root cron Post: 24218

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Top Forums UNIX for Dummies Questions & Answers root cron Post 24218 by Perderabo on Monday 8th of July 2002 02:10:06 PM

07-08-2002

Administrator Emeritus

Quote:

Originally posted by RTM
I have never heard of a maximum capacity for cron - I doubt you could reach it since those of us who have mistakenly created incorrect entries in cron which have created multiple processes in seconds can tell you, it won't be the crontab that gets to a maximum but the server - with runaway processes in endless loops. The most entries I have seen in a crontab to date: 171 (a DBA who has so much running in cron, I don't think he even needs to come to work anymore).

Modern versions of cron do have a limit. At least both HP-UX and Solaris do. And I have bumped into that limit many times. I have never heard on a limit of the number of entries in a crontab or a collection of crontabs. But there is a limit on the max number of sumultaneous cron jobs running. When you exceed this you get a message like this:
! c queue max run limit reached Sat Jul 6 04:00:00 2002
! rescheduling a cron job Sat Jul 6 04:00:00 2002
And yes, I copied that message from the log file on one of our servers.

The message is a little cryptic. If you look at your queuedefs file, you will probably find two lines, one for "at" jobs, and one for "batch" jobs. A third line that starts with a "c" could exist and it would be for cron jobs. In the absence of such a line, the defaults apply. That is 100, so you can have no more than 100 cron jobs running at once. But you can override this by editing your queuedefs file. And this is documented, see "man queuedefs".

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queuedefs

queuedefs(4)						     Kernel Interfaces Manual						      queuedefs(4)

NAME

       queuedefs - queue description file for at, batch, and crontab

SYNOPSIS

DESCRIPTION

       The  file  describes  the characteristics of the queues managed by (see cron(1M)).  Each non-comment line in this file describes one queue.
       The format of the lines are as follows:

	      [njob[nice[nwait

       The fields in this line are:

	      q 	The name of the queue, such that is the default queue for jobs started by (see at(1)), is the queue for  jobs  started	by
			(see  at(1)),  and  is	the  queue  for jobs run from a file (see crontab(1)).	Queue names through designate user-defined
			queues.

	      njob	The maximum number of jobs that can be run simultaneously in that queue.  Although any number can be specified here,  (see
			cron(1M))  by  default limits the number of jobs that can be run on all the queues to 100.  This limitation can be removed
			by setting the variable to 1 in the file.

	      nice	The value to give to all jobs in that queue that are not run with a user ID of	super-user  (see  nice(1)).   The  default
			value is 2.

	      nwait	The number of seconds to wait before rescheduling a job that was deferred because more than njob jobs were running in that
			job's queue, or because more than 100 jobs were running in all the queues (see njob above).

EXAMPLES

       Consider the following file:

       The file is interpreted as follows:

	      The	     queue, for jobs (see at(1)), can have up to 4 jobs running simultaneously, and those jobs will be run with a value of
			     1.

			     Since  no	nwait value is given, if a job cannot be run because too many other jobs are running, will wait 60 seconds
			     before trying again to run it (see cron(1M)).

	      The	     queue, for jobs (see at(1)), can have up to 2 jobs running simultaneously.  Those jobs will be run with a value of 2.
			     If a job cannot be run because too many other jobs are running, will wait 90 seconds before trying again to run it.

       All  other  queues  can have up to 100 jobs running simultaneously.  They will be run with a value of 2, and if a job cannot be run because
       too many other jobs are running, will wait 60 seconds before trying again to run it.

SEE ALSO

       at(1), nice(1), crontab(1), cron(1M), proto(4).

STANDARDS CONFORMANCE

																      queuedefs(4)

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