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[libvirt] Question : Configuring a VM with backing 1G huge pages across 2 NUMA nodes


Hi Michal,


‘have a kernel+qemu+libvirt setup with all recent upstream bits on a given host & was trying to configure a VM with backing 1G huge pages…spanning 2 NUMA nodes.


The host had 3 1G huge pages on each of the 2 NUMA nodes :


# cat /sys/devices/system/node/node0/hugepages/hugepages-1048576kB/nr_hugepages



# cat /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages




And I had the following in the /etc/fstab 


hugetlbfs      /hugepages_1G    hugetlbfs  pagesize=1GB     0     0


I added the following entries in the xml file for the 4G/4vcpu VM




      <page size='1048576' unit='KiB' nodeset='0'/>

      <page size='1048576' unit='KiB' nodeset='1'/>




  <vcpu placement='static'>4</vcpu>


    <vcpupin vcpu='0' cpuset='0'/>

    <vcpupin vcpu='1' cpuset='1'/>

    <vcpupin vcpu='2' cpuset='8'/>

    <vcpupin vcpu='3' cpuset='9'/>




    <memory node=”strict” nodeset=”0-1”/>





      <cell id='0' cpus='0-1' memory='2097152'/>

      <cell id='1' cpus='2-3' memory='2097152'/>





The resulting qemu command looked like this :


/usr/local/bin/qemu-system-x86_64 -name vm1 -S -machine pc-i440fx-2.2,accel=kvm,usb=off \

-m 4096 -realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 \

-object memory-backend-file,prealloc=yes,mem-path=/hugepages_1G/libvirt/qemu,size=2048M,id=ram-node0,host-nodes=0-1,policy=bind -numa node,nodeid=0,cpus=0-1,memdev=ram-node0 \

-object memory-backend-file,prealloc=yes,mem-path=/hugepages_1G/libvirt/qemu,size=2048M,id=ram-node1,host-nodes=0-1,policy=bind -numa node,nodeid=1,cpus=2-3,memdev=ram-node1 \



There were 3 1G pages available on each NUMA node on the host as shown above... and I noticed that the VM got backed by 3 1G pages from node0 and 1 1G page from node1.


#cat /sys/devices/system/node/node0/hugepages/hugepages-1048576kB/free_hugepages



# cat /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/free_hugepages




Not sure if this was expected behavior given the options I specified in the xml file ? If yes…Is there some additional option to specify (in the XML file) such that only a given number of 1Gig huge pages per node are picked to back the VM (i.e. in the above case just 2 1G from each node) ?





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