Re: [PATCH v4 0/3] KVM: Dynamic Halt-Polling

From: Wanpeng Li
Date: Sat Aug 29 2015 - 19:55:16 EST

On 8/30/15 6:26 AM, Peter Kieser wrote:
Thanks, Wanpeng. Applied this to Linux 3.18 and seeing much higher CPU usage (200%) for qemu 2.4.0 process on a Windows 10 x64 guest. qemu parameters:

Thanks for the report. If Paolo's patch "kvm: add halt_poll_ns module parameter" is applied on your 3.18? Btw, do you test the linux guest?

Wanpeng Li

qemu-system-x86_64 -enable-kvm -name arwan-20150704 -S -machine pc-q35-2.2,accel=kvm,usb=off -cpu Haswell,hv_time,hv_relaxed,hv_vapic,hv_spinlocks=0x1000 -m 8192 -realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 -uuid 7c2fc02d-2798-4fc9-ad04-db5f1af92723 -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/arwan-20150704.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=localtime -no-shutdown -boot strict=on -device i82801b11-bridge,id=pci.1,bus=pcie.0,addr=0x1e -device pci-bridge,chassis_nr=2,id=pci.2,bus=pci.1,addr=0x1 -device nec-usb-xhci,id=usb1,bus=pci.2,addr=0x4 -device virtio-serial-pci,id=virtio-serial0,bus=pci.2,addr=0x5 -drive file=/dev/mapper/crypt-arwan-20150704,if=none,id=drive-virtio-disk0,format=raw,cache=none,discard=unmap,aio=native -device virtio-blk-pci,scsi=off,bus=pci.2,addr=0x3,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=2 -drive file=/usr/share/virtio-win/virtio-win.iso,if=none,media=cdrom,id=drive-sata0-0-2,readonly=on,format=raw -device ide-cd,bus=ide.2,drive=drive-sata0-0-2,id=sata0-0-2,bootindex=1 -netdev tap,fds=31:32:33:34,id=hostnet0,vhost=on,vhostfds=35:36:37:38 -device virtio-net-pci,guest_csum=off,guest_tso4=off,guest_tso6=off,mq=on,vectors=10,netdev=hostnet0,id=net0,mac=52:54:00:f3:6b:c4,bus=pci.2,addr=0x2 -chardev socket,id=charchannel0,path=/var/lib/libvirt/qemu/channel/target/,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0 -chardev spicevmc,id=charchannel1,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=com.redhat.spice.0 -vnc -device qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,vgamem_mb=16,bus=pcie.0,addr=0x1 -device virtio-balloon-pci,id=balloon0,bus=pci.2,addr=0x1 -msg timestamp=on

I revert patch, qemu shows 17% CPU usage on host. Thoughts?


On 2015-08-29 3:21 PM, Wanpeng Li wrote:
Hi Peter,
On 8/30/15 5:18 AM, Peter Kieser wrote:
Hi Wanpeng,

Do I need to set any module parameters to use your patch, or should halt_poll_ns automatically tune with just your patch series applied?

You don't need any module parameters.

Wanpeng Li


On 2015-08-27 2:47 AM, Wanpeng Li wrote:
v3 -> v4:
* bring back grow vcpu->halt_poll_ns when interrupt arrives and shrinks
when idle VCPU is detected

v2 -> v3:
* grow/shrink vcpu->halt_poll_ns by *halt_poll_ns_grow or /halt_poll_ns_shrink
* drop the macros and hard coding the numbers in the param definitions
* update the comments "5-7 us"
* remove halt_poll_ns_max and use halt_poll_ns as the max halt_poll_ns time,
vcpu->halt_poll_ns start at zero
* drop the wrappers
* move the grow/shrink logic before "out:" w/ "if (waited)"

v1 -> v2:
* change kvm_vcpu_block to read halt_poll_ns from the vcpu instead of
the module parameter
* use the shrink/grow matrix which is suggested by David
* set halt_poll_ns_max to 2ms

There is a downside of halt_poll_ns since poll is still happen for idle
VCPU which can waste cpu usage. This patchset add the ability to adjust
halt_poll_ns dynamically, grows halt_poll_ns if an interrupt arrives and
shrinks halt_poll_ns when idle VCPU is detected.

There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and halt_poll_ns_shrink.

Test w/ high cpu overcommit ratio, pin vCPUs, and the halt_poll_ns of
halt-poll is the default 500000ns, the max halt_poll_ns of dynamic
halt-poll is 2ms. Then watch the %C0 in the dump of Powertop tool.
The test method is almost from David.

| | | |
| w/o halt-poll | w/ halt-poll | dynamic halt-poll |
| | | |
| ~0.9% | ~1.8% | ~1.2% |
The always halt-poll will increase ~0.9% cpu usage for idle vCPUs and the
dynamic halt-poll drop it to ~0.3% which means that reduce the 67% overhead
introduced by always halt-poll.

Wanpeng Li (3):
KVM: make halt_poll_ns per-VCPU
KVM: dynamic halt_poll_ns adjustment
KVM: trace kvm_halt_poll_ns grow/shrink

include/linux/kvm_host.h | 1 +
include/trace/events/kvm.h | 30 ++++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 50 +++++++++++++++++++++++++++++++++++++++++++---
3 files changed, 78 insertions(+), 3 deletions(-)

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