Re: [PATCH] hostmem: change default prealloc threads number

[David Hildenbrand]

On 27.09.21 15:19, Nan Wang wrote:
> From: "wangnan.light" <wangnan.light@bytedance.com>
>
> the default number of prealloc threads is 1, for huge memory backend
> file, single thread touch page is really slow.
> We can adjust thread number by prealloc-threads property, but if the
> default value updated to MachineState::smp::cpus may be better.
> For example, old version of qemu(prealloc-threads have not been
> introduced yet), the value of threads num is MachineState::smp::cpus,
> so if someone use the same commandline to start current verion of qemu
> and old version of qemu which will lead to different behaviors.

The introducing patch mentions:

commit ffac16fab33bb42f17e47624985220c1fd864e9d
Author: Igor Mammedov <imammedo@redhat.com>
Date:   Wed Feb 19 11:09:50 2020 -0500

    hostmem: introduce "prealloc-threads" property

    the property will allow user to specify number of threads to use
    in pre-allocation stage. It also will allow to reduce implicit
    hostmem dependency on current_machine.
    On object creation it will default to 1, but via machine
    compat property it will be updated to MachineState::smp::cpus
    to keep current behavior for hostmem and main RAM (which is
    now also hostmem based).

So it looks like we want to do the latter via compat properties eventually.

However, I'd like to note that more prealloc threads might be good for 
large backends, and might be bad for small backends. To me, it feels 
like a workload that relies on this should really do this manually. So I 
am still not sure if this is the right thing to do.

Note that qapi/qom.json:

"@prealloc-threads: number of CPU threads to use for prealloc (default: 
1", so that doc would be wrong now.

Why exactly can't workload that cares not simply set this manually? 
Performance tuning smells like something to be done manually for a 
specific workload.

> Signed-off-by: wangnan.light <wangnan.light@bytedance.com>
> ---
>   backends/hostmem.c | 2 +-
>   hw/core/machine.c  | 5 +++++
>   2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/backends/hostmem.c b/backends/hostmem.c
> index 4c05862ed5..c4a249b7e6 100644
> --- a/backends/hostmem.c
> +++ b/backends/hostmem.c
> @@ -273,7 +273,7 @@ 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(machine);
>   }
>   
>   static void host_memory_backend_post_init(Object *obj)
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 067f42b528..95ba5b1477 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -1065,6 +1065,11 @@ bool machine_dump_guest_core(MachineState *machine)
>       return machine->dump_guest_core;
>   }
>   
> +bool machine_smp_cpus(MachineState *machine)
> +{
> +    return machine->smp.cpus;
> +}
> +
>   bool machine_mem_merge(MachineState *machine)
>   {
>       return machine->mem_merge;


--
Thanks,

David / dhildenb