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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 15:13:55 -0400
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On Friday 04 May 2012 15:02:27 John Kearney wrote:
> Am 04.05.2012 20:53, schrieb Mike Frysinger:
> > 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 ...
> 
> That won't work I don't think.

seemed to work fine for me

> I think you meant something more like this?

no.  i want to sleep the parent indefinitely and fork a child asap (hence the 
`wait`), not busy wait with a one second delay.  the `set -m` + SIGCHLD 
interrupted the `wait` and allowed it to return.
-mike

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