briggs%gburg.DECnet@BLUTO.SCC.COM ("GBURG::BRIGGS") (07/13/87)
>I've been submitting CPU intensive batch jobs on our VAX 8650 with the >priority set at 1. It uses some pretty big arrays so it tends to page >ALOT. When I run my job people say that they notice a degredation to >the system. I say that my job shouldn't pose a problem because it's >priority is lower than theirs. >Am I wrong in assumming that since my job's priority is set at 1, it >shouldn't interfere with anybody with a higher priority? A job that pages for this kind of reason can indeed cause performance problems on the system as a whole. I would guess that the main impact in your case is disk I/O as the swapper writes modified pages back to the page file. There is probably some secondary impact because pages belonging to the batch job will fill up the free page list. This will reduce the "hit rate" of other processes page faults and the ratio of hard faults to soft faults will degrade. If you don't have adequate memory on the free page list then things get really bad and the batch application will start doing hard page faults. The disk I/O required to satisfy these faults would seriously degrade the performance of your paging disk. One solution to the problem is to modify the batch application so that it uses its arrays differently. If you can manage to access the arrays sequentially, page faults should go down. Sometimes, this is simply a matter of transposing the subscripts on a two dimensional array. The other solution is to increase the working set on the batch job until the pathological page faulting goes away. This could be done by increasing the working set size on the batch queue. Make sure that the SYSGEN parameter WSMAX is set at least as high as the working set you need. ------