Re: [PATCH] KVM/x86: Increase max vcpu number to 352

[Lan Tianyu]

On 2017å08æ12æ 03:35, Konrad Rzeszutek Wilk wrote:
> On Fri, Aug 11, 2017 at 03:00:20PM +0200, Radim KrÄmÃÅ wrote:
>> 2017-08-11 10:11+0200, David Hildenbrand:
>>> On 11.08.2017 09:49, Lan Tianyu wrote:
>>>> Hi Konrad:
>>>> 	Thanks for your review.
>>>>
>>>> On 2017å08æ11æ 01:50, Konrad Rzeszutek Wilk wrote:
>>>>> On Thu, Aug 10, 2017 at 06:00:59PM +0800, Lan Tianyu wrote:
>>>>>> Intel Xeon phi chip will support 352 logical threads. For HPC usage
>>>>>> case, it will create a huge VM with vcpu number as same as host cpus. This
>>>>>> patch is to increase max vcpu number to 352.
>>>>>
>>>>> Why not 1024 or 4096?
>>>>
>>>> This is on demand. We can set a higher number since KVM already has
>>>> x2apic and vIOMMU interrupt remapping support.
>>>>
>>>>>
>>>>> Are there any issues with increasing the value from 288 to 352 right now?
>>>>
>>>> No found.
>>
>> Yeah, the only issue until around 2^20 (when we reach the maximum of
>> logical x2APIC addressing) should be the size of per-VM arrays when only
>> few VCPUs are going to be used.
> 
> Migration with 352 CPUs all being busy dirtying memory and also poking
> at various I/O ports (say all of them dirtying the VGA) is no problem?

This depends on what kind of workload is running during migration. I
think this may affect service down time since there maybe a lot of dirty
memory data to transfer after stopping vcpus. This also depends on how
user sets "migrate_set_downtime" for qemu. But I think increasing vcpus
will break migration function.

> 
> 
>>
>>>>> Also perhaps this should be made in an Kconfig entry?
>>>>
>>>> That will be anther option but I find different platforms will define
>>>> different MAX_VCPU. If we introduce a generic Kconfig entry, different
>>>> platforms should have different range.
> 
> 
> By different platforms you mean q35 vs the older one, and such?

I meant x86, arm, sparc and other vendors' code define different max
vcpu number.

> Not whether the underlaying accelerator is tcg, Xen, KVM, or bHyve?
> 
> What I was trying to understand whether it makes even sense for
> the platforms to have such limits in the first place - and instead the
> accelerators should be the ones setting it?
> 
> 
>>>>
>>>> Radim & Paolo, Could you give some input? In qemu thread, we will set
>>>> max vcpu to 8192 for x86 VM. In KVM, The length of vcpu pointer array in
>>>> struct kvm and dest_vcpu_bitmap in kvm_irq_delivery_to_apic() are
>>>> specified by KVM_MAX_VCPUS. Should we keep align with Qemu?
>>
>> That would be great.
>>
>>> commit 682f732ecf7396e9d6fe24d44738966699fae6c0
>>> Author: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>> Date:   Tue Jul 12 22:09:29 2016 +0200
>>>
>>>     KVM: x86: bump MAX_VCPUS to 288
>>>
>>>     288 is in high demand because of Knights Landing CPU.
>>>     We cannot set the limit to 640k, because that would be wasting space.
>>>
>>> I think we want to keep it small as long as possible. I remember a patch
>>> series from Radim which would dynamically allocate memory for these
>>> arrays (using a new VM creation ioctl, specifying the max # of vcpus).
>>> Wonder what happened to that (I remember requesting a simply remalloc
>>> instead of a new VM creation ioctl :] ).
>>
>> Eh, I forgot about them ...  I didn't like the dynamic allocation as we
>> would need to protect the memory, which would result in a much bigger
>> changeset, or fragile macros.
>>
>> I can't recall the disgust now, so I'll send a RFC with the dynamic
>> version to see how it turned out.
>>
>> Thanks.


-- 
Best regards
Tianyu Lan