malloc: Optimize the number of arenas for better application performance

[Florian Weimer]

* Yang Yanchao via Libc-alpha:

> At Kunpeng920 platform, tpcc-mysql scores decreased by about 11.2% 
> between glibc-2.36 and glibc2.28.
> Comparing the code, I find that the two commits causes performance 
> degradation.
> 11a02b035b46 (misc: Add __get_nprocs_sched)
> 97ba273b5057 (linux: __get_nprocs_sched: do not feed CPU_COUNT_S with 
> garbage [BZ #28850])
>
> These two patches modify the default behavior.
> However, my machine is 96 cores and I have 91 cores bound.
> It means that perhaps the current way of computing arenas is not optimal.
> So I roll back some of the code submitted by 11a02b035(misc: Add 
> __get_nprocs_sched).
>
> ---
>   malloc/arena.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/malloc/arena.c b/malloc/arena.c
> index 0a684a720d..a1ee7928d3 100644
> --- a/malloc/arena.c
> +++ b/malloc/arena.c
> @@ -937,7 +937,7 @@ arena_get2 (size_t size, mstate avoid_arena)
>               narenas_limit = mp_.arena_max;
>             else if (narenas > mp_.arena_test)
>               {
> -              int n = __get_nprocs_sched ();
> +              int n = __get_nprocs ();
>
>                 if (n >= 1)
>                   narenas_limit = NARENAS_FROM_NCORES (n);

How many threads does tpcc-mysql create?

I wonder if all threads get their own arena with the larger count, and
there is some arena sharing with the smaller count.

Thanks,
Florian