Re: Spreading parallel across nodes on HPC system

[Ken Mankoff]

I'll try to simplify my original question...

If I run

parallel -s-slf hostfile -j 1000 <script> ::: $(seq 1000)

And hostfile has some hosts that have 1 CPU, and some hosts that have 100s of 
CPUs, does parallel take care of handling this?

I've now just read the man page in more detail and above --slf under the -S 
documentation I see

> GNU parallel will determine the number of CPUs on the remote computers
> and run the number of jobs as specified by -j.

So I *think* that if I leave "-j" off the command line, parallel will use the 
maximum number of available CPUs. This all sounds good.

Last question, which I may be able to figure out with trial-and-error testing. 
Does parallel detect the total number of CPUs on host, or the number of CPUs 
allocated to me and my job? I only have access to the latter...

Thanks,

   -k.


On 2022-11-10 at 20:49 +01, Ken Mankoff <mankoff@gmail.com> wrote:
> Hello,
>
> I'm trying to run parallel on multiple nodes. Each node may have a
> different number of CPUs. It appears the best syntax for this is from
> the man page --slf section:
>
> 8/my-8-cpu-server.example.com
> 2/my_other_username@my-dualcore.example.net
>
> My problem is that I'm running in the SLURM environment. I can get the
> hostnames with
>
> scontrol show hostnames $SLURM_JOB_NODELIST > nodelist.0
>
> But I cannot easily get the CPUS-per-node. From the SLURM docs,
>
> SLURM_JOB_CPUS_PER_NODE: Count of CPUs available to the job on the
> nodes in the allocation, using the format
> CPU_count[(xnumber_of_nodes)][,CPU_count [(xnumber_of_nodes)] ...].
> For example: SLURM_JOB_CPUS_PER_NODE='72(x2),36' indicates that on the
> first and second nodes (as listed by SLURM_JOB_NODELIST) the
> allocation has 72 CPUs, while the third node has 36 CPUs.
>
> So, parsing '72(x2),36' seems complicated.
>
> If I requested a total of 1000 tasks, but have no control over how
> many nodes, can I just call parallel with -j1000 and pass it a
> hostfile without the "CPUs/" prepended to the hostname? Would parallel
> then start however many jobs it can per node, and if for some reason I
> was allocated 1000 CPUS on 1 node, that would work fine, as would 1
> CPU on 1000 different nodes?
>
> Thanks,
>
>   -k.