Support for hugepages in KVM VMs

[akrasnov-drv]

The required feature described as a wish

We use hugepages and some other tweaks to optimize performance of KVM VMs.
I managed to use our KVM VM config in CloudStack by updating VM xml via hook.
The problem is that CloudStack checks free memory of Linux and ignores free hugepages.
As I understand, in the earlier version of CloudStack it used shell to get free memory info, but now it uses Java calls that I can't change without patching and rebuilding agent code.

The following changes should do what I miss:
I'd like to have config option (in Cloudstack config, or in agent.properties) to enable hugepages (defaults to off). IMHO, it's quite easy to do. The option will

• add the following block to each KVM VM created

  <memoryBacking>
    <hugepages/>
  </memoryBacking>

• change check of free memory from MemFree to HugePages_Free × Hugepagesize

Something like the following

long hpFree  = map.getOrDefault(HUGEPAGE_FREE_KEY, 0L);
    long hpSize  = map.getOrDefault(HUGEPAGE_SIZE_KEY, 1024L);  // KiB
    return (hpFree * hpSize) + memFree;

Another option, though less preferable, is to allow using external script (e.g. host.stats.command.script) to override internally acquired values (e.g. MemFree)

[NuxRo]

@akrasnov-drv this can be done with extra config

https://www.shapeblue.com/cloudstack-feature-first-look-enable-sending-of-arbitrary-configuration-data-to-vms/

[akrasnov-drv]

First of all I did not manage to add it via extraconfig, because CS blocks it.
CS wrote for that extraconfig that hugepages are supported only for DPDK.

But that's not the main issue. I can add hugepages block via hook. The issue is that CS checks free memory in host without considering hugepages. If I have 256Gb of RAM and set 240Gb to be in huge pages, CS rejects to start 32Gb VM on that host, even if all hugepages are free and VM should use hugepages.
And opposite, if it fills all hugepages, but still there are some regular memory, CS thinks there is space for more VMs but actually there is no. I could possibly solve this issue by adding some reserved memory to CS config, but there is just single percent value that should match all hosts.
In any case, making CS to recognize and. use hugepages (as I wrote in the description) would be a proper solution.