On Friday, 04/17/2015 at 02:37 EDT, "Pavelka, Tomas" <[email protected]> wrote: > I just want to ensure I am understanding correctly what your are saying: Is it > true that waits under software SIE burn CPU where the same waits under hardware > SIE do not? That would explain a lot of the performance problems I have seen > but I am sure there are caveats.
It depends on how the wait is done. If you set a timer to pop when you want to wake up, there is a lot of variation between 1st and 2nd level, depending on how far in the future the timer goes off. The derivative and worst case is to keep waking up every few milliseconds to see if it's time to wake up. Imagine hitting the snooze button on your alarm clock every 7 minutes just to see if it's time to get up. Right. You'd be cranky, too. When the 2nd level guest issues the SIE instruction, the 1st level system converts the 2nd level SIE into 1st level SIE. I call this the "SIE Pancake Effect." This includes a time slice of a time slice and just a few pages of 3rd level guest resident memory. Or CP simulates an instruction. But the more often do you those things, the more overhead is incurred. And if all you're doing is going back to sleep, then it's a lot of CPU burn for no benefit. The miracle is the 3rd level guests run as well as they do. It's the speed of the CPU that save us and is what has allowed 2nd level systems to operate in production. This polling issue helps illustrate why future performance improvements must come primarily from improvements in software - we're nearing the end of the acceleration lane on the current processor technology. Alan Altmark Senior Managing z/VM and Linux Consultant Lab Services System z Delivery Practice IBM Systems & Technology Group ibm.com/systems/services/labservices office: 607.429.3323 mobile; 607.321.7556 [email protected] IBM Endicott ---------------------------------------------------------------------- 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 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
