Re: hdd kills vm

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> Sent: Tuesday, October 24, 2023 at 5:28 PM
> From: "Martin Kletzander" <mkletzan@xxxxxxxxxx>
> To: "daggs" <daggs@xxxxxxx>
> Cc: libvir-list@xxxxxxxxxx
> 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@xxxxxxxxxx>
> >> To: "daggs" <daggs@xxxxxxx>
> >> Cc: libvir-list@xxxxxxxxxx
> >> 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?

>
> >>
> >> * 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?

>
> >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.

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





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