Re: [PATCH v1] softmmu/vl: make default prealloc-threads work w/o -mem-prealloc

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 11 Mar 2021 11:59:18 +0300
Denis Plotnikov <den-plotnikov@yandex-team.ru> wrote:

> Preallocation in memory backends can be specified with either global
> QEMU option "-mem-prealloc", or with per-backend property
> "prealloc=true".  In the latter case, however, the default for the
> number of preallocation threads is not set to the number of vcpus, but
> remains at 1 instead.
> 
> Fix it by setting the "prealloc-threads" sugar property of
> "memory-backend" to the number of vcpus unconditionally.

well, number of prealloc-threads are policy that should be set by user.
it was kept equal to vcpus for compat reasons with "-mem-prealloc",
for if one goes with supplying memory backends explicitly, one is supposed
to set prealloc-threads explicitly as well if default(==1) doesn't
suit user. It could be equal to #vcpus or be a different numbers
depending on host CPU configuration. That was intent when 
commit ffac16fab3 was introduced.

Imagine one has a VM with a lot of VCPUs but not so much of of RAM,
setting prealloc-threads to #VCPUs will be wasting CPU resources
at least.


> Fixes: ffac16fab3 ("hostmem: introduce "prealloc-threads" property")
> 
> Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
> ---
>  softmmu/vl.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index ff488ea3e7db..e392e226a2d3 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -2300,14 +2300,17 @@ static void qemu_validate_options(void)
>  
>  static void qemu_process_sugar_options(void)
>  {
> -    if (mem_prealloc) {
> -        char *val;
> +    char *val;
>  
> -        val = g_strdup_printf("%d",
> -                 (uint32_t) qemu_opt_get_number(qemu_find_opts_singleton("smp-opts"), "cpus", 1));
> -        object_register_sugar_prop("memory-backend", "prealloc-threads", val,
> -                                   false);
> -        g_free(val);
> +    val = g_strdup_printf("%d",
> +              (uint32_t) qemu_opt_get_number(
> +                             qemu_find_opts_singleton("smp-opts"), "cpus", 1));
> +
> +    object_register_sugar_prop("memory-backend", "prealloc-threads", val,
> +                                false);
> +    g_free(val);
> +
> +    if (mem_prealloc) {
>          object_register_sugar_prop("memory-backend", "prealloc", "on", false);
>      }
>