Re: hdd kills vm

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.

>> >> * 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 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.

>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.

>> >> >Thanks, >> > >> >Dagg. >> > >> >