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

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 17 May 2022 14:38:58 +0200
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).

current behavior in QEMU is intentionally conservative. threads
number is subject to host configuration and limitations management
layer puts on it and it's not QEMU job to conjure magic numbers that
are host/workload depended.
If user needs more prealloc threads they need to specify it explicitly
for each memory backend (i.e. convince management to do it or fix your
scripts to so).

CCing Michal, as he recently looked into similar topic.

To behave it the old way you need to use legacy -mem-prealloc option.


> 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;
pls, do not add more dependencies to random external objects to memory backends.

If you have to do that, use machine compat properties instead, but then
the essence of the issue stays the same (user shall define optimal threads
number and provide it to qemu explicitly)

>  }
>  
>  static void host_memory_backend_post_init(Object *obj)