On 17/03/18 04:05, Stefan Hajnoczi wrote: > On Thu, Mar 15, 2018 at 09:47:35PM +1300, Richard Hector wrote: >> Apologies to those who saw this earlier on debian-user. >> >> I generally (currently) use KVM with libvirt on Debian Stretch. >> >> When I configure a KVM guest to have 2 vcpus, will that be 2 full cores? >> Or will it give the guest both threads on the same real core? Or might >> it use half of each of 2 different cores? >> >> I guess the same applies to physical CPUs, too - there's presumably an >> advantage in giving a VM a set of cores all on the same CPU, to take >> advantage of shared caching - is that dealt with automatically? >> >> I've always assumed that I should allocate even numbers of vcpus on an >> HT capable machine, so that it keeps the threads together. >> >> Does any of this matter? > > Yes, performance is affected by vcpu placement and topology. > > There are two things going on: > > 1. vcpu topology. This is virtual. You decide how many sockets, > cores, and hardware threads the guest sees. On the host side they > are just a bunch of threads in the QEMU process and the host Linux > scheduler decides when and where they execute. > > 2. vcpu placement (affinity). This lets you control which host CPUs the > vcpu threads run on. You can force a vcpu to run on a specific host > CPU or you can give it a set of host CPUs where the host Linux > scheduler will run it. The default is that vcpu threads are not > bound to any specific host CPU and could run anywhere! > > Nothing is dealt with automatically if you are directly using libvirt > (virsh, virt-manager, etc). A popular configuration is to mimic the > host CPU topology to the guest and then pin vcpu threads 1:1 onto their > host CPUs. This way the guest kernel can make proper scheduling > decisions. > > At what point does pinning matter for performance? If you have a small > VM with few vcpus it might not be important. But if you want to get > consistent performance, especially on larger hosts, it is probably a > good idea to configure the vcpu topology and placement. Thanks Stefan - I'll have to think about this a bit more :-) What's the best place to read more about it? BTW, doesn't the kernel try to maintain a process/thread's affinity to a CPU anyway? So the VM should benefit from that regardless? Thanks, Richard
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