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Re: gcc 3.1 is still very slow, compared to 2.95.3


On Sat, May 18, 2002 at 01:12:23PM +0100, Richard Earnshaw wrote:
> Memory use efficiency:  I suspect we have many partially used pages, since 
> each page of memory is only used for objects of a single size we end up 
> with many pages with just a few items in them; in particular persistent 
> objects can now be scattered anywhere across that memory, rather than 
> being gathered in a single block.  We now have to allocate far more pages 
> for a small compilation than we did before; I rarely see compilation of a 
> C file requiring less than 8M now, it used to be around 3-4M for a typical 
> file in GCC.  To make matters worse we regularly touch most of those pages 
> rather than just a subset of them, which means the OS can't usefully page 
> any of them out.

I haven't looked at the code closely, but if your suspicions are right,
it's likely that rounding up object sizes would help... doing some stats
on pages occupancy will help.  I'll check if I can instrument some things.

Right now, my compile times may have to do with cache hits...  I have at
least one killer box where paging cannot possibly be an issue.
Paradoxically, investigating slowdowns is best run on real fast boxes.

I'm going to try building m68k crosses when I can, and see whether there
are similar slowdowns on m68k, considering Jan's remark about the frontends
being vastly differnet.


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