.VM memory preallocation using multiple threads
You can now define multiple CPU threads for virtual machine (VM) memory allocation in the domain XML configuration, for example as follows:
<memoryBacking>
<allocation threads='8'/>
</memoryBacking>
This ensures that more than one thread is used for loading memory pages when starting a VM. As a consequence, VMs with multiple allocation threads configured start significantly faster, especially if the VMs has large amounts of RAM assigned and backed by hugepages.
Preverified with scratch build on rhel8.6:
libvirt-8.0.0-6.el8_rc.5b05e15dc0.x86_64
qemu-kvm-6.2.0-11.module+el8.6.0+14707+5aa4b42d.x86_64
Test steps:
1. Setup host hugepages
sysctl vm.nr_hugepages=1024
2. Define domain xml with memorybacking with below:
<memoryBacking>
<hugepages>
<page size='2048' unit='KiB'/>
</hugepages>
<allocation threads='8'/>
</memoryBacking>
3. Start the guest vm
4. Check the guest vm can boot up and works well.
5. Check the qemu cmdline, that multithread pass through libvirt layer to qemu:
-object {"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/dev/hugepages/libvirt/qemu/2-vm1","x-use-canonical-path-for-ramblock-id":false,"prealloc":true,"prealloc-threads":8,"size":2147483648}
6. Also check the other scenarios, and did not find issue:
6.1 no huge page
6.2 no multi-thread setting
6.3 working with memfd,anonymous,file source
6.4 working with immdiate, ondemand mode
6.5 negative thread setting with minus, zero, not number, too big number
6.6 working with big thread number
6.7 virsh define --validate
Verified with:
libvirt-8.0.0-5.1.module+el8.6.0+15027+c62943ab.x86_64
qemu-kvm-6.2.0-11.module+el8.6.0+14707+5aa4b42d.x86_64
Test steps:
1. Setup host hugepages
sysctl vm.nr_hugepages=1024
2. Define domain xml with memorybacking with below:
<memoryBacking>
<hugepages/>
<allocation threads='8'/>
</memoryBacking>
3. Start the guest vm
4. Check the guest vm can boot up and works well.
5. Check the qemu cmdline, that multithread pass through libvirt layer to qemu:
-object {"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/dev/hugepages/libvirt/qemu/18-vm1","x-use-canonical-path-for-ramblock-id":false,"prealloc":true,"prealloc-threads":8,"size":2147483648}
6. Also check the other scenarios, and did not find issue:
6.1 no huge page
6.2 no multi-thread setting
6.3 working with memfd,anonymous,file source
6.4 working with immdiate, ondemand mode
6.5 negative thread setting with minus, zero, not number, too big number
6.6 working with big thread number
6.7 virsh define --validate
6.8 working with dimm memory devices
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (Moderate: virt:rhel and virt-devel:rhel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHSA-2022:5821
Preverified with scratch build on rhel8.6: libvirt-8.0.0-6.el8_rc.5b05e15dc0.x86_64 qemu-kvm-6.2.0-11.module+el8.6.0+14707+5aa4b42d.x86_64 Test steps: 1. Setup host hugepages sysctl vm.nr_hugepages=1024 2. Define domain xml with memorybacking with below: <memoryBacking> <hugepages> <page size='2048' unit='KiB'/> </hugepages> <allocation threads='8'/> </memoryBacking> 3. Start the guest vm 4. Check the guest vm can boot up and works well. 5. Check the qemu cmdline, that multithread pass through libvirt layer to qemu: -object {"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/dev/hugepages/libvirt/qemu/2-vm1","x-use-canonical-path-for-ramblock-id":false,"prealloc":true,"prealloc-threads":8,"size":2147483648} 6. Also check the other scenarios, and did not find issue: 6.1 no huge page 6.2 no multi-thread setting 6.3 working with memfd,anonymous,file source 6.4 working with immdiate, ondemand mode 6.5 negative thread setting with minus, zero, not number, too big number 6.6 working with big thread number 6.7 virsh define --validate