Why is parallel –nice not setting the niceness

gnu-parallel

If I run a command with nice, then I can see its process having the expected niceness value:

In one terminal:

nice sleep 17

and in another one:

$ ps -aoni,comm | grep sleep
 10 sleep

But trying to do the same with GNU parallel (version 20161222, Debian 9.3), I fail:

parallel --nice 10 sleep ::: 17

$ ps -aoni,comm | grep sleep
  0 sleep

I am probably missing something obvious, but what ?

update: perhaps it is just a bug, because it worked with older versions…

Best Answer

You have found a bug. Thanks.

It was introduced in parallel-20160522, and until now did not have any automated testing to check that --nice was working locally.

Next release will have both testing and --nice working.

The workaround for local jobs is to run parallel with nice:

nice -n 18 parallel bzip2 '<' ::: /dev/zero /dev/zero

The bug only affects local jobs: Remote jobs are niced as you would expect.

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It is due to GNU Parallel --pipe being slow.

cat bigfile |  parallel --pipe -L1000 --round-robin grep -f regexp.txt -

maxes out at around 100 MB/s.

In the man page example you will also find:

parallel --pipepart --block 100M -a bigfile grep -f regexp.txt

which does close to the same, but maxes out at 20 GB/s on a 64 core system.

parallel --pipepart --block 100M -a bigfile -k grep -f regexp.txt

should give exactly the same result as grep -f regexp.txt bigfile

Can GNU parallel output stdout before the program has exited

I think you're looking for --ungroup. The man page says:

--group  Group output. Output from each jobs is grouped 
         together and is only printed when the command is finished. 

         --group is the default. Can be reversed with -u.

-u of course is a synonym for --ungroup.

Best Answer

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