02-20-2009
nohup has a terrible bad performance compared with interactive command, why?
I have a strange situation. I'm running a shell script containing several data uploads (using Oracle sqlloader utility). This script is being run on a Red Hat server.
I tried to run it in background:
$ nohup upload.sh &
This script uploads some thousands files. After several hours I checked what the nohup.out was showing and I was quite surprised to see it was very and extremely slow, because it just processed around less than a hundred files. Probably it would took more than 3 days to complete...
I ran the script interactively, I mean direct from command line showing output to screen. Surprise! the script ran several times better that when using nohup, finishing less than an hour.
I suspicted this was because the process using nohup has a slower priority than the one being run interactively. That was not true, both processes had the same priority.
Why this behaviour? I'm not a linux expert. Please help.
Thanks
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NOHUP(1) BSD General Commands Manual NOHUP(1)
NAME
nohup -- invoke a command immune to hangups
SYNOPSIS
nohup utility [arg ...]
DESCRIPTION
The nohup command allows the specified utility to be protected from termination if the user should become logged out (for example, due to a
modem line or TCP/IP connection being dropped). To do this, nohup sets the SIGHUP signal(3) (``terminal line hangup'') to be ignored, then
executes utility along with any arguments.
If the standard output is a terminal, the standard output is appended to the file nohup.out in the current directory. If standard error is a
terminal, it is directed to the same place as the standard output. If the output file nohup.out cannot be created in the current directory,
nohup attempts to create the file in the user's home directory. If the file nohup.out cannot be created, either in the current directory or
the user's home directory, nohup will exit without invoking utility, with an exit value as described below.
ENVIRONMENT
The following variable is used by nohup.
HOME User's home directory.
EXIT STATUS
The nohup utility exits with one of the following values:
126 The utility was found but could not be invoked.
127 The utility could not be found or an error occurred in nohup.
Otherwise, the exit status of nohup will be that of utility.
SEE ALSO
signal(3)
STANDARDS
The nohup command is expected to be IEEE Std 1003.2 (``POSIX.2'') compatible.
BSD
July 15, 2005 BSD