cgroup: status-quo and userland efforts

Kay Sievers kay at
Wed Jul 3 00:44:31 UTC 2013

On Wed, Jul 3, 2013 at 1:57 AM, Thomas Gleixner <tglx at> wrote:
> On Sun, 30 Jun 2013, Lennart Poettering wrote:
>> On 29.06.2013 05:05, Tim Hockin wrote:
>> > But that's not my point.  It seems pretty easy to make this cgroup
>> > management (in "native mode") a library that can have either a thin
>> > veneer of a main() function, while also being usable by systemd.  The
>> > point is to solve all of the problems ONCE.  I'm trying to make the
>> > case that systemd itself should be focusing on features and policies
>> > and awesome APIs.
>> You know, getting this all right isn't easy. If you want to do things
>> properly, then you need to propagate attribute changes between the units you
>> manage. You also need something like a scheduler, since a number of
>> controllers can only be configured under certain external conditions (for
>> example: the blkio or devices controller use major/minor parameters for
>> configuring per-device limits. Since major/minor assignments are pretty much
>> unpredictable these days -- and users probably want to configure things with
>> friendly and stable /dev/disk/by-id/* symlinks anyway -- this requires us to
>> wait for devices to show up before we can configure the parameters.) Soo...
>> you need a graph of units, where you can propagate things, and schedule things
>> based on some execution/event queue. And the propagation and scheduling are
>> closely intermingled.
> you are confusing policy and mechanisms.
>     The access to cgroupfs is mechanism.
>     The propagation of changes, the scheduling of cgroupfs access and
>     the correlation to external conditions are policy.
> What Tim is asking for is to have a common interface, i.e. a library
> which implements the low level access to the cgroupfs mechanism
> without imposing systemd defined policies to it (It might implement a
> set of common useful policies, but that's a different discussion).
> That's definitely not an unreasonable request, because he wants to
> implement his own set of policies which are not necessarily the same
> as those which are implemented by systemd.
> You are simply ignoring the fact, that Linux is used in other ways
> than those which you are focussed on. That's true for Google's way to
> manage its gazillion machines and that's equally true for the other
> end of the spectrum which is deep embedded or any other specialized
> use case. Just face it: running Linux on your laptop and on some RHT
> lab machines is covering about 1% of the use cases.
> Nevertheless you repeatedly claim, that systemd is the only way to
> deal with system startup and system management, is covering _ALL_ use
> cases and the interfaces you expose are sufficient.
> Did you ever work on specialized embedded or big data use cases? I
> really doubt that, but I might be wrong as usual.
> So I invite you to prove that you can beat an existing setup for an
> automotive use case with your magic systemd foo. I refund you fully,
> if you can beat the mark of a functional system less than 800ms after
> reset release on a 200MHz ARM machine. Functional is defined by the
> use case requirements and means:
>     - Basic cgroups management working
>     - GUI up and running
>     - Main communication interface (CAN bus) up and running
> The rest of the system is starting up after that including a more
> complex cgroup management.
> According to your claim that systemd is covering everything and some
> more, this should take you a few hours. I grant you a full week to
> work on that.
> The use case Tim is talking about is different, but has similar
> constraints which are completely driven by his particular use case
> scenario. I'm sure, that Tim can persuade his management to setup a
> similar contest to prove your expertise on the other extreme of the
> Linux world.
> Before answering please think about the relevance of your statements
> "getting this all right isn't easy", "something like a scheduler",
> "users probably want ..."  and "stable /dev/disk/by-id/* symlinks" in
> those contexts.

I don't think anybody needs your money.

But it's sure an improvement over last time when you wanted to use a
"Kantholz" to make your statement.


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