On 2017年08月12日 03:35, Konrad Rzeszutek Wilk wrote: > On Fri, Aug 11, 2017 at 03:00:20PM +0200, Radim Krčmář 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 2017年08月11日 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 Krčmář <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
