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| QEMU/KVM internals https://forum.osdev.org/viewtopic.php?f=15&t=55923 |
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| Author: | cianfa72 [ Tue Oct 12, 2021 11:27 am ] |
| Post subject: | QEMU/KVM internals |
Hi, I'd like to ask some details about QEMU/KVM implementation. On my Linux server (dual socket CPU w/ 12 core/24 HT each) I run a VM via qemu-system-x86_64: Code: root@eve-ng03:~# numactl --hardware available: 2 nodes (0-1) node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 24 25 26 27 28 29 30 31 32 33 34 35 node 0 size: 64180 MB node 0 free: 22819 MB node 1 cpus: 12 13 14 15 16 17 18 19 20 21 22 23 36 37 38 39 40 41 42 43 44 45 46 47 node 1 size: 64491 MB node 1 free: 51078 MB node distances: node 0 1 0: 10 21 1: 21 10 root@eve-ng03:~# root@eve-ng03:~# root@eve-ng03:~# ps -wwf -p 45802 UID PID PPID C STIME TTY TIME CMD root 45802 45786 99 Jun18 ? 153-02:15:51 /opt/qemu/bin/qemu-system-x86_64 -nographic -device virtio-net-pci,addr=3.0,multifunction=on,netdev=net0,mac=50:00:00:07:00:00 -netdev tap,id=net0,ifname=vunl0_7_0,script=no -device virtio-net-pci,addr=3.1,multifunction=on,netdev=net1,mac=50:00:00:07:00:01 -netdev tap,id=net1,ifname=vunl0_7_1,script=no -device virtio-net-pci,addr=3.2,multifunction=on,netdev=net2,mac=50:00:00:07:00:02 -netdev tap,id=net2,ifname=vunl0_7_2,script=no -device virtio-net-pci,addr=3.3,multifunction=on,netdev=net3,mac=50:00:00:07:00:03 -netdev tap,id=net3,ifname=vunl0_7_3,script=no -device virtio-net-pci,addr=3.4,multifunction=on,netdev=net4,mac=50:00:00:07:00:04 -netdev tap,id=net4,ifname=vunl0_7_4,script=no -device virtio-net-pci,addr=3.5,multifunction=on,netdev=net5,mac=50:00:00:07:00:05 -netdev tap,id=net5,ifname=vunl0_7_5,script=no -device virtio-net-pci,addr=3.6,multifunction=on,netdev=net6,mac=50:00:00:07:00:06 -netdev tap,id=net6,ifname=vunl0_7_6,script=no -device virtio-net-pci,addr=3.7,multifunction=on,netdev=net7,mac=50:00:00:07:00:07 -netdev tap,id=net7,ifname=vunl0_7_7,script=no -device virtio-net-pci,addr=4.0,multifunction=on,netdev=net8,mac=50:00:00:07:00:08 -netdev tap,id=net8,ifname=vunl0_7_8,script=no -device virtio-net-pci,addr=4.1,multifunction=on,netdev=net9,mac=50:00:00:07:00:09 -netdev tap,id=net9,ifname=vunl0_7_9,script=no -device virtio-net-pci,addr=4.2,multifunction=on,netdev=net10,mac=50:00:00:07:00:0a -netdev tap,id=net10,ifname=vunl0_7_10,script=no -smp 4 -m 8192 -name R_02 -uuid a368f2e7-eebf-418b-9ffd-5c190134a670 -drive file=virtioa.qcow2,if=virtio,bus=0,unit=0,cache=none -enable-kvm -smbios type=1,manufacturer=cisco,product=Cisco IOS XRv 9000,uuid=97fc351b-431d-4cf2-9c01-43c283faf2a3 -machine type=pc,accel=kvm,usb=off -serial mon:stdio -nographic -nodefconfig -nodefaults -rtc base=utc,driftfix=slew -global kvm-pit.lost_tick_policy=discard -no-hpet -realtime mlock=off -no-shutdown -boot order=c -serial null -serial null -serial null -cpu host root@eve-ng03:~# root@eve-ng03:~# root@eve-ng03:~# root@eve-ng03:~# ps -T -p 45802 PID SPID TTY TIME CMD 45802 45802 ? 14-05:23:18 qemu-system-x86 45802 812 ? 00:00:00 qemu-system-x86 45802 824 ? 7-07:15:51 qemu-system-x86 45802 825 ? 7-06:48:23 qemu-system-x86 45802 826 ? 8-04:20:31 qemu-system-x86 45802 827 ? 116-01:07:17 qemu-system-x86 45802 658 ? 00:00:00 qemu-system-x86 45802 678 ? 00:00:00 qemu-system-x86 45802 681 ? 00:00:00 qemu-system-x86 45802 733 ? 00:00:00 qemu-system-x86 45802 734 ? 00:00:00 qemu-system-x86 45802 761 ? 00:00:00 qemu-system-x86 root@eve-ng03:~# As you can see there are 12 threads that run inside the QEMU process (PID 45802). Digging inside qemu monitor (Ctrl+A C) Code: QEMU 2.4.0 monitor - type 'help' for more information I can see the four vCPU threads (as expected since the VM started with 4 vCPUs) and the main thread -- it should be the thread having Thread-ID (SPID) the same as the Process-ID (45802).(qemu) info cpus * CPU #0: pc=0xffffffff8d0454e6 (halted) thread_id=824 CPU #1: pc=0xffffffff8d0454e6 (halted) thread_id=825 CPU #2: pc=0xffffffff8d0454e6 (halted) thread_id=826 CPU #3: pc=0x00007f7b8a64f3ff thread_id=827 (qemu) My question is: which are actually the tasks in charge of all other (non main or vCPUs) threads inside the QEMU process ? Thank you. |
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| Author: | cianfa72 [ Thu Oct 14, 2021 11:21 am ] |
| Post subject: | Re: QEMU/KVM internals |
Any feedback ? Thank you. |
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| Author: | reapersms [ Thu Oct 14, 2021 11:49 am ] |
| Post subject: | Re: QEMU/KVM internals |
A random, mostly uneducated guess is that QEMU spawns up a thread for every physical CPU no matter what, and allocates those as needed to the VM. The other 8 all having a time of 00:00:00 suggests they all immediately went idle. If you spawned up another 4 CPU VM, you'd probably see another 4 wake up. |
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| Author: | cianfa72 [ Thu Oct 14, 2021 2:11 pm ] |
| Post subject: | Re: QEMU/KVM internals |
reapersms wrote: A random, mostly uneducated guess is that QEMU spawns up a thread for every physical CPU no matter what, and allocates those as needed to the VM. I take it as QEMU spawns up 12 threads since there are 12 core per socket on my system. Then 8 of them immediately go idle. Does it make sense ? |
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| Author: | reapersms [ Thu Oct 14, 2021 2:40 pm ] |
| Post subject: | Re: QEMU/KVM internals |
Yes. |
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