Re: [Qemu-devel] [PATCH for-1.5] qom: optimize casting to leaf class and parent class

[Aurelien Jarno]

On Fri, May 10, 2013 at 01:47:55PM -0500, Anthony Liguori wrote:
> Most QOM types use type_register_static but we still strdup the
> passed data.  However, the original pointers are useful because
> GCC is pretty good about collapsing strings so its very likely any
> use of the pointer will end up being that same address.
> 
> IOW, with a little trickery, we can compare types by just comparing
> strings and in fact that's what we do here.
> 
> We do this for the two most common cases, casting to a leaf class
> or to the parent class.
> 
> With these two changes, I see a decrease from around 2 hash table
> lookups to only a thousand with no run time lookups at all.
> 
> Cc: Paolo Bonzini <address@hidden>
> Cc: Aurelien Jarno <address@hidden>
> Cc: Andreas Färber <address@hidden>
> Reported-by: Aurelien Jarno <address@hidden>
> Signed-off-by: Anthony Liguori <address@hidden>
> ---
> Aurelien, could you please try this patch with your PPC test case?
> ---
>  qom/object.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/qom/object.c b/qom/object.c
> index 75e6aac..5ecfd28 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -132,7 +132,13 @@ TypeImpl *type_register(const TypeInfo *info)
>  
>  TypeImpl *type_register_static(const TypeInfo *info)
>  {
> -    return type_register(info);
> +    TypeImpl *impl;
> +
> +    impl = type_register(info);
> +    impl->name = info->name;
> +    impl->parent = info->parent;
> +
> +    return impl;
>  }
>  
>  static TypeImpl *type_get_by_name(const char *name)
> @@ -449,10 +455,16 @@ Object *object_dynamic_cast_assert(Object *obj, const 
> char *typename)
>  ObjectClass *object_class_dynamic_cast(ObjectClass *class,
>                                         const char *typename)
>  {
> -    TypeImpl *target_type = type_get_by_name(typename);
> +    TypeImpl *target_type;
>      TypeImpl *type = class->type;
>      ObjectClass *ret = NULL;
>  
> +    if (type->name == typename || type->parent == typename) {
> +        return class;
> +    }
> +
> +    target_type = type_get_by_name(typename);
> +
>      if (!target_type) {
>          /* target class type unknown, so fail the cast */
>          return NULL;

Unfortunately it doesn't fix the problem. I only see a 0.5% improvement,
which might be in the noise. I still see g_hash_table_lookup and
g_str_hash quite high in perf top.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net