More than 64 vCPU on Debian11 VM and AMD EPYC

alexredston

@olivierlambert Interestingly, so far I've seen about a 40% increase in write performance and IOPS from adjusting the scheduler in dom0 by adding elevator=noop as a kernel parameter and a further 10% from repeating the same on the VM.

I'm going to experiment next with migrating the disks so that the mirror is achieved in the VM with three separate pifs instead of in dom0. Then may try other more radical approaches like passthrough.

olivierlambert

That's a very nice increase. Indeed, noop is the best option for NVMe devices.

alexredston

@olivierlambert will repeat on everything!

alexredston

@olivierlambert Thanks to everyone's great advice. I've now managed a further more than 20 fold increase by using PCI passthrough on the 3 x NVMe drives, machine is only PCIe 3.x but still I'm getting 10.5GB /s reading on the test with fio and just over 1GB/s write.

My bottleneck for compiling is now once again the CPUs.

I seem to be unable to exceed 128 CPUs, was hoping to assign more as the host has 176 but it is struggling, at the moment my build is pinning those 128 at 100% CPU for 30 minutes so this could potentially offer a fairly significant improvement.

Overall quite pleased to be squeezing this much performance out of some old HPE Gen 9 hardware. May look at adding another disk to the mirror, but at some point the write penalty may outweigh the excellent read performance. I've put chosen slots based on ensuring each NVMe's PCI lanes are connected to a different host CPU.

May try another experiment with smaller PCIe devices and bifurication and see if I can test the upper limits of the throughput. 9 slots to play with!

olivierlambert

Indeed, PCI passthrough helps tremendously to reach near bare metal performances (on the storage part). Now indeed, the CPU will have issues to keep up. You can try statically partition your hardware at CPU level, ie pinning vCPUs to real CPUs, ideally for all your VMs so you will be 100% sure Xen scheduler will never affect your VM performance.

alexredston

@olivierlambert

Now attempting to push this further - when I go beyond 128 CPUs on the VM configuration I am getting the following:

vm.start
{
  "id": "d9b39e2d-a95b-b8bf-dc5f-01d176c49c70",
  "bypassMacAddressesCheck": false,
  "force": false
}
{
  "code": "INTERNAL_ERROR",
  "params": [
    "xenopsd internal error: Xenctrl.Error(\"22: Invalid argument\")"
  ],
  "call": {
    "method": "VM.start",
    "params": [
      "OpaqueRef:82abc808-84b8-4bc5-9db9-2e6ef20a5e4a",
      false,
      false
    ]
  },
  "message": "INTERNAL_ERROR(xenopsd internal error: Xenctrl.Error(\"22: Invalid argument\"))",
  "name": "XapiError",
  "stack": "XapiError: INTERNAL_ERROR(xenopsd internal error: Xenctrl.Error(\"22: Invalid argument\"))
    at Function.wrap (file:///opt/xo/xo-builds/xen-orchestra-202401131411/packages/xen-api/_XapiError.mjs:16:12)
    at file:///opt/xo/xo-builds/xen-orchestra-202401131411/packages/xen-api/transports/json-rpc.mjs:35:21"
}

olivierlambert

You can't go beyond 128 vCPUs at the moment, I think it's a Qemu limitation in XCP-ng (or something like that, pinging @andSmv )

andSmv

@alexredston Hey, sorry I'm a little bit late here. So, with regard of VCPUs - there's a hardcoded limit of 128 actually in XEN hypervisor. Moreover XEN toolstack (when creating a guest) will check that guest VCPU limit is below physical CPUs available on the platform.
Bypassing the VCPU 128 limit will require some rather important adjustements in XEN Hypervisor (basically the restrictions go with IOREQ server from QEMU and how LAPIC id are affected in XEN). So with the next XEN version this limit could potentially be increased (there's an ongoing work on this).

The things you also probably would like to know about this VCPUs limit

not all of the guests can handle this VCPUs number (e.g. Windows will certainely crash)
when you gives a VM such a big VCPU number (basically more than 32) the VM can potentially provoke the DoS on the whole platform (this is related how some routines are "serialized" in XEN Hypervisor). So, when you do this - be aware that if your guest is broken, pawned, whatever... your whole platform can potentially become unresponsive.

rarturas

Hi Guys, any of you got lock to configure more than 64 CPUs for Windows OS VM? if I set apci to 0 then vm would no longer boot

andSmv

@rarturas I'm not sure it's actually doable to run Windows with more than 64 VCPUs. I'm not surprised neither you VM isn't booting when you turn ACPI off.

We're actually in the middle of investigation what's the VCPU limit could be for Windows VM and especially what could be the gap to get 128 VCPUs for Windows. We're most probably will discuss this topic with community on Xen Summit (in a couple of weeks)

Stay tuned!

rarturas

@andSmv thanks for coming back on this. Actually you are right it is a limit on Windows OS as per documentation here https://learn.microsoft.com/en-us/windows-server/get-started/editions-comparison-windows-server-2019?tabs=full-comparison. Should have checked this before I asked the question