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Re: Parallelism a la make -j <n> / GNU parallel


From: Mike Frysinger
Subject: Re: Parallelism a la make -j <n> / GNU parallel
Date: Fri, 4 May 2012 14:53:31 -0400
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On Friday 04 May 2012 13:46:32 Andreas Schwab wrote:
> Mike Frysinger <address@hidden> writes:
> > i wish there was a way to use `wait` that didn't block until all the pids
> > returned.  maybe a dedicated option, or a shopt to enable this, or a new
> > command.
> > 
> > for example, if i launched 10 jobs in the background, i usually want to
> > wait for the first one to exit so i can queue up another one, not wait
> > for all of them.
> 
> If you set -m you can trap on SIGCHLD while waiting.

awesome, that's a good mitigation

#!/bin/bash
set -m
cnt=0
trap ': $(( --cnt ))' SIGCHLD
for n in {0..20} ; do
        (
                d=$(( RANDOM % 10 ))
                echo $n sleeping $d
                sleep $d
        ) &
        : $(( ++cnt ))
        if [[ ${cnt} -ge 10 ]] ; then
                echo going to wait
                wait
        fi
done
trap - SIGCHLD
wait

it might be a little racy (wrt checking cnt >= 10 and then doing a wait), but 
this is good enough for some things.  it does lose visibility into which pids 
are live vs reaped, and their exit status, but i more often don't care about 
that ...
-mike

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