Re: RFA: Don't use obsavestring in dwarf2read

Elena Zannoni
2004-02-05T20:37:33Z

Daniel Jacobowitz writes:
> On Thu, Feb 05, 2004 at 02:26:44PM -0500, Elena Zannoni wrote:
> > Daniel Jacobowitz writes:
> > > On Sun, Jan 11, 2004 at 08:57:26PM -0500, Daniel Jacobowitz wrote:
> > > > This patch is pretty self-explanatory, and pretty effective: With -readnow
> > > > to force immediate loading of full symbols, this is good for 3% startup time
> > > > and 30% memory savings (that's 100MB out of 330MB!) for a gdb session
> > > > against "monotone". We already rely on the lifetimes of this data, so
> > > > there's no point in duplicating it onto another obstack with the exact same
> > > > lifetime.
> > > >
> > > > OK?
> > > >
> > > > [My current C++ work may have significant memory and startup time impact.
> > > > I'm trying to clean house at the same time, so that I don't introduce a net
> > > > loss. This is low-hanging fruit; higher-hanging fruit will take somewhat
> > > > longer.]
> > >
> > > Updated for Michael's comments, and to fix merge issues (and a new
> > > introduction of obsavestring). I also updated the leading comment to
> > > mention that symbols and types can now point into each other's
> > > obstacks.
> >
> >
> > I am not comfortable with this micro-optimization.
> >
> > The purpose and design of the objfile obstacks would become confusing.
> > TYPE_TAG_NAME, for instance, would be now allocated on the
> > type_obstack in all files except for dwarf2read.c. And the
> > crosspollination between different obstacks also is perplexing. I
> > don't think that assuming that they will always have the same lifetime
> > is safe, given they are intentionally separate.
> >
> > However you do raise some good points. Do we need 3 separate obstacks for
> > each object file? If they all have the same lifetime, maybe not.
> > Also are the obstacks a good idea in general?
>
> The obstacks themselves are probably a good idea. Once upon a time,
> Peter informed me, there was a plan to free the psymbol obstack when
> all symbols had been read in; but that doesn't seem like a useful
> optimization, and I can't think offhand of any use for separate symbol
> and type obstacks. I wouldn't object to having a per-objfile obstack
> instead, and un-seperating them.

I think it would be worthwhile to see how much doing that would save us.

>
> > [BTW why are only few obstack properly initialized?]
>
> Which do you mean?
>

I grepped for obstack_init, and only a few obstacks call that
function. Form the obstack doco, it seems that it needs to be
called. I wonder if the function was introduced later on in libiberty,
as an afterthought.

> > How do you get to 30% savings from these changes?
>
> Take a look at how much of the memory usage of GDB on a large C++
> application is for storing names. For monotone, .debug_str is almost
> three times the size of .debug_info, at a whopping 40MB. That's where
> the biggest savings comes from: using pointers directly into
> .debug_str. Because of the GNU LD string merging optimizations, that
> probably accounts for 80MB or so of the savings.
>

Ah, ok, it's because of the nature of the program you were handling. I
was trying to imagine how the overhead of obstack themselves could be
that large. It seems to me that this is a good argument for an 'on
demand' symbol reading implementaion. But, yes the various dwarf2
sections are already in the psymbol_obstack. And we are duplicating
that again on the type_obstack. :-(

> Another large portion comes from not duplicating the names of types in
> the typedef symbols associated with the type. One was on type_obstack,
> the other on symbol_obstack.
>

Right; this would also go away if we unify the obstacks.