How to set up KVM guests to use HugePages?
- Red Hat Enterprise Linux (RHEL) 9
- Red Hat Enterprise Linux (RHEL) 8
- Red Hat Enterprise Linux (RHEL) 7
- Red Hat Enterprise Linux (RHEL) 6
- Red Hat Enterprise Linux (RHEL) 5
- KVM Virtualization
- How to set up KVM guests to use HugePages?
Mount the HugeTLB filesystem on the host
You may use any mountpoint desired, here we have created
mkdir -p /hugepages mount -t hugetlbfs hugetlbfs /hugepages
This is also possible via an entry in
/etc/fstab, for example
hugetlbfs /hugepages hugetlbfs defaults 0 0
Increase the memory lock limit on the host
Alter the following values in
/etc/security/limits.conf depending on your required memory usage
# Lock max 8Gb soft memlock 8388608 hard memlock 8388608
Reserve HugePages and give the KVM group access to them
Alter to following lines in
/etc/sysctl.conf depending on your required memory usage
vm.nr_hugepages = 4096 vm.hugetlb_shm_group = 36
Add HugePage backing to the KVM guest definition
Add the following to the guest config of an existing KVM guest. This can be done with
virsh edit <guestname> or
virsh define <guest.xml>
<memoryBacking> <hugepages/> </memoryBacking>
Restart the host
This is required to re-allocate contigous memory to HugePages
Start a guest
Confirm the guest has HugePage backing
qemu-kvm process associated with that guest for the presence of
-mem-path in the run command
ps -ef | grep qemu root 4182 1 1 17:35 ? 00:00:42 /usr/libexec/qemu-kvm -S -M rhel5.4.0 -m 1024 -mem-prealloc -mem-path /hugepages/libvirt/qemu -smp 1 -name vm1 -uuid 3f1f3a98-89f8-19ac-b5b5-bf496e2ed9be -no-kvm-pit-reinjection -monitor pty -pidfile /var/run/libvirt/qemu//vm1.pid -boot c -drive file=/vmimages/vm1,if=ide,index=0,boot=on,cache=none -drive file=,if=ide,media=cdrom,index=2 -net nic,macaddr=54:52:00:00:00:01,vlan=0 -net tap,fd=15,script=,vlan=0,ifname=vnet0 -serial pty -parallel none -usb -vnc 127.0.0.1:0 -k en-us
Confirm HugePage use on the system
Here we can see HugePages are being allocated at startup, as well as used/reserved for the guests
cat /proc/meminfo | grep Huge HugePages_Total: 4096 HugePages_Free: 873 HugePages_Rsvd: 761 Hugepagesize: 2048 kB
The default method of allocating memory for KVM guests is to use regular 4k pages. This can result in
- large page tables which occupy unnecessary and inefficient amounts of memory
- increased memory fragmentation which can slow down some kernel-based actions which require contigous memory (eg: disk writes, network access)
- increasing page faults which can slow down all applications
- risking swapping components of virtual guests out to disk which would cause a large performance hit
Using HugePages, page table sizes are dramatically reduced, contigous areas of memory are mapped, and HugePages cannot be swapped by design.
Note: These steps are not necessary with KVM on RHEL6, which uses Transparent HugePages to dynamically map contigous 2Mb areas of memory but also allows that memory to be broken up into 4k pages to be merged with KSM or swapped when the system is under memory pressure.
The above steps can be applied to RHEL6 if HugePages are desired over Transparent HugePages.
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