Re: [PATCH v5 1/1] Created tunable to force small pages on stack allocation.

[Cupertino Miranda <cupertino.miranda@oracle.com>]

Hi Wilco,

Exactly my remark on the patch. ;)

I think the tunable is benefitial when we care to allocate hugepages for
malloc, etc. But still be able to force small pages for stack.

Imagine a scenario were you create lots of threads. Most threads
barelly use any stack, however there is one that somehow requires a lot
of it to do some crazy recursion. :)

Most likely the heuristic would detect that hugepages would be useful
based on the stack size requirement, but it would never predict that it
only brings any benefit to 1% of the threads created.

Regards,
Cupertino



Wilco Dijkstra writes:

> Hi Adhemerval,
>
> I agree doing this automatically sounds like a better solution.
> However:
>
> +static __always_inline int
> +advise_thp (void *mem, size_t size, size_t guardsize)
> +{
> +  enum malloc_thp_mode_t thpmode = __malloc_thp_mode ();
> +  if (thpmode != malloc_thp_mode_always)
> +    return 0;
> +
> +  unsigned long int thpsize = __malloc_default_thp_pagesize ();
> +  if ((uintptr_t) mem % thpsize != 0
> +      || size % thpsize != 0
> +      || (size - guardsize) % thpsize != 0)
> +    return 0;
>
> Isn't the last part always true currently given the guard page size is based on
> the standard page size? IIRC the issue was the mmap succeeds but the guard
> page is taken from the original mmap which then causes the decomposition.
>
> So you'd need something like:
>
> || guardsize % thpsize == 0)
>
> Ie. we return without the madvise if the size and alignment is wrong for a huge
> page or it is correct and the guardsize is a multiple of a huge page (in which
> case it shouldn't decompose).
>
> +  return __madvise (mem, size, MADV_NOHUGEPAGE);
> +}
>
> Cheers,
> Wilco