Total of all shared memory vs. Total Physical memory

Hi listers, We have 8 instances of INFORMIX (-OnLine Version 7.23.UC4) on a HP-UX 10.20, 4 CPUs, 1.5GB Physical memory box. I am aware (thanks to this list) of the need to monitor with onstat -g seg and adjust SHMVIRTSIZE and SHMADD so as to avoid getting more than 3 shared memory segments for any given instance. But currently SHMTOTAL is 0 in all cases ===> unlimited. I am in favour of setting SHMTOTAL for each instance (starting conservatively with a high value, gradually shaving down to a 'sensible' level, preferably working on only 1 instance at a time). But my colleagues want to just leave SHMTOTAL as 0 for all instances. Dumb question time: Given that we have SHMTOTAL set to 0 for all 8 instances and thus no upper limit on the total amount of shared memory that they can grab, what happens if the total of all shared memory exceeds the total physical memory of 1.5GB ? Answers, flames, suggestions not to quit my day job etc. all welcomed ;-) TIA Michael michael_martin@komatsu.co.jp