malloc: Optimize the number of arenas for better application performance

[Adhemerval Zanella]

> On 28 Jun 2022, at 15:56, DJ Delorie <dj@redhat.com> wrote:
> 
> 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.

I think this patch make sense mainly because we changed to use the 
schedulable cores without much though either.  Maybe we can revert
to previous semantic and investigate that using the schedulable 
number makes more sense.