Re: 9 TiB vm memory creation

[David Hildenbrand]

On 15.02.22 10:48, Ani Sinha wrote:
> On Tue, Feb 15, 2022 at 3:14 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 15.02.22 10:40, Ani Sinha wrote:
>>> On Tue, Feb 15, 2022 at 2:08 PM David Hildenbrand <david@redhat.com> 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.
>>>> And QE seems to be using virtual numa nodes:
>>>>
>>>> Each of the 32 virtual numa nodes receive a:
>>>>
>>>>   -object memory-backend-ram,id=ram-node20,size=309237645312,host-
>>>>    nodes=0-31,policy=bind
>>>>
>>>> which results in a dedicated KVM memslot (just like each DIMM would)
>>>>
>>>>
>>>> 32 * 309237645312 == 9 TiB :)
>>>
>>> ah, I should have looked closely at the other commandlines before
>>> shooting off the email. Yes the limitation is per mem-slot and they
>>> have 32 slots one per node.
>>> ok so should we do
>>> kvm_set_max_memslot_size(KVM_SLOT_MAX_BYTES);
>>> from i386 kvm_arch_init()?
>>
>>
>> As I said, I'm not a friend of these workarounds in user space.
> 
> Oh ok, did not realize you were against s390x like workarounds.
> 

s390x doesn't support DIMMs so it was "easy" for them to just do it that
way :)

-- 
Thanks,

David / dhildenb