Forking / Multi-Threaded Processes | Bash
BashShellForkBash Problem Overview
I would like to make a section of my code more efficient. I'm thinking of making it fork off into multiple processes and have them execute 50/100 times at once, instead of just once.
For example (pseudo):
for line in file;
do
foo;
foo2;
foo3;
done
I would like this for loop to run multiple times. I know this can be done with forking. Would it look something like this?
while(x <= 50)
parent(child pid)
{
fork child()
}
child
{
do
foo; foo2; foo3;
done
return child_pid()
}
Or am I thinking about this the wrong way?
Thanks!
Bash Solutions
Solution 1 - Bash
In bash scripts (non-interactive) by default JOB CONTROL is disabled so you can't do the the commands: job, fg, and bg.
Here is what works well for me:
#!/bin/sh
set -m # Enable Job Control
for i in `seq 30`; do # start 30 jobs in parallel
sleep 3 &
done
# Wait for all parallel jobs to finish
while [ 1 ]; do fg 2> /dev/null; [ $? == 1 ] && break; done
The last line uses "fg" to bring a background job into the foreground. It does this in a loop until fg returns 1 ($? == 1), which it does when there are no longer any more background jobs.
Solution 2 - Bash
I don't know of any explicit fork call in bash. What you probably want to do is append &
to a command that you want to run in the background. You can also use & on functions that you define within a bash script:
do_something_with_line()
{
line=$1
foo
foo2
foo3
}
for line in file
do
do_something_with_line $line &
done
EDIT: to put a limit on the number of simultaneous background processes, you could try something like this:
for line in file
do
while [`jobs | wc -l` -ge 50 ]
do
sleep 5
done
do_something_with_line $line &
done
Solution 3 - Bash
I don't like using wait because it gets blocked until the process exits, which is not ideal when there are multiple process to wait on as I can't get a status update until the current process is done. I prefer to use a combination of kill -0 and sleep to this.
Given an array of pids to wait on, I use the below waitPids() function to get a continuous feedback on what pids are still pending to finish.
declare -a pids
waitPids() {
while [ ${#pids[@]} -ne 0 ]; do
echo "Waiting for pids: ${pids[@]}"
local range=$(eval echo {0..$((${#pids[@]}-1))})
local i
for i in $range; do
if ! kill -0 ${pids[$i]} 2> /dev/null; then
echo "Done -- ${pids[$i]}"
unset pids[$i]
fi
done
pids=("${pids[@]}") # Expunge nulls created by unset.
sleep 1
done
echo "Done!"
}
When I start a process in the background, I add its pid immediately to the pids array by using this below utility function:
addPid() {
local desc=$1
local pid=$2
echo "$desc -- $pid"
pids=(${pids[@]} $pid)
}
Here is a sample that shows how to use:
for i in {2..5}; do
sleep $i &
addPid "Sleep for $i" $!
done
waitPids
And here is how the feedback looks:
Sleep for 2 -- 36271
Sleep for 3 -- 36272
Sleep for 4 -- 36273
Sleep for 5 -- 36274
Waiting for pids: 36271 36272 36273 36274
Waiting for pids: 36271 36272 36273 36274
Waiting for pids: 36271 36272 36273 36274
Done -- 36271
Waiting for pids: 36272 36273 36274
Done -- 36272
Waiting for pids: 36273 36274
Done -- 36273
Waiting for pids: 36274
Done -- 36274
Done!
Solution 4 - Bash
With GNU Parallel you can do:
cat file | parallel 'foo {}; foo2 {}; foo3 {}'
This will run one job on each cpu core. To run 50 do:
cat file | parallel -j 50 'foo {}; foo2 {}; foo3 {}'
Watch the intro videos to learn more:
Solution 5 - Bash
haridsv's approach is great, it gives the flexibility to run a processor slots setup where a number of processes can be kept running with new jobs submitting as jobs complete, keeping the overall load up. Here are my mods to haridsv's code for an n-slot processor for a 'grid' of ngrid 'jobs' ( I use it for grids of simulation models ) Followed by test output for 8 jobs 3 at a time, with running totals of running, submitted, completed and remaining
#!/bin/bash
########################################################################
# see haridsv on forking-multi-threaded-processes-bash
# loop over grid, submitting jobs in the background.
# As jobs complete new ones are set going to keep the number running
# up to n as much as possible, until it tapers off at the end.
#
# 8 jobs
ngrid=8
# 3 at a time
n=3
# running counts
running=0
completed=0
# previous values
prunning=0
pcompleted=0
#
########################################################################
# process monitoring functions
#
declare -a pids
#
function checkPids() {
echo ${#pids[@]}
if [ ${#pids[@]} -ne 0 ]
then
echo "Checking for pids: ${pids[@]}"
local range=$(eval echo {0..$((${#pids[@]}-1))})
local i
for i in $range; do
if ! kill -0 ${pids[$i]} 2> /dev/null; then
echo "Done -- ${pids[$i]}"
unset pids[$i]
completed=$(expr $completed + 1)
fi
done
pids=("${pids[@]}") # Expunge nulls created by unset.
running=$((${#pids[@]}))
echo "#PIDS :"$running
fi
}
#
function addPid() {
desc=$1
pid=$2
echo " ${desc} - "$pid
pids=(${pids[@]} $pid)
}
########################################################################
#
# Loop and report when job changes happen,
# keep going until all are completed.
#
idx=0
while [ $completed -lt ${ngrid} ]
do
#
if [ $running -lt $n ] && [ $idx -lt ${ngrid} ]
then
####################################################################
#
# submit a new process if less than n
# are running and we haven't finished...
#
# get desc for process
#
name="job_"${idx}
# background execution
sleep 3 &
addPid $name $!
idx=$(expr $idx + 1)
#
####################################################################
#
fi
#
checkPids
# if something changes...
if [ ${running} -gt ${prunning} ] || \
[ ${completed} -gt ${pcompleted} ]
then
remain=$(expr $ngrid - $completed)
echo " Running: "${running}" Submitted: "${idx}\
" Completed: "$completed" Remaining: "$remain
fi
# save counts to prev values
prunning=${running}
pcompleted=${completed}
#
sleep 1
#
done
#
########################################################################
Test output:
job_0 - 75257
1
Checking for pids: 75257
#PIDS :1
Running: 1 Submitted: 1 Completed: 0 Remaining: 8
job_1 - 75262
2
Checking for pids: 75257 75262
#PIDS :2
Running: 2 Submitted: 2 Completed: 0 Remaining: 8
job_2 - 75267
3
Checking for pids: 75257 75262 75267
#PIDS :3
Running: 3 Submitted: 3 Completed: 0 Remaining: 8
3
Checking for pids: 75257 75262 75267
Done -- 75257
#PIDS :2
Running: 2 Submitted: 3 Completed: 1 Remaining: 7
job_3 - 75277
3
Checking for pids: 75262 75267 75277
Done -- 75262
#PIDS :2
Running: 2 Submitted: 4 Completed: 2 Remaining: 6
job_4 - 75283
3
Checking for pids: 75267 75277 75283
Done -- 75267
#PIDS :2
Running: 2 Submitted: 5 Completed: 3 Remaining: 5
job_5 - 75289
3
Checking for pids: 75277 75283 75289
#PIDS :3
Running: 3 Submitted: 6 Completed: 3 Remaining: 5
3
Checking for pids: 75277 75283 75289
Done -- 75277
#PIDS :2
Running: 2 Submitted: 6 Completed: 4 Remaining: 4
job_6 - 75298
3
Checking for pids: 75283 75289 75298
Done -- 75283
#PIDS :2
Running: 2 Submitted: 7 Completed: 5 Remaining: 3
job_7 - 75304
3
Checking for pids: 75289 75298 75304
Done -- 75289
#PIDS :2
Running: 2 Submitted: 8 Completed: 6 Remaining: 2
2
Checking for pids: 75298 75304
#PIDS :2
2
Checking for pids: 75298 75304
Done -- 75298
#PIDS :1
Running: 1 Submitted: 8 Completed: 7 Remaining: 1
1
Checking for pids: 75304
Done -- 75304
#PIDS :0
Running: 0 Submitted: 8 Completed: 8 Remaining: 0
Solution 6 - Bash
Let me try example
for x in 1 2 3 ; do { echo a $x ; sleep 1 ; echo b $x ; } & done ; sleep 10
And use jobs to see what's running.
Solution 7 - Bash
Based on what you all shared I was able to put this together:
#!/usr/bin/env bash
VAR1="192.168.1.20 192.168.1.126 192.168.1.36"
for a in $VAR1; do { ssh -t -t $a -l Administrator "sudo softwareupdate -l"; } & done;
WAITPIDS="$WAITPIDS "$!;...; wait $WAITPIDS
echo "Script has finished"
Exit 1
This lists all the updates on the mac on three machines at once. Later on I used it to perform a software update for all machines when i CAT my ipaddress.txt
Solution 8 - Bash
Here's my thread control function:
#!/bin/bash
# This function just checks jobs in background, don't do more things.
# if jobs number is lower than MAX, then return to get more jobs;
# if jobs number is greater or equal to MAX, then wait, until someone finished.
# Usage:
# thread_max 8
# thread_max 0 # wait, until all jobs completed
thread_max() {
local CHECK_INTERVAL="3s"
local CUR_THREADS=
local MAX=
[[ $1 ]] && MAX=$1 || return 127
# reset MAX value, 0 is easy to remember
[ $MAX -eq 0 ] && {
MAX=1
DEBUG "waiting for all tasks finish"
}
while true; do
CUR_THREADS=`jobs -p | wc -w`
# workaround about jobs bug. If don't execute it explicitily,
# CUR_THREADS will stick at 1, even no jobs running anymore.
jobs &>/dev/null
DEBUG "current thread amount: $CUR_THREADS"
if [ $CUR_THREADS -ge $MAX ]; then
sleep $CHECK_INTERVAL
else
return 0
fi
done
}