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Re: [RFA] update ggc_min_heapsize_heuristic()

On 2017.04.09 at 21:10 +0200, Markus Trippelsdorf wrote:
> On 2017.04.09 at 21:25 +0300, Alexander Monakov wrote:
> > On Sun, 9 Apr 2017, Markus Trippelsdorf wrote:
> > 
> > > The minimum size heuristic for the garbage collector's heap, before it
> > > starts collecting, was last updated over ten years ago.
> > > It currently has a hard upper limit of 128MB.
> > > This is too low for current machines where 8GB of RAM is normal.
> > > So, it seems to me, a new upper bound of 1GB would be appropriate.
> > 
> > While amount of available RAM has grown, so has the number of available CPU
> > cores (counteracting RAM growth for parallel builds). Building under a
> > virtualized environment with less-than-host RAM got also more common I think.
> > 
> > Bumping it all the way up to 1GB seems excessive, how did you arrive at that
> > figure? E.g. my recollection from watching a Firefox build is that most of
> > compiler instances need under 0.5GB (RSS).
> 1GB was just a number I've picked to get the discussion going. 
> And you are right, 512MB looks like a good compromise.
> > > Compile times of large C++ projects improve by over 10% due to this
> > > change.
> > 
> > Can you explain a bit more, what projects you've tested?.. 10+% looks
> > surprisingly high to me.
> I've checked LLVM build times on ppc64le and X86_64.

Here are the ppc64le numbers (llvm+clang+lld Release build):

--param ggc-min-heapsize=131072 :
 ninja -j60  15951.08s user 256.68s system 5448% cpu 4:57.46 total

--param ggc-min-heapsize=524288 :
 ninja -j60  14192.62s user 253.14s system 5527% cpu 4:21.34 total


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