Re: 9 TiB vm memory creation

[Daniel P . Berrangé]

On Tue, Feb 15, 2022 at 09:38:34AM +0100, David Hildenbrand wrote:
> On 15.02.22 09:12, Ani Sinha wrote:
> > On Tue, Feb 15, 2022 at 1:25 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 15.02.22 08:00, Ani Sinha wrote:
> >>>
> >>>
> >>> On Mon, 14 Feb 2022, David Hildenbrand wrote:
> >>>
> >>>> On 14.02.22 13:36, Igor Mammedov wrote:
> >>>>> On Mon, 14 Feb 2022 10:54:22 +0530 (IST)
> >>>>> Ani Sinha <ani@anisinha.ca> wrote:
> >>>>>
> >>>>>> Hi Igor:
> >>>>>>
> >>>>>> I failed to spawn a 9 Tib VM. The max I could do was a 2 TiB vm on my
> >>>>>> system with the following commandline before either the system
> >>>>>> destabilized or the OOM killed killed qemu
> >>>>>>
> >>>>>> -m 2T,maxmem=9T,slots=1 \
> >>>>>> -object 
> >>>>>> memory-backend-file,id=mem0,size=2T,mem-path=/data/temp/memfile,prealloc=off
> >>>>>>  \
> >>>>>> -machine memory-backend=mem0 \
> >>>>>> -chardev file,path=/tmp/debugcon2.txt,id=debugcon \
> >>>>>> -device isa-debugcon,iobase=0x402,chardev=debugcon \
> >>>>>>
> >>>>>> I have attached the debugcon output from 2 TiB vm.
> >>>>>> Is there any other commandline parameters or options I should try?
> >>>>>>
> >>>>>> thanks
> >>>>>> ani
> >>>>>
> >>>>> $ truncate -s 9T 9tb_sparse_disk.img
> >>>>> $ qemu-system-x86_64 -m 9T \
> >>>>>   -object 
> >>>>> memory-backend-file,id=mem0,size=9T,mem-path=9tb_sparse_disk.img,prealloc=off,share=on
> >>>>>  \
> >>>>>   -machine memory-backend=mem0
> >>>>>
> >>>>> works for me till GRUB menu, with sufficient guest kernel
> >>>>> persuasion (i.e. CLI limit ram size to something reasonable) you can 
> >>>>> boot linux
> >>>>> guest on it and inspect SMBIOS tables comfortably.
> >>>>>
> >>>>>
> >>>>> With KVM enabled it bails out with:
> >>>>>    qemu-system-x86_64: kvm_set_user_memory_region: 
> >>>>> KVM_SET_USER_MEMORY_REGION failed, slot=1, start=0x100000000, 
> >>>>> size=0x8ff40000000: Invalid argument
> >>>>>
> >>>
> >>> I have seen this in my system but not always. Maybe I should have dug
> >>> deeper as to why i do see this all the time.
> >>>
> >>>>> all of that on a host with 32G of RAM/no swap.
> >>>>>
> >>>
> >>> My system in 16 Gib of main memory, no swap.
> >>>
> >>>>
> >>>> #define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1)
> >>>>
> >>>> ~8 TiB (7,999999)
> >>>
> >>> That's not 8 Tib, thats 2 GiB. But yes, 0x8ff40000000 is certainly greater
> >>> than 2 Gib * 4K (assuming 4K size pages).
> >>
> >> "pages" don't carry the unit "GiB/TiB", so I was talking about the
> >> actual size with 4k pages (your setup, I assume)
> > 
> > yes I got that after reading your email again.
> > The interesting question now is how is redhat QE running 9 TiB vm with kvm?
> 
> As already indicated by me regarding s390x only having single large NUMA
> nodes, x86 is usually using multiple NUMA nodes with such large memory.

Yes, this is a documented requirement for KVM limits:

     https://access.redhat.com/articles/906543

   "3. Note that virtualized guests larger than 8 TB currently 
    require explicit virtual NUMA configuration, because the 
    maximum virtual NUMA node size is 8 TB."

Regards,
Daniel
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