>Assuming that I have a workload which will run effectively in 31 bit >mode, does anybody know if it will run more efficiently in 31 bit mode >than in 64 bit mode. That is, does 64 bit mode on System z have more >overhead (either hardware or software) than 31 bit mode? Or is this >another "it depends" question?
This is not a simple either 31-bit or 64-bit decision. The latest Linux on System z distributions like RHEL5 and SLES10 are only available as 64-bit operating system. So they require a 64-bit z/VM guest to run. As already mentioned in this thread there are applications which benefit from address spaces larger than 2 GB. If you think of SAP application servers, various database servers or bigger webservers for example. If you have a smaller server running an application which has no benefit of running in 64-bit you could run the 31-bit application binary in the build-in 31-bit emulation layer in 64-bit Linux. This doesn't prevent all of the 64-bit extra cost but gives you a better performance / cost ratio. Often tests showed for such small 31-bit applications an performance advantage and a better performance to cost ratio. If you have the source of your application available you can compile the application with and without the '-m31' parameter and test the performance in your special case. Then sometimes the answer is "it depends" of the scenario. Mit freundlichen Gruessen/Regards Mario Held System performance engineer IBM Germany Lab, Boeblingen ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
