[libvirt-users] HugePages - can't start guest that requires them

[Michal Privoznik]

On 09.02.2015 18:19, G. Richard Bellamy wrote:
> First I'll quickly summarize my understanding of how to configure numa...
> 
> In "//memoryBacking/hugepages/page[@nodeset]" I am telling libvirt to
> use hugepages for the guest, and to get those hugepages from a
> particular host NUMA node.

No, @nodeset refers to guest NUMA nodes.

> 
> In "//numatune/memory[@nodeset]" I am telling libvirt to pin the
> memory allocation to the guest from a particular host numa node.

The <memory/> element tells what to do with not explicitly pinned guest
NUMA nodes.

> In "//numatune/memnode[@nodeset]" I am telling libvirt which guest
> NUMA node (cellid) should come from which host NUMA node (nodeset).

Correct. This way you can explicitly pin guest onto host NUMA nodes.

> 
> In "//cpu/numa/cell[@id]" I am telling libvirt how much memory to
> allocate to each guest NUMA node (cell).

Yes. Each <cell/> creates guest NUMA node. It interconnects vCPUs and
guest memory - which vCPUs should lie in which guest NUMA node, and how
much memory should be available for that particular guest NUMA node.

> 
> Basically, I thought "nodeset", regardless of where it existed in the
> domain xml, referred to the host's NUMA node, and "cell" (<cell id=/>
> or @cellid) refers to the guest's NUMA node.
> 
> However....
> 
> Atlas [1] starts without issue, prometheus [2] fails with "libvirtd[]:
> hugepages: node 2 not found". I found a patch that contains the code
> responsible for throwing this error [3],
> 
> +        if (def->cpu && def->cpu->ncells) {
> +            /* Fortunately, we allow only guest NUMA nodes to be continuous
> +             * starting from zero. */
> +            pos = def->cpu->ncells - 1;
> +        }
> +
> +        next_bit = virBitmapNextSetBit(page->nodemask, pos);
> +        if (next_bit >= 0) {
> +            virReportError(VIR_ERR_XML_DETAIL,
> +                           _("hugepages: node %zd not found"),
> +                           next_bit);
> +            return -1;
> +        }
> 
> Without digging too deeply into the actual code, and just inferring
> from the above, it looks like we are reading the number of cells set
> in "//cpu/numa" with def->cpu->ncells, and comparing it to the number
> of nodesets in "//memoryBacking//hugepages". I think this means that I
> misunderstand what the nodeset is for in that element...
> 
> Of note is the fact that my host has non-contiguous NUMA node numbers:
> 2015-02-09 08:53:06
> root at eanna i ~ # numastat
>                            node0           node2
> numa_hit               216225024       440311113
> numa_miss                      0          795018
> numa_foreign              795018               0
> interleave_hit             15835           15783
> local_node             214029815       221903122
> other_node               2195209       219203009
> 
> Thanks again for any help.
> 

Libvirt should be perfectly able to cope with noncontinuous host NUMA
nodes. However, noncontinuous guest NUMA nodes are not supported yet -
but it shouldn't matter since users have full control over creating
guest NUMA nodes.

Anyway, if you find the documentation incomplete in any sense, any part,
or you feel that rewording some paragraphs may help, feel free to
propose a patch and I'll review it.

Michal