Messages in this thread | | | Date | Thu, 31 Mar 2011 12:30:23 +0200 | From | Avi Kivity <> | Subject | Re: [RFC 2/2] Make x86 calibrate_delay run in parallel. |
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On 03/31/2011 11:57 AM, Ingo Molnar wrote: > > > > I am not trying to be argumentative. I never got an understanding of > > what was going wrong with that earlier patch and am hoping for some > > understanding now. > > Well, if calibrate_delay() calls run in parallel then different hyperthreads > will impact each other.
It's different but not more wrong. If delay() later runs on a thread whose sibling is busy, it will in fact give more accurate results.
> > Why does it spectacularly miscalibrate? Can anything be done to correct > > that miscalibration? Doesn't this patch indicate another problem with > > the calibration for hotplug cpus? Isn't there already a problem if > > you boot a cpu normally, then hot-remove a hyperthread of a cpu, run a > > userland task which fully loads up all the cores on that socket, then > > hot-add that hyperthread back in? If the lpj value is that volatile, > > what value does it really have? > > The typical CPU hotplug usecase is suspend/resume, where we bring down the CPUs > in a more or less controlled manner. > > Yes, you could achieve something similar by frobbing /sys/*/*/online but that's > a big difference to *always* running the calibration loops in parallel. > > I'd argue for the opposite direction: only calibrate a physical CPU once per > CPU per bootup - this would also make CPU hotplug faster btw. > > ( Virtual CPUs (KVM, etc.) need a recalibration per bringup, because the new > CPU could be running on different hardware - but that's a detail: 4096 UV > CPUs are not in this category. )
Virtual cpus change their performance dynamically due to overcommit, live migration, the host scheduler rearranging them, etc.
> Really, there's no good reason why every CPU should be calibrated on a system > running identical CPUs, right? Mixed-frequency systems are rather elusive on > x86.
Good point. And udelay() users are probably not sensitive to accuracy anyway (which changes with load and thermal conditions).
-- error compiling committee.c: too many arguments to function
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