[vfio-users] KVM options and the effect on performance

[Eric Hattemer]

I'm interested in this too.  I think the things to focus on might be 
virtio, networking, accel, and cpu options.  As far as I can tell, the 
'chipset' doesn't relate to performance.  All it does is pre-configure a 
bunch of -device ...bus entries so that you can assume those buses are 
there instead of adding the buses yourself.  I switched to starting with 
-nodefaults and defining the buses myself.  Right now, when I have a 
game running with high video and cpu usage, things are running real 
smoothly.  But when a game is downloading and patching, things get 
really bad and my sound starts skipping.  I'm thinking it's the ahci 
sata device that's to blame.  I'll try to do something like vioscsi and 
keep you posted.

I just read through http://www.linux-kvm.org/page/Tuning_KVM and it says 
not to use btrfs for storing image files, but it doesn't say why.  I'm 
not sure I'm willing to give up btrfs just yet, though.  Maybe I'll test 
with chattr +C on the image files and play with some of the tuning like 
the extent sizes.

-Eric Hattemer


On 10/29/2015 09:50 AM, Ryan Flagler wrote:
> Hey everyone, sorry if I'm doing this wrong, this is my first time 
> using a mailing list. (Side note, if anyone has a better way to view 
> historical emails than the web page, please let me know)
>
> I've been tinkering with KVM for a bit on my system and had some 
> general performance questions to ask. I see a lot of people doing VGA 
> passthrough using the q35 chipset instead of the i440FX chipset. I've 
> personally had no luck getting q35 to be stable for me, and I've seen 
> some people say it's not worth the headache. But the big question to 
> me, is there a performance difference with CPU, VGA, memory etc. using 
> q35? I'm not looking for specifics, but I'm curious about the 
> following qemu parameters.
>
> Which chipset emulation performs better and in what areas?
> q35 vs i440fx
>
> What is the best way to pass a disk through to a VM to get the most 
> performance?
> .img file, /dev/sd[x] disk, virtio-scsi, etc.
>
> What is the best way to handle networking?
> virtio-nic, hardware passthrough, bridge, nat, etc.
>
> What is the best way to assign CPUs?
> cpu pinning, assigning host cpu parameters, etc.
>
> Does the BIOS have an effect on performance?
> seabios vs OVMF?
>
> CPU/Chipset IOMMU support - Not necessarily performance related, but 
> stability?
> e5 vs e3 vs i7 vs cpu architecture etc. What things are good to look 
> for, what are bad? Etc.
>
> What would be interesting, especially as a new KVM/Qemu user, would be 
> to see an entire wiki/performance page with examples and specifics. 
> It's hard to filter through all the various pages of VM options where 
> people don't really explain why they're doing something the way they are.
>
> Examples:
>
> Disk Options
> Best
> -device virtio-scsi-pci,id=scsi
> -drive file=/dev/sd[x],id=disk,format=ls raw,if=none -device 
> scsi-hd,drive=disk
>
> Better
> -device virtio-scsi-pci,id=scsi
> -drive file=/opt/[vm_name].img,id=disk,format=raw -device 
> scsi-hd,drive=disk
>
> Good
> -drive file=/opt/[vm_name].img,id=disk,format=raw -device 
> ide-hd,bus=ide.0,drive=disk
>
> And maybe an overall explanation of why one is better over the other. 
> I know this may not exist and I'm not asking a single person to do the 
> leg work, but being new to this, it's hard to focus on the pieces that 
> matter vs just using the first thing I find that works. If there is a 
> "right" place to start something like this I'd be happy to setup a 
> generic page where more experienced people could easily contribute.
>
> Thanks - Ryan
>
>
>
> _______________________________________________
> vfio-users mailing list
> vfio-users at redhat.com
> https://www.redhat.com/mailman/listinfo/vfio-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/vfio-users/attachments/20151029/22f6fbf2/attachment.htm>