[slurm-users] X11 forwarding and VNC?

Marcus Wagner wagner at itc.rwth-aachen.de
Fri Mar 29 18:45:24 UTC 2019

Hi Loris,

Am 29.03.2019 um 14:01 schrieb Loris Bennett:
> Hi Marcus,
> Marcus Wagner <wagner at itc.rwth-aachen.de> writes:
>> Hi Loris,
>> On 3/25/19 1:42 PM, Loris Bennett wrote:
>>>> 3. salloc works fine too without --x11, subsequent srun with a x11 app works great
>>> Doing 'salloc' followed by 'ssh -X'  works for us too, which is surprising to me.
>>> This last option currently seems to me to be the best option for users,
>>> being slightly less confusing than logging into the login node again
>>> from the login node, which is our current workaround.
>>> Still, it's all a bit odd.
>> I assume, you use pam_slurm_adopt?
> Yes.
>> Then it is clear, that this is working and has nothing to do with the x11
>> forwarding feature of slurm. This is plain ssh X11-forwarding in this case.
> OK, I  see that, but if I don't need --x11 with salloc, what is it
> for?  Just to control to control on which nodes forwarding is done
> viz. --x11[=<all|first|last>]?  What might be a use-case for not having
> X11 forwarding for all the nodes, which is the default?

The default is (according to the manpage) 'batch', which means the node, 
where the batchscript will be executed (the first of the allocation, I 
I do not know what first or last should be intended.
In fact I do not have a use case for x11-forwarding to all nodes, might 
have to think a little bit more about that one.

>> Please keep in mind, that processes started with an adopted ssh session are in
>> the jobs cgroup (good), but are accounted in the 'extern' step of the job.
>> e.g.
>> * sbatch --wrap "sleep 10m"
>> * ssh to compute-node
>> * do some work in the compute node
>> after job is done
>> * sacct -j <jobid> -o JobID,JobName,MaxRSS,CPUTime,TotalCPU
>>         JobID    JobName     MaxRSS    CPUTime   TotalCPU
>> ------------ ---------- ---------- ---------- ----------
>> 1053837            wrap              00:01:42  02:00.159
>> 1053837.bat+      batch       412K   00:01:43  00:00.158
>> 1053837.ext+     extern    543880K   00:01:42  02:00.001
> That's interesting, although is there any advantage/difference compared
> with just doing
>    srun --x11 --pty bash
> ?
srun --x11 --pty bash
the accounting will be in the batch step of the job, that is the only 
difference I'm aware of at the moment.
With LSF we used that kind of mechanism to start e.g. vtune directly out 
of the job. Without the X11-Forwarding feature of Slurm you would have 
to salloc some hosts and then ssh to the nodes with x11 forwarding 
enabled to then start vtune.
So it is a little bit more to do for the user if you do not do 
X11-Forwarding the SLURM style.

> Cheers,
> Loris

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