Hugepagesize is not increasing to 1G in VM - centos

Question

I am using CentOS VM in ESXi Server. I want to increase the Huge page size to 1G.
I followed the link:
http://dpdk-guide.gitlab.io/dpdk-guide/setup/hugepages.html
I executed the small script to check if the size of 1 GB is supported:
[root#localhost ~]# if grep pdpe1gb /proc/cpuinfo >/dev/null 2>&1; then echo "1GB supported."; fi
1GB supported.
[root#localhost ~]#
I added default_hugepagesz=1GB hugepagesz=1G hugepages=4 to /etc/default/grub.
grub2-mkconfig -o /boot/grub2/grub.cfg
Rebooted the VM.
But still I can see 2048 KB (2MB) for the Huge page size.
[root#localhost ~]# cat /proc/meminfo | grep -i huge
AnonHugePages: 8192 kB
HugePages_Total: 1024
HugePages_Free: 1024
HugePages_Rsvd: 0
HugePages_Surp: 0
**Hugepagesize: 2048 kB**
[root#localhost ~]#
The following are details of VM:
[root#localhost ~]# uname -a
Linux localhost.localdomain 3.10.0-514.10.2.el7.x86_64 #1 SMP Fri Mar 3 00:04:05 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
[root#localhost ~]#
[root#localhost ~]# cat /proc/cpuinfo | grep -i flags
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts nopl xtopology tsc_reliable nonstop_tsc aperfmperf pni pclmulqdq vmx ssse3 cx16 pcid sse4_1 sse4_2 x2apic popcnt aes xsave avx hypervisor lahf_lm ida arat epb pln pts dtherm tpr_shadow vnmi ept vpid
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts nopl xtopology tsc_reliable nonstop_tsc aperfmperf pni pclmulqdq vmx ssse3 cx16 pcid sse4_1 sse4_2 x2apic popcnt aes xsave avx hypervisor lahf_lm ida arat epb pln pts dtherm tpr_shadow vnmi ept vpid
[root#localhost ~]#
8GB of Memory and 2 CPUs are allocated to VM.

Answer 1

CPU flag of 1gb hugepage support and guest OS support/enabling are not enough to get 1 gb hugepages working in virtualized environment.
The idea of huge pages both on PMD (2MB or 4 MB before PAE and x86_64) and on PUD level (1 GB) is to create mapping from aligned virtual region of huge size into some huge region of physical memory (As I understand, it should be aligned too). With additional virtualization level of hypervisor there are now three (or four) memory levels: virtual memory of app in guest OS, some memory which is considered as physical by guest OS (it is the memory managed by virtualization solution: ESXi, Xen, KVM, ....), and the real physical memory. It's reasonable to assume that hugepage idea should have the same size of huge region in all three levels to be useful (generate less TLB miss, use less Page Table structures to describe a lot of memory - grep "Need bigger than 4KB pages" in the DickSites's "Datacenter Computers: modern challenges in CPU design", Google, Feb2015).
So, to use huge page of some level inside Guest OS, you should already have same sized huge pages in physical memory (in your Host OS) and in your virtualization solution. You can't effectively use huge page inside Guest when they are not available for you host OS and Virtualization software. (Some like qemu or bochs may emulate them but this will be from slow to very slow.) And when you want both 2 MB and 1 GB huge pages: Your CPU, Host OS, Virtual System and Guest OS all should support them (and host system should have enough aligned continuous physical memory to allocate 1 GB page, you probably can't split this page over several Sockets in NUMA).
Don't know about ESXi, but there as some links for
RedHat and some(?) linux virtualization solutions (with libvirtd). In "Virtualization Tuning and Optimization Guide" manual hugepages are listed for Host OS: ⁠8.3.3.3. Enabling 1 GB huge pages for guests at boot or runtime
:
Procedure 8.2. Allocating 1 GB huge pages at boot time
To allocate different sizes of huge pages at boot, use the following command, specifying the number of huge pages. This example allocates 4 1 GB huge pages and 1024 2 MB huge pages: 'default_hugepagesz=1G hugepagesz=1G hugepages=4 hugepagesz=2M hugepages=1024' Change this command line to specify a different number of huge pages to be allocated at boot.
Note The next two steps must also be completed the first time you allocate 1 GB huge pages at boot time.
Mount the 2 MB and 1 GB huge pages on the host:
# mkdir /dev/hugepages1G
# mount -t hugetlbfs -o pagesize=1G none /dev/hugepages1G
# mkdir /dev/hugepages2M
# mount -t hugetlbfs -o pagesize=2M none /dev/hugepages2M
Restart libvirtd to enable the use of 1 GB huge pages on guests:
# service restart libvirtd
1 GB huge pages are now available for guests.
For Ubuntu and KVM: "KVM - Using Hugepages"
By increasing the page size, you reduce the page table and reduce the pressure on the TLB cache. ... vm.nr_hugepages = 256 ... Reboot the system (note: this is about physical reboot of host machine and host OS) ... Set up Libvirt to use Huge Pages KVM_HUGEPAGES=1 ... Setting up a guest to use Huge Pages
For Fedora and KVM (old manual about 2MB pages): https://fedoraproject.org/wiki/Features/KVM_Huge_Page_Backed_Memory
ESXi 5 had support of 2MB pages, which should be manually enabled: How to Modify Large Memory Page Settings on ESXi
For "VMware’s ESX server" of unknown version, from March 2015 paper: BQ Pham, "Using TLB Speculation to Overcome Page Splintering in Virtual Machines", Rutgers University Technical Report DCS-TR-713, March 2015:
Lack of hypervisor support for large pages: Finally, hypervisor vendors can take a few production cycles before fully adopting large pages. For example, VMware’s ESX server currently has no support for 1GB large pages in the hypervisor,
even though guests on x86-64 systems can use them.
Newer paper, no direct conclusion about 1GB pages: https://rucore.libraries.rutgers.edu/rutgers-lib/49279/PDF/1/
We find that large pages are conflicted with lightweight memory management across a range of hypervisors (e.g., ESX, KVM) across architectures (e.g., ARM, x86-64) and container-based technologies.
Old pdf from VMWare: "Large Page Performance. ESX Server 3.5 and ESX Server 3i v3.5". https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/techpaper/large_pg_performance.pdf --- only 2MB huge pages are listed as supported
VMware ESX Server 3.5 and VMware ESX Server 3i v3.5 introduce 2MB large page support to the virtualized
environment. In earlier versions of ESX Server, guest operating system large pages were emulated using small
pages. This meant that, even if the guest operating system was using large pages, it did not get the
performance benefit of reducing TLB misses. The enhanced large page support in ESX Server 3.5 and ESX
Server 3i v3.5 enables 32‐bit virtual machines in PAE mode and 64‐bit virtual machines to make use of large
pages.

Answer 2

Passthrough host cpu to VM work for me, which gives VM pdpe1gb cpu flag.
I use Qemu + libvirt, enable 1G hugepagesz on host.

Answer 3

Maybe it is helpful. set the cpu fuature in the xml described the vm as following:
<cpu mode='custom' match='exact' check='partial'>
<model fallback='allow'>Broadwell</model>
<feature policy='force' name='pdpe1gb'/>
</cpu>