Re: hdd kills vm

> Sent: Thursday, October 26, 2023 at 9:50 AM > From: "Martin Kletzander" <mkletzan(a)redhat.com&gt; > To: "daggs" <daggs(a)gmx.com&gt; > Cc: libvir-list(a)redhat.com > Subject: Re: hdd kills vm > > On Wed, Oct 25, 2023 at 03:06:55PM +0200, daggs wrote: > >> Sent: Tuesday, October 24, 2023 at 5:28 PM > >> From: "Martin Kletzander" <mkletzan(a)redhat.com&gt; > >> To: "daggs" <daggs(a)gmx.com&gt; > >> Cc: libvir-list(a)redhat.com > >> Subject: Re: hdd kills vm > >> > >> On Mon, Oct 23, 2023 at 04:59:08PM +0200, daggs wrote: > >> >Greetings Martin, > >> > > >> >> Sent: Sunday, October 22, 2023 at 12:37 PM > >> >> From: "Martin Kletzander" <mkletzan(a)redhat.com&gt; > >> >> To: "daggs" <daggs(a)gmx.com&gt; > >> >> Cc: libvir-list(a)redhat.com > >> >> Subject: Re: hdd kills vm > >> >> > >> >> On Fri, Oct 20, 2023 at 02:42:38PM +0200, daggs wrote: > >> >> >Greetings, > >> >> > > >> >> >I have a windows 11 vm running on my Gentoo using libvirt (9.8.0) + qemu (8.1.2), I'm passing almost all available resources to the vm > >> >> >(all 16 cpus, 31 out of 32 GB, nVidia gpu is pt), but the performance is not good, system lags, takes long time to boot. > >> >> > >> >> There are couple of things that stand out to me in your setup and I'll > >> >> assume the host has one NUMA node with 8 cores, each with 2 threads as, > >> >> just like you set it up in the guest XML. > >> >thats correct, see: > >> >$ lscpu | grep -i numa > >> >NUMA node(s): 1 > >> >NUMA node0 CPU(s): 0-15 > >> > > >> >however: > >> >$ dmesg | grep -i numa > >> >[ 0.003783] No NUMA configuration found > >> > > >> >can that be the reason? > >> > > >> > >> no, this is fine, 1 NUMA node is not a NUMA, technically, so that's > >> perfectly fine. > >thanks for clarifying it for me > > > >> > >> >> > >> >> * When you give the guest all the CPUs the host has there is nothing > >> >> left to run the host tasks. You might think that there "isn't > >> >> anything running", but there is, if only your init system, the kernel > >> >> and the QEMU which is emulating the guest. This is definitely one of > >> >> the bottlenecks. > >> >I've tried with 12 out of 16, same behavior. > >> > > >> >> > >> >> * The pinning of vCPUs to CPUs is half-suspicious. If you are trying to > >> >> make vCPU 0 and 1 be threads on the same core and on the host the > >> >> threads are represented as CPUs 0 and 8, then that's fine. If that is > >> >> just copy-pasted from somewhere, then it might not reflect the current > >> >> situation and can be source of many scheduling issues (even once the > >> >> above is dealt with). > >> >I found a site that does it for you, if it is wrong, can you point me to a place I can read about it? > >> > > >> > >> Just check what the topology is on the host and try to match it with the > >> guest one. If in doubt, then try it without the pinning. > >I can try to play with it, what I don't know is what should be the mapping logic? > > > > Threads on the same core in the guest should map to threads on the same > core in the host. Since there is no NUMA that should be enough to get > the best performance. But even misconfiguration of this will not > introduce lags in the system if it has 8 CPUs. So that's definitely not > the root cause of the main problem, it just might be suboptimal. > > >> > >> >> > >> >> * I also seem to recall that Windows had some issues with systems that > >> >> have too many cores. I'm not sure whether that was an issue with an > >> >> edition difference or just with some older versions, or if it just did > >> >> not show up in the task manager, but there was something that was > >> >> fixed by using either more sockets or cores in the topology. This is > >> >> probably not the issue for you though. > >> >> > >> >> >after trying a few ways to fix it, I've concluded that the issue might be related to the why the hdd is defined at the vm level. > >> >> >here is the xml: https://bpa.st/MYTA > >> >> >I assume that the hdd sits on the sata ctrl causing the issue but I'm not sure what is the proper way to fix it, any ideas? > >> >> > > >> >> > >> >> It looks like your disk is on SATA, but I don't see why that would be an > >> >> issue. Passing the block device to QEMU as VirtIO shouldn't cause that > >> >> much of a difference. Try measuring the speed of the disk on the host > >> >> and then in the VM maybe. Is that SSD or NVMe? I presume that's not > >> >> spinning rust, is it. > >> >as seen, I have 3 drives, 2 cdroms as sata and one hdd pt as virtio, I read somewhere that if the controller of the virtio > >> >device is sata, than it doesn't uses the virtio optimally. > >> > >> Well it _might_ be slightly more beneficial to use virtio-scsi or even > >> <disk type='block' device='lun'>, but I can't imagine that would make > >> the system lag. I'm not that familiar with the details. > >configure virtio-scsi and sata-scai at the same time? > > > > Yes, forgot that, sorry. Try virtio-scsi. You could also go farther > and pass through the LUN or the whole HBA (if you don't need to access > any other disk on it) to the VM. Try the information presented here: > > https://libvirt.org/formatdomain.html#usb-pci-scsi-devices > > >> > >> >it is a spindle, nvmes are too expensive where I live, frankly, I don't need lightning fast boot, the other BM machines running windows on spindle > >> >run it quite fast and they aren't half as fast as this server > >> > > >> > >> That might actually be related. The guest might think it is a different > >> type of disk and use completely suboptimal scheduling. This might > >> actually be solved by passing it as <disk device='lun'..., but at this > >> point I'm just guessing. > >I'll look into that, thanks. so bottom line, you suggest the following: 1. remove the manual cpu pin, let qemu sort that out.

2. add a virtio scsi controller and connect the os hdd to it 3. pass the hss via scsi pt and not dev node 4. if I able to do #3, no need to add device='lun' as it won't use the disk option

Dagg.