Cores CPUs Threads

[krystofr]

I'm in a muddle with setting the number of cores, CPUs and Threads

I use proxmox on a machine with 1x Intel Xeon D-1520 processor. This is a 4 core but 8 threads. I have only 1 VM running.

Proxmox 5.1 summary for the node reports 8 CPUs
However when setting the options for the processor in a VM it asks for sockets and cores. I only have a physical CPU so I set it to 1 socket, but should I select 4 cores, as per the intel spec, or should I select 8 cores as in the threads, or CPU's reported on the node?

Thank you in advance,

 

[PigLover]

KVM/qemu always presents a simulation of a hardware configuration to the VM. It can make the hardware appear in almost any configuration you want. If you want to have 4 virtual CPUs available to the VM you can tell it that it has 1 socket with 4 cores, two sockets with 2 cores each or 4 sockets with 1 core each. When running, KVM will attempt to schedule actual CPUs that make sense for the configuration, but that is not always guaranteed.

Whether or not it matters depends a lot on what you are running in the VM and how much it "knows" about and/or depends upon the CPU/NUMA archtecture.

 

[nexusguy59]

So also remember that with 4 core 8 threads in any operating system it presents as an 8 core server. Found this out when I bought an intel NUC and couldn't figure out why it reported 8 cpus instead of 4. 4 core with 4 threads would be 4 core, 4 core with 8 threads 8 core...I don't understand it totally but it's in the way they do the math.

 

[Joel61]

I'm in the same boat. Thanks for clearing this up.
So if I have an 8 core 16 thread CPU and wanted to allocate 100% of it to a VM, I would give the VM 16 Cores? (Since it considers threads and cores the same thing)

 

[nexusguy59]

I'm in the same boat. Thanks for clearing this up.
So if I have an 8 core 16 thread CPU and wanted to allocate 100% of it to a VM, I would give the VM 16 Cores? (Since it considers threads and cores the same thing)

Sorry it took me so long to answer, I didn't get an alert that you responded to me. Yes you would in effect be giving the VM 16 cores. Let me know if you need anything else.

Michael

 

[Aleksej]

Hi. I was seeking for this question for a long time.
You can manually set cpu topology, but it consumes resources heavily.
as an example for 1x Intel Xeon D-1520 use

args: -cpu 'host,-hypervisor,+kvm_pv_unhalt,+kvm_pv_eoi,hv_spinlocks=0x1fff,hv_vapic,hv_time,hv_reset,hv_vpindex,hv_runtime,hv_relaxed,kvm=off,hv_vendor_id=intel,+vmx,+ht', -smp 4,cores=2,threads=2,sockets=1,maxcpus=4

for my E5-2630v4 the result is:

user@vpn1:~$ lscpu
Архитектура:x86_64
CPU op-mode(s):        32-bit, 64-bit
Порядок байтов:Little Endian
CPU(s):                4
On-line CPU(s) list:   0-3
Потоков на ядро:2
Ядер на сокет:2
Сокет(ы):        1
NUMA node(s):          1
Vendor ID:             GenuineIntel
Семейство CPU:6
Модель:          79
Model name:            Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz
Stepping:              1
CPU МГц:            2200.036
BogoMIPS:              4400.07
Виртуализация:VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              4096K
L3 cache:              16384K
NUMA node0 CPU(s):     0-3
Flags:                 fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon rep_good nopl xtopology pni pclmulqdq vmx ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch invpcid_single ssbd ibrs ibpb kaiser tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm rdseed adx smap xsaveopt arat

as you can see VM have ht, vmx, gpu, lm and so on...

but i cannot explain why the cache is 16384, but CPU have 25Mb cache...

 

[AntonCrack]

It does not say that you have a 16384Mb cache, it says you have a 16384 kilobyte cache, if you divide by 1000 to convert to megabytes, that equals 16.384 megabytes for your L3 cache, added together, they will be 25Mb.

 

[Aleksej]

I mean that E5-2630v4 have 25Mb cache but reported only 16Mb.

Also:
"args: -cpu "
replaces all cpu flags. So consider to use "host" then view
"qm config VMID --verbose" and use all cpu flags from there and only then add -hypervisor, +ht (or +svm for AMD)

But on AMD processors -smp threads not working

 

[DC-CA1]

KVM/qemu always presents a simulation of a hardware configuration to the VM. It can make the hardware appear in almost any configuration you want. If you want to have 4 virtual CPUs available to the VM you can tell it that it has 1 socket with 4 cores, two sockets with 2 cores each or 4 sockets with 1 core each. When running, KVM will attempt to schedule actual CPUs that make sense for the configuration, but that is not always guaranteed.

Whether or not it matters depends a lot on what you are running in the VM and how much it "knows" about and/or depends upon the CPU/NUMA archtecture.

so if set 1 VM with 1 socket and 4 core , so total of 4 Vcu , the Host will handle the load of that Socket on his 2 socket Cpu right ?