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

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(a)redhat.com&gt; --- 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mem; + warn_report("Parameter -numa node,mem is obsolete," + " use -numa node,memdev instead");

} 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numa_auto_assign_ram(mc, numa_info, nb_numa_nodes, ram_size); + warn_report("Default splitting of RAM between nodes is obsolete," + " Use '-numa node,memdev' to explicitly define RAM" + " allocation per node"); } numa_total = 0; diff --git a/qemu-options.hx b/qemu-options.hx index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 -@samp{mem} assigns a given RAM amount to a node. @samp{memdev} -assigns RAM from a given memory backend device to a node. If -@samp{mem} and @samp{memdev} are omitted in all nodes, RAM is -split equally between them. +@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