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

[Hadrien Grasland]

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.

In the following discussion, I'll focus on capping the read bandwidth of 
/dev/sda for the sake of keeping my examples short, but if I can get 
this to work, the idea would be to cap the read and write bandwidth of 
all storage devices.

>From 
https://www.freedesktop.org/software/systemd/man/systemd.resource-control.html 
, I understand that I should be able to achieve the intended goal by...

  * Running something like `systemctl set-property --runtime user.slice
    IOReadBandwidthMax='/dev/sda 1M'` before the benchmark
  * Running something like `systemctl set-property --runtime user.slice
    IOReadBandwidthMax=` after the benchmark.

However, this is not effective, as can be checked by running `hdparm 
-t`, which still observes the full disk bandwidth.

I have tried the following variants:

  * `systemd-run -p IOReadBandwidthMax='/dev/sda 1M' -t bash` works
    (hdparm only sees 1 MB/s within the resulting shell)
  * `systemd-run -t bash` followed by `systemctl set-property --runtime
    <new unit> IOReadBandwidthMax='/dev/sda 1M'` also works (hdparm only
    sees 1 MB/s).
  * More specifically targeting individual users' slices
    (user-<uid>.slice) doesn't work.

This looks like a cgroups or systemd bug to me, but I thought I would 
cross-check with you before reporting this to my distribution's 
bugtracker (my distro packages systemd 246, which is just below your 
minimal version criterion for upstream bug reports).

Should I be able to set I/O bandwidth caps on a top-level slice like 
user.slice, or is it expected that I can only do it on individual services?

Cheers,
Hadrien

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