Cron (was: Re: [Tig] employment ad policy)

Rob Lingelbach rob
Mon Jul 15 14:07:03 BST 2002

On Mon, Jul 15, 2002 at 08:17:39AM +0100, Dick Hobbs wrote:
> All right. I'll ask. Why is the tig announce digest sent out at 17.49 UTC?
> Why not 17.53, a far superior moment time in my view?

you ask and you shall receive an explanation.  The almighty Cron, the
Unix scheduling daemon, runs jobs throughout the day/year/epoch.  In a 
system as busy as the one at, which physical host contains,
among others, the following virtual domains:,,,,, the
crontab (cron table file) has things happening almost all the time.  Good
system administration practice is to spread the load somewhat so processes
don't bog the machine down unneccessarily, though that doesn't create any
problems other than the (slight) bogging.  The tig-announce digest mailing,
going out as it does to 1400 addresses, fires up 10 parallel instances of
exim, the mail transport agent, and in a paean to symmetry and speed
delivers all 1400 messages in less than one minute.  It's substantially an
asynchronous process for which we are asking synchronicity.  The statistics
for deliveries in the last 24 hours:

Time spent on the queue: all messages

Under   1m     6271  91.7%   91.7%
        5m        6   1.3%   93.0%
       15m        8   1.7%   94.8%   
       30m        1   0.2%   95.0%
        1h        4   0.9%   95.9%
        3h        5   1.1%   96.9%
       12h        2   0.4%   97.4%
        1d        1   0.2%   97.6%
Over    1d       11   2.4%  100.0%   

If I were to change the moment of delivery initiation to one more propitious
for Mr. Hobbs, say 17.53 UTC, it might impinge on another CPU-intensive (10
simultaneous exims's at once do load the processor somewhat ) process and a
user such as I now logged into that host typing this message might even feel
a certain slowness.

Individual messages to the regular TIG that hit the fan of the
mailman/python/exim mailinglist redirection are subject to a niceness factor
in how they interact with ongoing processes, such that they are prioritized 
without regard to synchronicity.  

That table above by the way shows that if an individual message hits at e.g. 17.53 UTC, it will be delivered within 60 seconds
91.7% of the time.

I realize this is a lot of detail, but mail is one of my specialties. 
...and we didn't even touch on anacron, the newest Unix cron that adjusts
itself to your habits.

Rob Lingelbach      rob at

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