[libvirt] [PATCH v2] numa: warn if numa 'mem' option or default RAM splitting between nodes is used.

[Daniel P. Berrangé]

On Wed, Mar 06, 2019 at 06:33:25PM +0100, Igor Mammedov wrote:
> Amend -numa option docs and print warnings if 'mem' option or default RAM
> 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(-)
> 
> diff --git a/numa.c b/numa.c
> index 3875e1e..42838f9 100644
> --- a/numa.c
> +++ b/numa.c
> @@ -121,6 +121,8 @@ static void parse_numa_node(MachineState *ms, NumaNodeOptions *node,
>  
>      if (node->has_mem) {
>          numa_info[nodenr].node_mem = node->mem;
> +        warn_report("Parameter -numa node,mem is obsolete,"
> +                    " use -numa node,memdev instead");

My comments from v1 still apply. We must not do this as long as
libvirt has no choice but to continue using this feature.

>      }
>      if (node->has_memdev) {
>          Object *o;
> @@ -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 +            warn_report("Default splitting of RAM between nodes is obsolete,"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 explicitly define RAM"
> +                        " allocation per node");
>          }
>  
>          numa_total = 0;
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 1cf9aac..61035cb 100644
> --- a/qemu-options.hx
> +++ 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 the host side and only allow QEMU to fake
> +NUMA support which in practice could degrade VM performance.
> +It's advised to always explicitly configure NUMA RAM by using the @samp{memdev} option.
>  
>  @samp{mem} and @samp{memdev} are mutually exclusive. Furthermore,
>  if one node uses @samp{memdev}, all of them have to use it.
> -- 
> 2.7.4
> 

Regards,
Daniel
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