On 03/06/2007 03:53 PM, Thomas Gleixner wrote: >> 2) Virtual interrupts have a relatively high overhead as compared with >> native interrupts. So, in vmitime, we wanted to be able to lower the >> timer interrupt rate at runtime, even if HZ is a compile time constant >> (and set to something high, like 1000hz). While we could hack this in >> by using evt->min_delta_ns, it wouldn't really work since process time >> accounting would be wrong. Instead, we should allow the >> tick_sched_timer in cases (c) and (d) to have runtime configurable >> period, and then scale the time value accordingly before passing to >> account_system_time. This is probably something the Xen folks will want >> also, since I think Xen itself only gets 100hz hard timer, and so it can >> implement at best a oneshot virtual timer with 100hz resolution. Any >> objections to us doing something like this? > > Yes. It's gross hackery. > > 1) We want to have a cleanup of the tick assumptions _all_ over the > place and this is going to be real hard work. > > 2) As I said above. The time accounting for virtualization needs to be > fixed in a generic way. > > I'm not going to accept some weird hackery for virtualization, which is > of exactly ZERO value for the kernel itself. Quite the contrary it will > make the cleanup harder and introduce another hard to remove thing, > which will in the worst case last for ever. > Okay, to confirm I'm on the same page as you, you want to move process time accounting from being periodic sampled based to being trace based? i.e. at the system-call/interrupt boundaries, read clocksource and compute directly the amount of system/user/process time? Do you know if anyone has explored this? I thought there was a discussion about this a while back but it was rejected due to the sample-based approach having much lower overheads on high system call rate workloads. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxx https://lists.osdl.org/mailman/listinfo/virtualization
