On 8/29/07, David Kreuter <[EMAIL PROTECTED]> wrote: > Well, maybe. What about the case where, say, a WAS server is running 31-bit > and is fighting with itself with multiple address spaces for heap storage, > and all must below the 2G line? ... and can be thrashing around doing this > management all the time? That's not good.
This is getting a bit beyond the original question I think. If your WAS server is only certified by the vendor to run in 64-bit Linux and its so big it does not fit in 2GB, then that's different from "assume my workload runs in... " The original question imho is very legitimate and understanding, because on some hardware platforms the 64-bit CPU runs the 32-bit code slower than a 32-bit CPU with the same clock speed. From what I know, this is not the case with System z. There is no performance penalty for running in 32-bit mode on a CPU that could do 64-bit mode. > I really don't say a few grains of sand as a big deal for CP to build 64-bit > vs 31-bit page and segment tables. etc. That's purely related to the virtual machine size and the z/VM architecture, and does not change when the guest switches into 64-bit mode. The big difference is that all kind of things in Linux get twice as big. All memory management structures get bigger - because Linux can address more page frames and because process address spaces get bigger. Context change needs to save the double amount of register contents (though this one probably does not take twice as long). A lot of other things in Linux also changed from 32 bit to 64 bit. It takes more memory, and it takes more CPU cycles to manipulate those objects. And when the JVM becomes 64-bit, it will also have bigger objects to manipulate. When we try to compare apples with apples: when running in a 512M virtual machine, the 32-bit distribution leaves more free for the application that the 64-bit would. And a 64-bit application will leave less room for the workload than a 32-bit application would. And the same workload in the 64-bit case will take more CPU cycles than in a 32-bit environment. But this is a moot point when the application vendor decides for you. Rob -- Rob van der Heij Velocity Software, Inc http://velocitysoftware.com/ ---------------------------------------------------------------------- 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
