Re: instance-private-data issue

[Mathieu Lacage <mathieu_lacage myrealbox com>]

Le lun 04/08/2003 à 12:48, Owen Taylor a écrit :
> > > So, how do we fix the problem? Two ideas I can come up with:
> > > 
> > >  - increase GTypeInstance from 4 bytes to 8 bytes (on
> > >    32 bits), caching the real type *before* the public g_class field.
> 
> Actually, it's from 4 bytes to 12 bytes since the result of
> g_type_create_instance() needs to be maximally aligned - e.g,
> so we can do;
> 
>  struct MyA
>  {
>    GObject parent_instance;
>    gdouble foo;
>  };
> 
> > >  - Use thread-private data to keep track of a stack of
> > >    currently-being-initialized objects with their real types, and 
> > >    have g_type_instance_get_private() check that.
> > > 
> > 
> > I'd go for the thread-private stack: increasing the GTypeInstance size
> > feels like unneeded overhead at all times when there is need only for
> > specific overhead at one point in time (ie: initialization).
> > 
> > However, the cost in terms of additional implementation complexity might
> > not be worth it (given the beauty of the current codebase, it might be
> > worth not trying to make it any more beautiful :). Are there really
> > people around who rely on being able to create thousands of small GTypes
> 
> My concern is not beauty, but rather efficiency; looking up a
> thread-private data key isn't free (*), and
> g_type_instance_get_private()
> is intended to be just a few cycles. That already isn't the
> really the case, which is why we are putting ->priv pointers into new
> Gtk types, but I'm a little reluctant to make it worse.

Why did you started to do this ? Is it because it was too slow ? Did you
have any specific scenario usage which felt really badly slow ? 

I must say I don't see the point of having private instance pimples in
GType and not using them in GTK+.

> 
> Some timing is probably needed here.

I'll just repeat what I said: it feels like a very bad idea to trade
memory at all times for speed at one time. I must say I never expected
myself that g_type_instance_get_private would be just a few cycles long.
Is this something you see on your profiles ?

> 
> The other advantage of the "bloat GTypeInstance" approach is that
> this may not be the only problem of the form "G_TYPE_FROM_INSTANCE()
> produces unexpected results in instance_init()". 

My personal god (namely Darin) would laugh at me if I tried to justify a
patch with a sentence including the word "may". 

Mathieu
-- 
Mathieu Lacage <mathieu gnu org>