Re: MPS: weak hash tables

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Pip Cet <pipcet@protonmail.com> writes:

>> What do the timings say?
>
> Horrible things. Unacceptably slow. Sufficiently so that I looked into
> the generation parameters and randomly changed them, and now it's
> faster than vanilla Emacs?! Does that make sense to you, or is it more
> likely I'm being tricked by something silly like CPU speed?

Good tension building :-)

>
> All I did was change:
>
> -  mps_gen_param_s gens[] = { { 128000, 0.8 }, { 5 * 128000, 0.4 } };
> +  mps_gen_param_s gens[] = { { 64000, 0.8 }, { 5 * 64000, 0.4 }, { 10 * 64000, 0.3 }, };
>
> but that was a totally random guess...

Wow, one of the knobs I always wanted to play with but couldn't figure
out the rules :-).

> Again, I may just be doing something wrong.

Hm, what might be happening? I guess we can ignore the mortality because
MPS says these values are only initial estimates, and it computes
them itself over time.

So, we had 14 * 64M total before, and 16 * 64M afterwards. A bit more
makes sense since we now have GMP objects, but a dramatic improvement?

The number of generations? Maybe MPS gets away, in the 3 gens case, with
collecting gen1 and 2 most of the time if gen1 is not enough, which
means 6 * 64, while in the 2 gens case, when it runs out of space
in gen1 it directly has to handle gen1 + gen2 = 12 * 64M?

That's the only thing I can come up with ATM. Complete guesswork, of
course. I wonder how one would find an "optimal" configuration at all.

Helmut, do you perhaps have an idea?

And could one do something with telemetry to optimize generations? Do
they mention something like that?