Re: [RFC 2/2] Make x86 calibrate_delay run in parallel.

[Avi Kivity <avi@redhat.com>]

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).

-- 
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