RE: Network I/O performance
Subject: Re: Network I/O performance Fischer, Anna wrote: Subject: Re: Network I/O performance Fischer, Anna wrote: I am running KVM with Fedora Core 8 on a 2.6.23 32-bit kernel. I use the tun/tap device model and the Linux bridge kernel module to connect my VM to the network. I have 2 10G Intel 82598 network devices (with the ixgbe driver) attached to my machine and I want to do packet routing in my VM (the VM has two virtual network interfaces configured). Analysing the network performance of the standard QEMU emulated NICs, I get less that 1G of throughput on those 10G links. Surprisingly though, I don't really see CPU utilization being maxed out. This is a dual core machine, and mpstat shows me that both CPUs are about 40% idle. My VM is more or less unresponsive due to the high network processing load while the host OS still seems to be in good shape. How can I best tune this setup to achieve best possible performance with KVM? I know there is virtIO and I know there is PCI pass-through, but those models are not an option for me right now. How many cpus are assigned to the guest? If only one, then 40% idle equates to 100% of a core for the guest and 20% for housekeeping. No, the machine has a dual core CPU and I have configured the guest with 2 CPUs. So I would want to see KVM using up to 200% of CPU, ideally. There is nothing else running on that machine. Well, it really depends on the workload, whether it can utilize both vcpus. If this is the case, you could try pinning the vcpu thread (info cpus from the monitor) to one core. You should then see 100%/20% cpu load distribution. wrt emulated NIC performance, I'm guessing you're not doing tcp? If you were we might do something with TSO. No, I am measuring UDP throughput performance. I have now tried using a different NIC model, and the e1000 model seems to achieve slightly better performance (CPU goes up to 110% only though). I have also been running virtio now, and while its performance with 2.6.20 was very poor too, when changing the guest kernel to 2.6.30, I get a reasonable performance and higher CPU utilization (e.g. it goes up to 180-190%). I have to throttle the incoming bandwidth though, because as soon as I go over a certain threshold, CPU goes back down to 90% and throughput goes down too. Yes, there's a known issue with UDP, where we don't report congestion and the queues start dropping packets. There's a patch for tun queued for the next merge window; you'll need a 2.6.31 host for that IIRC (Herbert?) I have not seen this with Xen/VMware where I mostly managed to max out CPU completely before throughput performance did not go up anymore. I have also realized that when using the tun/tap configuration with a bridge, packets are replicated on all tap devices when QEMU writes packets to the tun interface. I guess this is a limitation of tun/tap as it does not know to which tap device the packet has to go to. The tap device then eventually drops packets when the destination MAC is not its own, but it still receives the packet which causes more overhead in the system overall. Right, I guess you'd see this with a real switch as well? Maybe have your guest send a packet out once in a while so the bridge can learn its MAC address (we do this after migration, for example). No, this is not about the bridge - packets are replicated by tun/tap as far as I can see. In fact I run two bridges, and attach my two tap interfaces to those (one tap per bridge to connect it to the external network). And packets that should actually only go to one bridge, are replicated on the other one, too. This is far off from being ideal, but I guess the issue is that the tun/tap interface is a 1-N mapping, so there is not much you can do. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
tun/tap and Vlans (was: Re: Network I/O performance)
Hi all, On a sidenote: I have also realized that when using the tun/tap configuration with a bridge, packets are replicated on all tap devices when QEMU writes packets to the tun interface. I guess this is a limitation of tun/tap as it does not know to which tap device the packet has to go to. The tap device then eventually drops packets when the destination MAC is not its own, but it still receives the packet which causes more overhead in the system overall. Right, I guess you'd see this with a real switch as well? Maybe have your guest send a packet out once in a while so the bridge can learn its MAC address (we do this after migration, for example). Does this mean that it is not possible for having each tun device in a seperate bridge that serves a seperate Vlan? We have experienced a strange problem that we couldn't yet explain. Given this setup: GuestHost kvm1 --- eth0 -+- bridge0 --- vlan1 \ | +-- eth0 kvm2 -+- eth0 -/ / \- eth1 --- bridge1 --- vlan2 + When sending packets through kvm2/eth0, they appear on both bridges and also vlans, also when sending packets through kvm2/eth1. When the guest has only one interface, the packets only appear on one bridge and one vlan as it's supposed to be. Can this be worked around? -- Lukas -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Network I/O performance
Herbert Xu wrote: Yes, there's a known issue with UDP, where we don't report congestion and the queues start dropping packets. There's a patch for tun queued for the next merge window; you'll need a 2.6.31 host for that IIRC (Herbert?) It should be in 2.6.30 in fact. However, this is for outbound traffic only since inbound traffic shouldn't have this problem of the guest sending faster than the wire. Is there a corresponding qemu change? Or is this a already handled by the existing code? -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Network I/O performance
Fischer, Anna wrote: Subject: Re: Network I/O performance Fischer, Anna wrote: I am running KVM with Fedora Core 8 on a 2.6.23 32-bit kernel. I use the tun/tap device model and the Linux bridge kernel module to connect my VM to the network. I have 2 10G Intel 82598 network devices (with the ixgbe driver) attached to my machine and I want to do packet routing in my VM (the VM has two virtual network interfaces configured). Analysing the network performance of the standard QEMU emulated NICs, I get less that 1G of throughput on those 10G links. Surprisingly though, I don't really see CPU utilization being maxed out. This is a dual core machine, and mpstat shows me that both CPUs are about 40% idle. My VM is more or less unresponsive due to the high network processing load while the host OS still seems to be in good shape. How can I best tune this setup to achieve best possible performance with KVM? I know there is virtIO and I know there is PCI pass-through, but those models are not an option for me right now. How many cpus are assigned to the guest? If only one, then 40% idle equates to 100% of a core for the guest and 20% for housekeeping. No, the machine has a dual core CPU and I have configured the guest with 2 CPUs. So I would want to see KVM using up to 200% of CPU, ideally. There is nothing else running on that machine. Well, it really depends on the workload, whether it can utilize both vcpus. If this is the case, you could try pinning the vcpu thread (info cpus from the monitor) to one core. You should then see 100%/20% cpu load distribution. wrt emulated NIC performance, I'm guessing you're not doing tcp? If you were we might do something with TSO. No, I am measuring UDP throughput performance. I have now tried using a different NIC model, and the e1000 model seems to achieve slightly better performance (CPU goes up to 110% only though). I have also been running virtio now, and while its performance with 2.6.20 was very poor too, when changing the guest kernel to 2.6.30, I get a reasonable performance and higher CPU utilization (e.g. it goes up to 180-190%). I have to throttle the incoming bandwidth though, because as soon as I go over a certain threshold, CPU goes back down to 90% and throughput goes down too. Yes, there's a known issue with UDP, where we don't report congestion and the queues start dropping packets. There's a patch for tun queued for the next merge window; you'll need a 2.6.31 host for that IIRC (Herbert?) I have not seen this with Xen/VMware where I mostly managed to max out CPU completely before throughput performance did not go up anymore. I have also realized that when using the tun/tap configuration with a bridge, packets are replicated on all tap devices when QEMU writes packets to the tun interface. I guess this is a limitation of tun/tap as it does not know to which tap device the packet has to go to. The tap device then eventually drops packets when the destination MAC is not its own, but it still receives the packet which causes more overhead in the system overall. Right, I guess you'd see this with a real switch as well? Maybe have your guest send a packet out once in a while so the bridge can learn its MAC address (we do this after migration, for example). -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Network I/O performance
Fischer, Anna wrote: I am running KVM with Fedora Core 8 on a 2.6.23 32-bit kernel. I use the tun/tap device model and the Linux bridge kernel module to connect my VM to the network. I have 2 10G Intel 82598 network devices (with the ixgbe driver) attached to my machine and I want to do packet routing in my VM (the VM has two virtual network interfaces configured). Analysing the network performance of the standard QEMU emulated NICs, I get less that 1G of throughput on those 10G links. Surprisingly though, I don't really see CPU utilization being maxed out. This is a dual core machine, and mpstat shows me that both CPUs are about 40% idle. My VM is more or less unresponsive due to the high network processing load while the host OS still seems to be in good shape. How can I best tune this setup to achieve best possible performance with KVM? I know there is virtIO and I know there is PCI pass-through, but those models are not an option for me right now. How many cpus are assigned to the guest? If only one, then 40% idle equates to 100% of a core for the guest and 20% for housekeeping. If this is the case, you could try pinning the vcpu thread (info cpus from the monitor) to one core. You should then see 100%/20% cpu load distribution. wrt emulated NIC performance, I'm guessing you're not doing tcp? If you were we might do something with TSO. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
RE: Network I/O performance
Subject: Re: Network I/O performance Fischer, Anna wrote: I am running KVM with Fedora Core 8 on a 2.6.23 32-bit kernel. I use the tun/tap device model and the Linux bridge kernel module to connect my VM to the network. I have 2 10G Intel 82598 network devices (with the ixgbe driver) attached to my machine and I want to do packet routing in my VM (the VM has two virtual network interfaces configured). Analysing the network performance of the standard QEMU emulated NICs, I get less that 1G of throughput on those 10G links. Surprisingly though, I don't really see CPU utilization being maxed out. This is a dual core machine, and mpstat shows me that both CPUs are about 40% idle. My VM is more or less unresponsive due to the high network processing load while the host OS still seems to be in good shape. How can I best tune this setup to achieve best possible performance with KVM? I know there is virtIO and I know there is PCI pass-through, but those models are not an option for me right now. How many cpus are assigned to the guest? If only one, then 40% idle equates to 100% of a core for the guest and 20% for housekeeping. No, the machine has a dual core CPU and I have configured the guest with 2 CPUs. So I would want to see KVM using up to 200% of CPU, ideally. There is nothing else running on that machine. If this is the case, you could try pinning the vcpu thread (info cpus from the monitor) to one core. You should then see 100%/20% cpu load distribution. wrt emulated NIC performance, I'm guessing you're not doing tcp? If you were we might do something with TSO. No, I am measuring UDP throughput performance. I have now tried using a different NIC model, and the e1000 model seems to achieve slightly better performance (CPU goes up to 110% only though). I have also been running virtio now, and while its performance with 2.6.20 was very poor too, when changing the guest kernel to 2.6.30, I get a reasonable performance and higher CPU utilization (e.g. it goes up to 180-190%). I have to throttle the incoming bandwidth though, because as soon as I go over a certain threshold, CPU goes back down to 90% and throughput goes down too. I have not seen this with Xen/VMware where I mostly managed to max out CPU completely before throughput performance did not go up anymore. I have also realized that when using the tun/tap configuration with a bridge, packets are replicated on all tap devices when QEMU writes packets to the tun interface. I guess this is a limitation of tun/tap as it does not know to which tap device the packet has to go to. The tap device then eventually drops packets when the destination MAC is not its own, but it still receives the packet which causes more overhead in the system overall. I have not yet experimented much with pinning VCPU threads to cores. I will do that as well. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
