malloc: Optimize the number of arenas for better application performance

[DJ Delorie]

Yang Yanchao <yangyanchao6@huawei.com> writes:
> However, my machine is 96 cores and I have 91 cores bound.

One benchmark on one uncommon configuration is not sufficient reason to
change a core tunable.  What about other platforms?  Other benchmarks?
Other percentages of cores scheduled?

I would reject this patch based solely on the lack of data backing up
your claims.

> -              int n = __get_nprocs_sched ();
> +              int n = __get_nprocs ();

I've heard complaints about how our code leads to hundreds of arenas on
processes scheduled on only two CPUs.  I think using the number of
*schedulable* cores makes more sense than using the number of *unusable*
cores.

I think this change warrants more research.