[libvirt] Problem with setting up KVM guests to use HugePages

[Michal Privoznik]

[please keep the list CC'ed]

On 10.06.2015 20:09, Clarylin L wrote:
> Hi Michal,
> 
> Thanks a lot.
> 
> If 100 hugepages are pre-allocated, the guest can start without decreasing
> number of hugepages. Since the guest requires 128 hugepages, it's kind of
> expected that the guest would not take memory from hugepages.
> 
> Before guest start,
> 
> [root at local ~]# cat /proc/meminfo | grep uge
> 
> AnonH*uge*Pages:         0 kB
> 
> H*uge*Pages_Total:     100
> 
> H*uge*Pages_Free:      100
> 
> H*uge*Pages_Rsvd:        0
> 
> H*uge*Pages_Surp:        0
> H*uge*pagesize:    1048576 kB
> 
> After:
> 
> [root at local ~]# cat /proc/meminfo | grep uge
> 
> AnonH*uge*Pages:  134254592 kB
> 
> H*uge*Pages_Total:     100
> 
> H*uge*Pages_Free:      100
> 
> H*uge*Pages_Rsvd:        0
> 
> H*uge*Pages_Surp:        0
> H*uge*pagesize:    1048576 kB
> 
> There is no -mem-prealloc and -mem-path options in qemu command

And there can't be. From the command line below, you are defining 2 NUMA
nodes for your guest. In order to instruct qemu to back their memory by
huge pages you need it to support memory-backend-file object which was
introduced in qemu-2.1.0.
The other option you have is to not use guest NUMA nodes, in which case
global -mem-path can be used.

> 
> [root at local ~]# ps -ef | grep qemu
> 
> *qemu*      3403     1 99 17:42 ?        00:36:42 /usr/libexec/*qemu*-kvm
> -name qvpc-di-03-sf -S -machine pc-i440fx-rhel7.0.0,accel=kvm,usb=off -cpu
> host -m 131072 -realtime mlock=off -smp 32,sockets=2,cores=16,threads=1
> -numa node,nodeid=0,cpus=0-15,mem=65536 -numa
> node,nodeid=1,cpus=16-31,mem=65536 -uuid
> e1b72349-4a0b-4b91-aedc-fd34e92251e4 -smbios type=1,serial=SCALE-SLOT-03
> -no-user-config -nodefaults -chardev
> socket,id=charmonitor,path=/var/lib/libvirt/*qemu*/qvpc-di-03-sf.monitor,server,nowait
> -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown
> -boot strict=on -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -drive
> file=/var/lib/libvirt/images/asr5700/qvpc-di-03-sf-hda.img,if=none,id=drive-ide0-0-0,format=qcow2
> -device
> ide-hd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=2 -drive
> if=none,id=drive-ide0-1-0,readonly=on,format=raw -device
> ide-cd,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0,bootindex=1
> -chardev pty,id=charserial0 -device
> isa-serial,chardev=charserial0,id=serial0 -chardev pty,id=charserial1
> -device isa-serial,chardev=charserial1,id=serial1 -vnc 127.0.0.1:0 -vga
> cirrus -device i6300esb,id=watchdog0,bus=pci.0,addr=0x3 -watchdog-action
> reset -device vfio-pci,host=08:00.0,id=hostdev0,bus=pci.0,addr=0x5 -device
> vfio-pci,host=09:00.0,id=hostdev1,bus=pci.0,addr=0x6 -device
> vfio-pci,host=0a:00.0,id=hostdev2,bus=pci.0,addr=0x7 -device
> virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 -msg timestamp=on
> 
> 
> If 140 hugepages are preallocated, the guest cannot start and it complained
> not enough memory.
> 
> 
> The libvirt version is shown as follows:
> 
> virsh # version
> 
> Compiled against library: libvirt 1.2.8
> 
> Using library: libvirt 1.2.8
> 
> Using API: QEMU 1.2.8
> 
> Running hypervisor: QEMU 1.5.3
> 
> 



> Also in guest configuration contains numa session. The hugepages are
> uniformly distributed to two nodes. In this case, do I need to make
> additional configurations to enable usage of hugepages?
> 
>   <numatune>
> 
>     <memory mode='strict' nodeset='0-1'/>
> 
>   </numatune>
> 

This says that all the memory for you guests should be pinned onto host
nodes 0-1. If you want to be more specific, you can explicitly wire
guest NUMA nodes onto host NUMA nodes in 1:N relationship (where N can
be even 1, in which case you will get 1:1), e.g.:

<memnode cellid='0' mode='preferred' nodeset='0'/>

Michal