Date: Fri, 18 Mar 2011 14:09:58 -0400 From: Jung-uk Kim <jkim@FreeBSD.org> To: freebsd-hackers@freebsd.org Cc: Kostik Belousov <kostikbel@gmail.com>, Bruce Evans <brde@optusnet.com.au>, Maxim Dounin <mdounin@mdounin.ru> Subject: Re: get_cyclecount(9) deprecation Message-ID: <201103181410.00726.jkim@FreeBSD.org> In-Reply-To: <20110319035138.H3038@besplex.bde.org> References: <201103171436.22283.jkim@FreeBSD.org> <20110319024813.N2581@besplex.bde.org> <20110319035138.H3038@besplex.bde.org>
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On Friday 18 March 2011 01:05 pm, Bruce Evans wrote: > On Sat, 19 Mar 2011, Bruce Evans wrote: > > On Fri, 18 Mar 2011, Kostik Belousov wrote: > >> We definitely do not support configurations with different > >> models of CPUs in SMP, this is what Simmetric is about. > >> Different as in frequency or stepping. > > > > ... > > Now there is even more asymmetry > > in core frequencies, with the hardware transiently slowing down > > or stopping cores independently, at least for cores in different > > packages. > > Also, with virtualization, the virtualizer cannot reasonably even > provide an invariant TSC that runs at the same rate on all cores. > It should provide an invariant TSC that claims to run at the same > rate on all cores, but then the cores cannot run at the same rate > except on average, since some of the cores will have to run the > virtualizer some of the time, and it is unreasonable to distribute > the overhead for this evenly except on average. Ah, virtualization... It is really painful to make it reasonably correct. For example, VMware claims that all timers (including TSC and excluding RTC) runs at "apparent time", which is concept of constant ticks in virtualized guest. It also means TSCs are always "virtually" constant and synchronized across all virtual cores in guest environment. If it loses periodic timer ticks, lost ticks are "compensated" later. This also means timecounter does not exactly scale well based on realtime and its frequency fluctuates so much, if my understanding is correct: http://www.vmware.com/files/pdf/Timekeeping-In-VirtualMachines.pdf I am not sure how others handle this but VirtualBox gives me really wacky TSC frequency sometimes. When it happens, it becomes unusably slow. So, I know something is really wrong there, too. However, Xen seems to do much smarter than that because it has its own concept of virtual TSC, thanks to its para-virtualization architecture. Jung-uk Kim
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