7

I have script I'd always like to run 'x' instances in parallel.

The code looks a like that:

for A in 
do
  for B in
  do
    (script1.sh $A $B;script2.sh $A $B) &
  done #B
done #A

The scripts itself run DB queries, so it would benefit from parallel running. Problem is

1) 'wait' doesn't work (because it finished all background jobs and starts new ones (even if I include a threadcounter), that wastes lots of time.

2) I couldn't figure out how to get parallel to do that. I only found examples where the same script gets run multiple times, but not with different parameters.

3) the alternative solution would be:

for A in 
do
  for B in
  do
    while threadcount>X 
    do
      sleep 60
    done
    (script1.sh $A $B;script2.sh $A $B) &
  done #B
done #A

But I didn't really figure out how to get the thread count reliable.

Some hints into the right direction are very much welcomed.


I'd love to use parallel, but the thing just doesn't work as the documentation tells me.

I do

parallel echo ::: A B C ::: D E F

(from the doc) and it tells me

parallel: Input is read from the terminal. Only experts do this on purpose. Press CTRL-D to exit.

and that is just the simplest example of the man pages.

1

2 Answers 2

9

Using GNU Parallel it looks like this:

parallel script1.sh {}';' script2.sh {} ::: a b c ::: d e f

It will spawn one job per CPU.

GNU Parallel is a general parallelizer and makes is easy to run jobs in parallel on the same machine or on multiple machines you have ssh access to. It can often replace a for loop.

If you have 32 different jobs you want to run on 4 CPUs, a straight forward way to parallelize is to run 8 jobs on each CPU:

Simple scheduling

GNU Parallel instead spawns a new process when one finishes - keeping the CPUs active and thus saving time:

GNU Parallel scheduling

Installation

If GNU Parallel is not packaged for your distribution, you can do a personal installation, which does not require root access. It can be done in 10 seconds by doing this:

(wget -O - pi.dk/3 || curl pi.dk/3/ || fetch -o - http://pi.dk/3) | bash

For other installation options see http://git.savannah.gnu.org/cgit/parallel.git/tree/README

Learn more

See more examples: http://www.gnu.org/software/parallel/man.html

Watch the intro videos: https://www.youtube.com/playlist?list=PL284C9FF2488BC6D1

Walk through the tutorial: http://www.gnu.org/software/parallel/parallel_tutorial.html

Sign up for the email list to get support: https://lists.gnu.org/mailman/listinfo/parallel

1
  • please see my edit at the bottom
    – Chris
    Commented Jun 24, 2015 at 23:26
1

I think you can do this with xargs:

for A in a b c
do
  for B in d e f
  do
    echo $A $B; done; done | xargs -I {} -P4 bash -c "script1.sh {}; script2.sh {}"

Where -P4 says to run 4 instances in parallel; you'll have to specify some value for that.

5
  • how sure are you about that? i tested your script (with echo ... in the script1/2.sh) and it is running them sequentially, no parallel processing here
    – Chris
    Commented Jun 24, 2015 at 2:46
  • Indeed, my looping was wrong. I've updated the answer to use a proper structure now.
    – techieb0y
    Commented Jun 24, 2015 at 3:38
  • the latest version runs them parallel, but only the first script, it does not run the second script. (or at least i can see the 'echo 2' from script2.sh, I can see the 'echo 1' from script1.sh)
    – Chris
    Commented Jun 24, 2015 at 11:16
  • Looks like you need to wrap the scripts in another shell layer (either by calling them both from a standalone script and having xargs run that, or inline as I've edited above).
    – techieb0y
    Commented Jun 24, 2015 at 12:55
  • last version was working, i needed to remove the ' from the {} otherwise it was using it as just one argument, not two, thanks
    – Chris
    Commented Jun 24, 2015 at 16:33

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