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Re: [RFA] update ggc_min_heapsize_heuristic()
- From: Markus Trippelsdorf <markus at trippelsdorf dot de>
- To: Alexander Monakov <amonakov at ispras dot ru>
- Cc: gcc at gcc dot gnu dot org
- Date: Sun, 9 Apr 2017 21:10:19 +0200
- Subject: Re: [RFA] update ggc_min_heapsize_heuristic()
- Authentication-results: sourceware.org; auth=none
- References: <20170409144125.GA10606@x4> <alpine.LNX.firstname.lastname@example.org>
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.
But you can observe the effect also with a single big C++ file like
tramp3d-v4.cpp. On my old machine:
--param ggc-min-heapsize=131072 : 26.97 secs / 711MB peak memory (current default)
--param ggc-min-heapsize=393216 : 26.04 secs / 886MB peak memory
--param ggc-min-heapsize=524288 : 25.37 secs / 983MB peak memory
--param ggc-min-heapsize=1000000 : 25.36 secs / 990MB peak memory
> > What do you think?
> I wonder if it's possible to reap most of the compile time benefit with a bit
> more modest gc threshold increase?
512MB looks like the sweet spot. And of course one is basically trading memory
usage for compile time performance.