[Halld-offline] short-term advantage to switching to centos7

[Mark Ito]

Folks,

Over the past week or so many of you have likely been experiencing long 
wait times for your jobs to start on the centos65 OS on the JLab farm. 
Currently, with centos7 having more cpu power, and halld being the 
majority user under centos7, halld is logging a lot of cpu time, 
globally. This counts against our priority, thus the long wait times for 
the centos65 jobs. The halld jobs under centos7 would experience the 
same long wait times, except that halld has relatively little 
competition for those nodes. That lack of competition allows us to rack 
up even more cpu time making the situation even worse for the centos65 
halld jobs.

In the long run, the others halls will be running under centos7 as well 
and our advantage on that side will disappear. Until that happens, halld 
jobs running on the centos65 side will be severely disadvantaged. We 
could help those jobs by stopping the running under centos7, but you can 
see that that would be counter-productive to the global effort.

Bottom line, making the switch will pay big dividends over the next few 
weeks until the other halls make the same move.

   -- Mark