how to write a process-pool bash shell

Question

I have more than 10 tasks to execute, and the system restrict that there at most 4 tasks can run at the same time.

My task can be started like: myprog taskname

How can I write a bash shell script to run these task. The most important thing is that when one task finish, the script can start another immediately, making the running tasks count remain 4 all the time.

Answer 1

This tested script runs 5 jobs at a time and will restart a new job as soon as it does (due to the kill of the sleep 10.9 when we get a SIGCHLD. A simpler version of this could use direct polling (change the sleep 10.9 to sleep 1 and get rid of the trap).

#!/usr/bin/bash

set -o monitor
trap "pkill -P $$ -f 'sleep 10\.9' >&/dev/null" SIGCHLD

totaljobs=15
numjobs=5
worktime=10
curjobs=0
declare -A pidlist

dojob()
{
  slot=$1
  time=$(echo "$RANDOM * 10 / 32768" | bc -l)
  echo Starting job $slot with args $time
  sleep $time &
  pidlist[$slot]=`jobs -p %%`
  curjobs=$(($curjobs + 1))
  totaljobs=$(($totaljobs - 1))
}

# start
while [ $curjobs -lt $numjobs -a $totaljobs -gt 0 ]
 do
  dojob $curjobs
 done

# Poll for jobs to die, restarting while we have them
while [ $totaljobs -gt 0 ]
 do
  for ((i=0;$i < $curjobs;i++))
   do
    if ! kill -0 ${pidlist[$i]} >&/dev/null
     then
      dojob $i
      break
     fi
   done
   sleep 10.9 >&/dev/null
 done
wait

Answer 2

You could probably do something clever with signals.

Note this is only to illustrate the concept, and thus not thoroughly tested.

#!/usr/local/bin/bash

this_pid="$$"
jobs_running=0
sleep_pid=

# Catch alarm signals to adjust the number of running jobs
trap 'decrement_jobs' SIGALRM

# When a job finishes, decrement the total and kill the sleep process
decrement_jobs()
{
  jobs_running=$(($jobs_running - 1))
  if [ -n "${sleep_pid}" ]
  then
    kill -s SIGKILL "${sleep_pid}"
    sleep_pid=
  fi
}

# Check to see if the max jobs are running, if so sleep until woken
launch_task()
{
  if [ ${jobs_running} -gt 3 ]
  then
    (
      while true
      do
        sleep 999
      done
    ) &
    sleep_pid=$!
    wait ${sleep_pid}
  fi

  # Launch the requested task, signalling the parent upon completion
  (
    "$@"
    kill -s SIGALRM "${this_pid}"
  ) &
  jobs_running=$((${jobs_running} + 1))
}

# Launch all of the tasks, this can be in a loop, etc.
launch_task task1
launch_task tast2
...
launch_task task99

Answer 3

Following @Parag Sardas' answer and the documentation linked here's a quick script you might want to add on your .bash_aliases.

Relinking the doc link because it's worth a read

#!/bin/bash
# https://stackoverflow.com/a/19618159
# https://stackoverflow.com/a/51861820
#
# Example file contents:
# touch /tmp/a.txt
# touch /tmp/b.txt

if [ "$#" -eq 0 ];  then
  echo "$0 <file> [max-procs=0]"
  exit 1
fi

FILE=${1}
MAX_PROCS=${2:-0}
cat $FILE | while read line; do printf "%q\n" "$line"; done | xargs --max-procs=$MAX_PROCS -I CMD bash -c CMD

I.e. ./xargs-parallel.sh jobs.txt 4 maximum of 4 processes read from jobs.txt

Answer 4

I chanced upon this thread while looking into writing my own process pool and particularly liked Brandon Horsley's solution, though I couldn't get the signals working right, so I took inspiration from Apache and decided to try a pre-fork model with a fifo as my job queue.

The following function is the function that the worker processes run when forked.

# \brief the worker function that is called when we fork off worker processes
# \param[in] id  the worker ID
# \param[in] job_queue  the fifo to read jobs from
# \param[in] result_log  the temporary log file to write exit codes to
function _job_pool_worker()
{
    local id=$1
    local job_queue=$2
    local result_log=$3
    local line=

    exec 7<> ${job_queue}
    while [[ "${line}" != "${job_pool_end_of_jobs}" && -e "${job_queue}" ]]; do
        # workers block on the exclusive lock to read the job queue
        flock --exclusive 7
        read line <${job_queue}
        flock --unlock 7
        # the worker should exit if it sees the end-of-job marker or run the
        # job otherwise and save its exit code to the result log.
        if [[ "${line}" == "${job_pool_end_of_jobs}" ]]; then
            # write it one more time for the next sibling so that everyone
            # will know we are exiting.
            echo "${line}" >&7
        else
            _job_pool_echo "### _job_pool_worker-${id}: ${line}"
            # run the job
            { ${line} ; } 
            # now check the exit code and prepend "ERROR" to the result log entry
            # which we will use to count errors and then strip out later.
            local result=$?
            local status=
            if [[ "${result}" != "0" ]]; then
                status=ERROR
            fi  
            # now write the error to the log, making sure multiple processes
            # don't trample over each other.
            exec 8<> ${result_log}
            flock --exclusive 8
            echo "${status}job_pool: exited ${result}: ${line}" >> ${result_log}
            flock --unlock 8
            exec 8>&-
            _job_pool_echo "### _job_pool_worker-${id}: exited ${result}: ${line}"
        fi  
    done
    exec 7>&-
}

You can get a copy of my solution at Github. Here's a sample program using my implementation.

#!/bin/bash

. job_pool.sh

function foobar()
{
    # do something
    true
}   

# initialize the job pool to allow 3 parallel jobs and echo commands
job_pool_init 3 0

# run jobs
job_pool_run sleep 1
job_pool_run sleep 2
job_pool_run sleep 3
job_pool_run foobar
job_pool_run foobar
job_pool_run /bin/false

# wait until all jobs complete before continuing
job_pool_wait

# more jobs
job_pool_run /bin/false
job_pool_run sleep 1
job_pool_run sleep 2
job_pool_run foobar

# don't forget to shut down the job pool
job_pool_shutdown

# check the $job_pool_nerrors for the number of jobs that exited non-zero
echo "job_pool_nerrors: ${job_pool_nerrors}"

Hope this helps!

Answer 5

I would suggest writing four scripts, each one of which executes a certain number of tasks in series. Then write another script that starts the four scripts in parallel. For instance, if you have scripts, script1.sh, script2.sh, script3.sh, and script4.sh, you could have a script called headscript.sh like so.

#!/bin/sh
./script1.sh & 
./script2.sh & 
./script3.sh & 
./script4.sh &

Answer 6

Using GNU Parallel you can do:

cat tasks | parallel -j4 myprog

If you have 4 cores, you can even just do:

cat tasks | parallel myprog

From http://git.savannah.gnu.org/cgit/parallel.git/tree/README:

Full installation

Full installation of GNU Parallel is as simple as:

./configure && make && make install

Personal installation

If you are not root you can add ~/bin to your path and install in ~/bin and ~/share:

./configure --prefix=$HOME && make && make install

Or if your system lacks 'make' you can simply copy src/parallel src/sem src/niceload src/sql to a dir in your path.

Minimal installation

If you just need parallel and do not have 'make' installed (maybe the system is old or Microsoft Windows):

wget http://git.savannah.gnu.org/cgit/parallel.git/plain/src/parallel
chmod 755 parallel
cp parallel sem
mv parallel sem dir-in-your-$PATH/bin/

Test the installation

After this you should be able to do:

parallel -j0 ping -nc 3 ::: foss.org.my gnu.org freenetproject.org

This will send 3 ping packets to 3 different hosts in parallel and print the output when they complete.

Watch the intro video for a quick introduction: https://www.youtube.com/playlist?list=PL284C9FF2488BC6D1