On 14 Apr 2012 at 9:45, John Wendel wrote:
Date sent: Sat, 14 Apr 2012 09:45:50 -0700
From: John Wendel <jwendel10@xxxxxxxxxxx>
To: users@xxxxxxxxxxxxxxxxxxxxxxx
Subject: Re: off topic: combined output of concurrent
processes
> On 04/14/2012 08:20 AM, Amadeus W.M. wrote:
> >> If you really would like to get output in sequence, write to a
> >> pipe, and have a reader process drain the pipe to a logfile. It's
> >> pretty easy; look at "mknod" with the 'p' option, or "mkfifo". I'd
> >> still suggest tagging each output line with an identifier and
> >> sequence number.
> >>
> > For the sake of the argument, assume I echo 500 As, 500 Bs and 500
> > Cs.
> >
> > I don't care which process the output is coming from. It doesn't
> > matter which order the As, Bs and Cs are output. All I care about is
> > that I don't get 349As followed by 245Bs, etc. I want to see blocks
> > of 500 each.
> >
> > I don't see how echoing into a pipe would change the problem.
> > Theoretically, if several processes (e.g. echo) are running in the
> > background, e.g. on a round robin basis, then potentially I could
> > see random sequences of As, Bs and Cs. It doesn't seem to be the
> > case in practice though. So which is it?
> >
> > This has to do with the operating system internals, it's not a
> > trivial question.
> >
> Actually it is semi-non-trivial. :-)
>
> Unix/Linux makes the following guarantee ...
>
> Multiple processes that open the same file for writing each maintain
> their own file positions, so they may overwrite the output of another
> process, unless the processes all open the file with the "O_APPEND"
> option. With the "O_APPEND" option, the system guarantees that the
> entire data from a single write by a process will be written to the
> end of the file as a indivisible block and will not be mixed with the
> output from another process. Without "O_APPEND", data from multiple
> processes may be intermixed in any order, or may seem to "disappear"
> (is overwritten by other data).
>
> Of course, if you didn't write the code that is doing the output,
> you'll have to examine the source to see if it uses the "O_APPEND"
> open option. This may be non-trivial.
>
Interesting thread. Did the following modified test to see what I
would get, and at the end it does get 100 100 600 for the As, Bs,
and Cs by using a file for the output instead of stdout.
The resulting file does a somewhat random pattern of the 5
character blocks. This was run on my quad core system.
#!/bin/bash
rm ./outtest
i=0
while [ $i -lt 100 ];
do
echo "AAAAA" >>./outtest & # e.g. 500 As
echo "BBBBB" >>./outtest &
echo "CCCCC" >>./outtest &
i=$(($i+1))
done
sleep 5
grep ^A ./outtest |wc
grep ^B ./outtest |wc
grep ^C ./outtest |wc
> Regards,
>
> John
>
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