Re: [RFA] update ggc_min_heapsize_heuristic()

[Markus Trippelsdorf <markus at trippelsdorf dot de>]

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.

-- 
Markus