On Thu, 11 Mar 2021 11:59:18 +0300
Denis Plotnikov <den-plotni...@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-plotni...@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);
> }
>
