[libvirt] [PATCH] numa: warn if numa 'mem' option or default RAM splitting between nodes is used.

[Eric Blake]

On 3/6/19 10:30 AM, Igor Mammedov wrote:
> Ammend -numa option docs and print warnings if 'mem' option or default RAM

s/Ammend/Amend/

> splitting between nodes is used. It's intended to discourage users from using
> configuration that allows only to fake NUMA on guest side while leading
> to reduced performance of the guest due to inability to properly configure
> VM's RAM on the host.
> 
> In NUMA case, it's recommended to always explicitly configure guest RAM
> using -numa node,memdev={backend-id} option.
> 
> Signed-off-by: Igor Mammedov <imammedo at redhat.com>
> ---
>  numa.c          |  5 +++++
>  qemu-options.hx | 12 ++++++++----
>  2 files changed, 13 insertions(+), 4 deletions(-)
> 

> @@ -407,6 +409,9 @@ void numa_complete_configuration(MachineState *ms)
>          if (i == nb_numa_nodes) {
>              assert(mc->numa_auto_assign_ram);
>              mc->numa_auto_assign_ram(mc, numa_info, nb_numa_nodes, ram_size);
> +            warn_report("Default splitting of RAM between nodes is obsolete,"
> +                        " Use '-numa node,memdev' to explictly define RAM"

s/explictly/explicitly/


> +++ b/qemu-options.hx
> @@ -206,10 +206,14 @@ For example:
>  -numa cpu,node-id=0,socket-id=0 -numa cpu,node-id=1,socket-id=1
>  @end example
>  
> - at samp{mem} assigns a given RAM amount to a node. @samp{memdev}
> -assigns RAM from a given memory backend device to a node. If
> - at samp{mem} and @samp{memdev} are omitted in all nodes, RAM is
> -split equally between them.
> + at samp{memdev} assigns RAM from a given memory backend device to a node.
> +
> +Legacy options/behaviour: @samp{mem} assigns a given RAM amount to a node.
> +If @samp{mem} and @samp{memdev} are omitted in all nodes, RAM is split equally
> +between them. Option @samp{mem} and default RAM splitting are obsolete as they
> +do not provide means to manage RAM on host side and only allow QEMU to fake

s/on host/on the host/

> +NUMA support which in practice could degrade VM performance.
> +It's advised to always explicitly configure NUMA RAM using @samp{memdev} option.

s/using/by using the/

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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