[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC
> >From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx] > > > >I implemented this as xen-unstable:21468. This represents a strict > >improvement on what was in xen-unstable before that (no tsc sync at > all, > >ever, because I deleted it about a week ago). Open to further > improvements, > >if we can get consensus. > > Thanks for the patch. I will get the system that support CPU hotplug > tomorrow morning, I can try it then. > > --jyh Hmmm... I'd be very surprised if this works ever, let alone always across all hardware. By "work", I mean that it will result in an "undetectably small difference" (less than a cache bounce), as defined and measured by the tsc warp test. Why? Because rdtsc is a long instruction (30-100 cycles) and I'll bet writing to the TSC is even longer. Plus, there's a cache bounce overhead added in due to the synchronization via the in-memory tsc_count variable. Our firmware guys say that TSC synchronization can't be implemented algorithmically in firmware... it requires a simultaneous "reset" signal to all sockets/cores, which is obviously not an option here. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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