Re: GCC 3.3, GCC 3.4

[Geoff Keating <geoffk@geoffk.org>]

Devang Patel <dpatel@apple.com> writes:

> On Monday, February 3, 2003, at 12:04 AM, Tim Josling wrote:
> 
> >
> >> On Thursday, January 30, 2003, at 03:38 PM, Neil Booth wrote:
> >>> I'm interested in improving the quality of GCC.  Your statment above
> >>> indicates you're interested in pointing to a number and saying "look,
> >>> it's lower", regardless of what that means long-term.  Hell, let's
> >>> not free anything at all and turn GC off altogether; GCC would be
> >>> faster
> >>> for 90% of files I imagine.
> >>
> >> Guess what?  For smaller programs, it already does that!  So, you are
> >> just describing status quo.
> >
> > It is one thing to stop collecting; it is another to turn off GC.
> >
> > Tests I did last year showed that even without doing collection, gcc
> > is slowed
> > down by the requirements of GC:
> >
> > 1. Allocating storage of similar sizes together, sacrificing locality.
> > 2. Wasting space by rounding up allocations to the next power of 2,
> > wasting
> > space and increasing working set sizes.
> > 3. Having bits of GC code all over the place impacting code locality
> > and
> > working set sizes.
> >
> > Turning off GC slows down the part of the build of GCC attributable
> > to the
> > xgcc/cc1/ld by 5%.
> >
> > However removing it and allocating sequentially (each allocation is
> > next to
> > the previous one) reduces build xgcc/cc1/ld compile time by about 5%.
> >
> > Add these together and the requirements of GC slow down the compiler
> > by about
> > 10% (though collecting gets half of it back)! And most compiles
> > don't need or
> > benefit from it!
> 
> Matt Austern at Apple did similar experiment by using ggc placeholder.
> (same interface as ggc, no collection, allocate memory using mmap in
> chunks and
> satisfy memory allocation requests from these chunks etc..).
> 
> He got around ~12% speedup for sample c++ source.

To be precise, Matt found that the speedup of using the ggc
placeholder was about equivalent to the speedup of using ggc-page but
disabling garbage collection.  This contradicts the results that Tim
Josling got above.

-- 
- Geoffrey Keating <geoffk@geoffk.org>