Re: [PATCH] hostmem: default the amount of prealloc-threads to smp-cpus

[Paolo Bonzini <pbonzini@redhat.com>]

On 5/17/22 14:38, dzejrou@gmail.com wrote:
> From: Jaroslav Jindrak <dzejrou@gmail.com>
> 
> Prior to the introduction of the prealloc-threads property, the amount
> of threads used to preallocate memory was derived from the value of
> smp-cpus passed to qemu, the amount of physical cpus of the host
> and a hardcoded maximum value. When the prealloc-threads property
> was introduced, it included a default of 1 in backends/hostmem.c and
> a default of smp-cpus using the sugar API for the property itself. The
> latter default is not used when the property is not specified on qemu's
> command line, so guests that were not adjusted for this change suddenly
> started to use the default of 1 thread to preallocate memory, which
> resulted in observable slowdowns in guest boots for guests with large
> memory (e.g. when using libvirt <8.2.0 or managing guests manually).
> 
> This commit restores the original behavior for these cases while not
> impacting guests started with the prealloc-threads property in any way.
> 
> Fixes: 220c1fd864e9d ("hostmem: introduce "prealloc-threads" property")
> Signed-off-by: Jaroslav Jindrak <dzejrou@gmail.com>
> ---
>   backends/hostmem.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/backends/hostmem.c b/backends/hostmem.c
> index a7bae3d713..624bb7ecd3 100644
> --- a/backends/hostmem.c
> +++ b/backends/hostmem.c
> @@ -274,7 +274,7 @@ static void host_memory_backend_init(Object *obj)
>       backend->merge = machine_mem_merge(machine);
>       backend->dump = machine_dump_guest_core(machine);
>       backend->reserve = true;
> -    backend->prealloc_threads = 1;
> +    backend->prealloc_threads = machine->smp.cpus;
>   }
>   
>   static void host_memory_backend_post_init(Object *obj)

Queued, thanks.

Paolo