Re: [Qemu-devel] [RFC PATCH v2] PPC: smp: autodetect numbers of threads per core

[Alexey Kardashevskiy]

On 01/11/2014 12:28 AM, Alexander Graf wrote:
> 
> On 10.01.2014, at 14:03, Mike Day <address@hidden> wrote:
> 
>> 
>> Alexey Kardashevskiy <address@hidden> writes:
>> 
>>> On 01/10/2014 10:40 AM, Alexander Graf wrote:
>>>> 
>>> 
>>>> What if we make the max thread count a property of our cpu class?
>>>> The we
>>> can add a threads=max option which will be identical between kvm and
>>> tcg.
>>> 
>>> 
>>> You lost me here :) Right now the sequence is: 1. smp_parse 2.
>>> config_accelerator 3. machine_init
>>> 
>>> I proposed 1. config_accelerator - reads max threads from KVM (and
>>> initializes "host" type) 2. smp_parse - does the parsing using
>>> smp_threads tweaked in 1) 3. machine_init - creates CPUs which may
>>> or may be not "host".
>> 
>> The patch as it its now is very simple and well-contained. I wonder
>> how much it would expand if we added a max thread count to the cpu
>> class. It seems like the need for a max thread count is idiomatic to
>> powerpc.
> 
> It's only ever useful on IBM POWER. Any other PowerPC system can
> partition vcpus by host threads, it's only IBM POWER hardware that's as
> broken as it is.
> 

> But really, what problem are you trying to solve here? Do you have users
> that don't understand the constraints they have? You will still have
> this even with this patch, as if you do threads=max as default for KVM
> (which is a bad idea, because it diverges from TCG) -smp 5 would still
> allocate 2 host cores on p7 and you're not effectively using your
> resources.

I am not changing the default here. I am adding an ability to choose the
maximum.


> With the patch you're also not taking compat thread count into account.
> A POWER7 compat system would still need to manually specify threads=4,
> no?

Nope. I will need to smp_threads=min(smp_threads, 4) (and I do this in my
patches which I think I already posted but I need to repost them) so if it
is 1 by default, it will be still 1.


> I do see that the user experience is slightly suboptimal, but by
> creating this special case there's a good chance you're doing more harm
> than good.

Again. I do not change the default. I add an additional option (which is
false by default) to use the maximum of the CPU. What harm can it possibly
make?

I am definitely missing your point, sorry.


> 
> 
> Alex
> 


-- 
Alexey