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Re: Spreading parallel across nodes on HPC system

From: Ken Mankoff
Subject: Re: Spreading parallel across nodes on HPC system
Date: Fri, 11 Nov 2022 08:24:54 +0100
User-agent: mu4e 1.8.10; emacs 27.1

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...



On 2022-11-10 at 20:49 +01, Ken Mankoff <> 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/
> 2/
> 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.

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