[ovirt-users] Question about Huge Pages

[Martin Polednik]

On 19/04/17 14:01 +0200, Gianluca Cecchi wrote:
>On Wed, Apr 19, 2017 at 8:03 AM, Michal Skrivanek <mskrivan at redhat.com>
>wrote:
>
>>
>> Why not reserving regular hugepages for VMs on boot?
>
>
>Do you mean at hypervisor level? In this case it is what I'm doing normally
>for physical servers where I install Oracle RDBMS
>
>
>> then you can use
>> it with vdsm hook for that Oracle VM.
>
>
>Which hook are you referring?
>This one:
>http://www.ovirt.org/develop/developer-guide/vdsm/hook/hugepages/
>?
>In case is it still current? In the sense that I need to mount the
>hugetblfs virtual file system at hst level?
>The hook description seems low detailed...
>Normally if I want oracle user able to use huge pages on physical server, I
>have to specify
>
>#
># Huge pages
>#
>vm.hugetlb_shm_group = 2000
># 18GB allocatable
>vm.nr_hugepages = 9216
>#
>
>where 2000 is the group id for dba group, the main group of oracle user
>
>How to map this with vrtualization?
>Eg:
>1) vm.hugetlb_shm_group at hypervisor side should be set to the group of
>the qemu user as the qemu-kvm process runs with it?
>2) Then I have to set VM for VM the hugepages=xxx value in the hook and
>that will bypass the sysctl.conf configuration in the guest?
>3) I presume I have to set the vm.hugetlb_shm_group parameter at guest
>level....

If you are using recent CentOS (or I guess Fedora), there isn't any
extra setup required. Just create the custom property:

On the host where engine is running:

$ engine-config -s "UserDefinedVMProperties=hugepages=^.*$"
$ service ovirt-engine restart

and you should see 'hugepages' when editing a VM under custom properties.
Set the number to (desired memory / 2048) and you're good to go. The
VM will run with it's memory backed by hugepages. If you need
hugepages even inside the VM, do whatever you would do on a physical
host.

mpolednik

>Thanks,
>Gianluca
>
>
>
>> It improves VM performance in
>> general, the only drawback is less flexibility since that memory can't
>> be used by others unless they specifically ask for  hugepages.
>>
>
>This seems to confirm that I have to set a statich sysctl.conf entry at
>hypervisor level such as
>vm.nr_hugepages = YYYY
>
>
>> Also, I suppose you disable KSM, and I'm not sure about ballooning,
>> unless you need it I'd disable it too.
>>
>
>I kept the defaults at the moment that I suppose should be
>
>a) KSM disabled
>
>ksm has been configured to start by default as normally, but ksmtuned has
>been disabled:
>
>[g.cecchi at ov300 ~]$ sudo systemctl status ksm
>● ksm.service - Kernel Samepage Merging
>   Loaded: loaded (/usr/lib/systemd/system/ksm.service; enabled; vendor
>preset: enabled)
>   Active: active (exited) since Tue 2017-04-11 11:07:28 CEST; 1 weeks 1
>days ago
>  Process: 976 ExecStart=/usr/libexec/ksmctl start (code=exited,
>status=0/SUCCESS)
> Main PID: 976 (code=exited, status=0/SUCCESS)
>   CGroup: /system.slice/ksm.service
>
>Apr 11 11:07:28 ov300.datacenter.polimi.it systemd[1]: Starting Kernel
>Samepage Merging...
>Apr 11 11:07:28 ov300.datacenter.polimi.it systemd[1]: Started Kernel
>Samepage Merging.
>
>[g.cecchi at ov300 ~]$ sudo systemctl status ksmtuned
>● ksmtuned.service - Kernel Samepage Merging (KSM) Tuning Daemon
>   Loaded: loaded (/usr/lib/systemd/system/ksmtuned.service; disabled;
>vendor preset: disabled)
>   Active: inactive (dead)
>[g.cecchi at ov300 ~]$
>
>
>b) ballooning enabled for a newly created VM unless I explicitly disable it
>(at least I see this happens in 4.1.1)
>
>What to do for a) and b) to not interfere with huge pages?
>
>
>> The hook is being improved right now in master, but it should be
>> usable in stable too.
>>
>>
>I will be happy to test and verify and contribute to its description, as
>soon as I understand its usage....
>
>Gianluca

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