Re: [Qemu-devel] [Qemu-ppc] [RFC PATCH] smp: autodetect numbers of threads per core

[Alexander Graf]

On 14.04.14 15:10, Alexey Kardashevskiy wrote:
> On 04/14/2014 09:15 PM, Alexander Graf wrote:
> > On 15.11.13 17:58, Alexey Kardashevskiy wrote:
> > > On 16.11.2013 0:15, Alexander Graf wrote:
> > > > Am 15.11.2013 um 00:12 schrieb Alexey Kardashevskiy <address@hidden>:
> > > >
> > > > > At the moment only a whole CPU core can be assigned to a KVM. Since
> > > > > POWER7/8 support several threads per core, we want all threads of a core
> > > > > to go to the same KVM so every time we run QEMU with -enable-kvm on
> > > > > POWER, we have to add -smp X,threads=(4|8)" (4 for POWER7 and
> > > > > 8 for POWER8).
> > > > >
> > > > > This patch tries to read smp_threads number from an accelerator and
> > > > > falls back to the default value (1) if the accelerator did not care
> > > > > to change it.
> > > > >
> > > > > Signed-off-by: Alexey Kardashevskiy <address@hidden>
> > > > > ---
> > > > >
> > > > > (!!!)
> > > > >
> > > > > The usual question - what would be the normal way of doing this?
> > > > > What does this patch break? I cannot think of anything right now.
> > > > Is this really what the user wants? On p7 you can run in no-smt, smt2
> > > > and smt4 mode. Today we simply default to no-smt. Changing defaults
> > > > is usually a bad thing.
> > > Defaulting to 1 thread on P7 is a bad thing (other threads stay unused -
> > > what is good about this?) and the only reason which I know why it is
> > > still threads=1 is that it is hard to make a patch for upstream to
> > > change this default.
> > threads=1 improves single-thread performance significantly. The thread
> > itself is faster when it runs in SMT1 mode. Also we don't have to kick
> > other threads out of the guest context, making every guest/host transition
> > faster.
> >
> > Overall, it's really just a random default. I'm not sure it makes a lot of
> > sense to change it.
> >
> > However, could we be really smart here? How does ppc64_cpu --smt=off work?
> > It only turns off the unused vcpus, right? Is there any way we could
> > actually not even enter unused vcpus at all? Then we could indeed always
> > expose the maximum available number of threads to the guest and let that
> > one decide what to do.
> Now sure I understood the idea.
>
> If the user runs QEMU with the maximum number of threads and then stops the
> threads other than the very first in the guest, the effect will be the same
> as with threads=1 - POWER7/8 can detect stalled CPU and assign the entire
> internal bus thingy to one thread.

We would still have to enter all vcpus when we go into the guest, no? 
And the reverse as well - get out of all vcpus when we go into host 
virtual mode.

> On the other hand after all these lessons I received last month about QEMU
> parameters and everything, I do not feel I want this feature anymore -
> looks like we have to let libvirt deal with it :)
;)


Alex