# Using julia -L startupfile.jl, rather than machinefiles for starting workers.

Re-posted from: http://white.ucc.asn.au/2017/08/17/starting-workers.html

If one wants to have full control over the worker process to method to use is addprocs and the -L startupfile.jl commandline arguement when you start julia
See the documentation for addprocs.

The simplest way to add processes to the julia worker is to invoke it with julia -p 4.
The -p 4 argument says start 4 worker processes, on the local machine.
For more control, one uses julia --machinefile ~/machines
Where ~/machines is a file listing the hosts.
The machinefile is often just a list of hostnames/IP-addresses,
but sometimes is more detailed.
Julia will connect to each host and start a number of workers on each equal to the number of cores.

Even the most detailed machinefile doesn’t give full control,
for example you can not specify the topology, or the location of the julia exectuable.

For full control, one shoud invoke addprocs directly,
and to do so, one should use julia -L startupfile.jl

Inkoking julia with julia -L startupfile.jl ...
causes julia to exectute startupfile.jl before all other things.
It can be invoked to start the REPL, or with a normal script afterwoods.
It is thus a good place to do addprocs rather than doing it at the top of the script being run,
since this allows the main script to be concerned only with the task.

On to a proper example,
let us assume one has 4 servers.

• host1 with 24 available cores
• host2 with 12 available cores
• host3 with 8 available cores

Doing this with machinefile would just be having file:

host1
host2
host3


Assuming the number of workers desired is equal to the number of cores;
and you don’t need anything fancy then using julia -m ~/machinefile is fine.
If you are working on a supercomputer or a cluster you likely have a method to generate a machinefile – it is the same as is used by MPI etc.

However, let us also say that you want to use the :master_slave topology,
so all workers are only allowed to talk to the master process,
not to each other.
This is common in many clusters.

We can start a number of local workers, using addproc(4).

for host in ["host1", "host2", "host3"]
end


There is also an overload for this which takes a vector of hostnames.
So this can be done as:

addproc(["host1", "host2", "host3"]; topology=:master_slave)


This method will use the default number of workers, i.e. equal to the core count.
It also takes an overload allowing one to specify the number of worker on each process:

addproc([("host1", 24), ("host2", 12), ("host3", 8)]; topology=:master_slave)


Further to this, instread of a hostname,
one can provide a line from a machine file.

Thus:
one can read every line from a machinefile by: collect(eachline("~/machinefile"))

addproc(collect(eachline("~/machinefile")); topology=:master_slave)


This is useful, if your system is providing you with a machine file.

The short of all this is,
using julia -L startup.jl is the most powerful,
and generally best way to do anything when it comes to setting up your distributed julia worker processes.