[systemd-devel] RFC: Setting TasksMax= by default

[Jóhann B. Guðmundsson]

On 11/13/2015 01:49 PM, Lennart Poettering wrote:
> Heya!
>
> So, I am tempted to make the following changes to systemd, and was
> wondering about opinions about it:
>
> a) The first change is rather uncontroversial I presume: I'd like to
>     set DefaultTasksAccounting=yes in system.conf by default. This
>     means we get accounting of the number of tasks per unit enabled by
>     default. Effectively this turns on the "pids" cgroup controller for
>     all units. According to the cgroups folks this is OK as this
>     specific controller is not particularly costly. This means that by
>     default "systemctl status" will show a "Tasks: " line then, showing
>     the number of tasks in the service. cgtop will be more efficient,
>     too, then.

Arguably this should be called system instead of default for system wide 
settings where applicable ( you need to keep a clear distinction which 
option belongs where and what it affects )

Will people be able to disable this per type unit (  set 
"UnitTasksAccounting=yes/no" )

Users sets SystemTasksAccounting=yes in system.conf but turns off task 
accounting for b.service while while keeping it for every other type unit ?

Or

Users sets SystemTasksAccounting=no in system.conf but enables task 
accounting for a.service while while keeping it disabled for every other 
type unit ?

>
> b) I'd like to introduce DefaultTasksMax= that controls the default
>     value of the per-unit TasksMax= by default, and would like it to
>     set to some value such 1024 out-of-the-box. This will mean that any
>     service or scope created will by default be limited to 1024
>     tasks. This of course is a change from before that has the
>     potential to break some daemons that maintain an excessive number
>     of processes or threads. However, I think it's a much better choice
>     to raise the limit for them, rather than stay unlimited for all
>     services by default. I think 1024 is not particularly low, but also
>     not particularly high. Note that the kernel by default limits the
>     number of processes to 32K in total anyway.

Should there not be two options "SystemTasksMax" which alters the kernel 
default limits and UnitTaskMax  which controls the default value of 
per-unit ?

>
> c) In logind.conf I intend to add a TasksMax= setting that sets the
>     number of tasks for user sessions, and overrides the systemd-wide
>     setting for user scopes. It would also be set out-of-the-box, and
>     default to something like 8K or so. (Note that this is very similar
>     to setting RLIMIT_NPROC via /etc/security/limits.conf, but has the
>     benefit of covering also suid binaries, being nicely queriable
>     via systemctl status and controllable during runtime via "systemctl
>     set-property" and so on)

Should not this be SessionTasksMax= setting? to clearly disquince this 
from other task mask settings ( accommodated by SessionTasksAccounting= )
>
> d) in systemd's own unit files we'll configure much lower settings by
>     default, since we know how many tasks they require.
>
>


Putting this altogether you have in systemd.conf something along the 
lines of

# Enable/disable system wide task accounting
SystemTasksAccounting=yes/no


# Set system wide task limit
SystemTasksMax=32768

# Enables/disable task accounting for type units
UnitTasksAccounting=yes/no

# Set type unit wide task limit
UnitTaskMax=1024

# Enable/disable session task accounting
SessionTasksAccounting=yes/no

# Set session task limit
SessionTasksMax=8000

For type units people could overwrite system wide defaults via

[Unit]
UnitTasksAccounting=yes/no
UnitTaskMax=64

For sessions people could overwrite system wide defaults ( logind.conf ) via
SessionTasksAccounting=yes/no
SessionTasksMax=12000

Status needs to show the current usage and max available and daemon 
reload should issue a warning on or fail if the max value for type units 
and sessions is set higher than the value of system tasks Max

JBG