system added, but what's the justification for removing the other option, especially when it can co-exist with theirs and provide capabilities that they don't offer?

For Rsyslog in Fedora specifically?

According to the Change page [1]:

1. Less disk and runtime footprint
2. Fewer packages in the default install
3. Faster booting

Now, you can argue whether those are *sufficient* justification for the removal of syslog from the default install. I'm sure people did.

At the end of the day, though, the vote was in that this Change should go ahead.

[1] https://fedoraproject.org/wiki/Changes/NoDefaultSyslog

Another daemon for managing control groups

Posted Jan 3, 2014 22:02 UTC (Fri) by raven667 (guest, #5198) [Link]

But they haven't removed rsyslog, and in fact rsyslog is required if you want to do central logging using syslog, systemd and the journal will pass through log messages to it. What they aren't doing is installing both logging systems by default. There is also no default sendmail in Fedora either.

Other distros make different decisions but rsyslog integration is still an important part of the journal, heck its even an important part of rsyslog which added support for the journal on-disk data format as I recall.

Another daemon for managing control groups

Posted Jan 3, 2014 21:46 UTC (Fri) by dlang (guest, #313) [Link]

> Really, how is this different from today where if you want to use this kernel feature you have to have your software talk to the kernel and twiddle it. How is it different than iproute2 or iptables?

iproute2 and iptables are happy to sit on a system and only be used for the parts that you want them to be used for. They don't try really hard to force you to do things a specific way and push you to replace a bunch of other tools if you use them.