[systemd-devel] Is it meant to be possible to set IO[Read|Write]BandwidthMax on a slice ?

[Hadrien Grasland]

Le 08/04/2021 à 18:08, Lennart Poettering a écrit :
> On Do, 08.04.21 17:19, Hadrien Grasland (hadrien.grasland at ijclab.in2p3.fr) wrote:
>
>> Le 08/04/2021 à 16:11, Lennart Poettering a écrit :
>>> On Do, 08.04.21 12:24, Hadrien Grasland (hadrien.grasland at ijclab.in2p3.fr) wrote:
>>>
>>>> Hi everyone,
>>>>
>>>> In a scenario where running benchmarks on dedicated hardware is not
>>>> possible, I'm trying to momentarily cap the I/O bandwidth used by
>>>> interactive user sessions while benchmarks are running, in order to improve
>>>> the stability of said benchmark's I/O performance.
>>> Is this on cgroupsv1 or cgroupsv2?
>>>
>>> IIRC there was some issue that the block io controller wasn't fully
>>> recursive on cgroupsv1. It should work on cgroupsv2.
>> This is on a hybrid cgroup configuration. I (perhaps mistakenly) assumed
>> that modern systemd (v246) will use the cgroups v2 hierarchy in that case,
>> even though cgroups v1 is still exposed for compatibility with older apps.
> No, hybrid mode means all controllers operate in cgroupsv1 mode. It's
> just that the controller-less cgroupsv2 hierarchy is also mounted.
>
> hybrid mode was a mistake, we should never have added that, it's just
> a massive maintainance burden for little gain.

I see, thanks for the clarification.

I also checked that setting block IO caps on a session-xyz.scope (which 
is the last systemd hierarchy node above the interactive processes that 
I want to cap) does indeed work, which is consistent with your former 
recollection that this might be an issue with cgroupsv1 block io 
controller not being recursive.

Which gives me a dirty workaround idea for a way to handle my use case 
until slurm get their cgroups v2 support in shape: "just" write a 
program that parses the output of systemd-cgls, enumerates every 
last-level unit below user.slice, and runs systemctl set-property on 
that. That's both ugly and TOCTOU-racy (won't handle newly spawned 
units, and must account for units that go away), but might work well 
enough for my purposes...

Hadrien