Re: [PATCH 2/3] Free large chunks in ggc

[Richard Guenther <richard.guenther@gmail.com>]

On Fri, Oct 21, 2011 at 7:52 AM, Andi Kleen <andi@firstfloor.org> wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> This implements the freeing back of large chunks in the ggc madvise path
> Richard Guenther asked for.  This way on systems with limited
> address space malloc() and other allocators still have
> a chance to get back at some of the memory ggc freed. The
> fragmented pages are still just given back, but the address space
> stays allocated.
>
> I tried freeing only aligned 2MB areas to optimize for 2MB huge
> pages, but the hit rate was quite low, so I switched to 1MB+
> unaligned areas. The target size is a param now.
>
> Passed bootstrap and testing on x86_64-linux
>
> gcc/:
> 2011-10-18  Andi Kleen  <ak@linux.intel.com>
>
>        * ggc-page (release_pages): First free large continuous
>        chunks in the madvise path.
>        * params.def (GGC_FREE_UNIT): Add.
>        * doc/invoke.texi (ggc-free-unit): Add.
> ---
>  gcc/doc/invoke.texi |    5 +++++
>  gcc/ggc-page.c      |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  gcc/params.def      |    5 +++++
>  3 files changed, 58 insertions(+), 0 deletions(-)
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 4f55dbc..e622552 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -8858,6 +8858,11 @@ very large effectively disables garbage collection.  Setting this
>  parameter and @option{ggc-min-expand} to zero causes a full collection
>  to occur at every opportunity.
>
> +@item  ggc-free-unit
> +
> +Continuous areas in OS pages to free back to OS immediately. Default is 256
> +pages, which is 1MB with 4K pages.
> +
>  @item max-reload-search-insns
>  The maximum number of instruction reload should look backward for equivalent
>  register.  Increasing values mean more aggressive optimization, making the
> diff --git a/gcc/ggc-page.c b/gcc/ggc-page.c
> index ba88e3f..eb0eeef 100644
> --- a/gcc/ggc-page.c
> +++ b/gcc/ggc-page.c
> @@ -972,6 +972,54 @@ release_pages (void)
>   page_entry *p, *start_p;
>   char *start;
>   size_t len;
> +  size_t mapped_len;
> +  page_entry *next, *prev, *newprev;
> +  size_t free_unit = PARAM_VALUE (GGC_FREE_UNIT) * G.pagesize;
> +
> +  /* First free larger continuous areas to the OS.
> +     This allows other allocators to grab these areas if needed.
> +     This is only done on larger chunks to avoid fragmentation.
> +     This does not always work because the free_pages list is only
> +     sorted over a single GC cycle. */

But release_pages is only called from ggc_collect, or what do you
mean with the above?  Would the hitrate using the quire size increase
if we change how we allocate from the freelist or is it real fragmentation
that causes it?

I'm a bit hesitant to approve the new param, I'd be ok if we just hard-code
quire-size / 2.

Richard.

> +
> +  p = G.free_pages;
> +  prev = NULL;
> +  while (p)
> +    {
> +      start = p->page;
> +      start_p = p;
> +      len = 0;
> +      mapped_len = 0;
> +      newprev = prev;
> +      while (p && p->page == start + len)
> +        {
> +          len += p->bytes;
> +         if (!p->discarded)
> +             mapped_len += p->bytes;
> +         newprev = p;
> +          p = p->next;
> +        }
> +      if (len >= free_unit)
> +        {
> +          while (start_p != p)
> +            {
> +              next = start_p->next;
> +              free (start_p);
> +              start_p = next;
> +            }
> +          munmap (start, len);
> +         if (prev)
> +           prev->next = p;
> +          else
> +            G.free_pages = p;
> +          G.bytes_mapped -= mapped_len;
> +         continue;
> +        }
> +      prev = newprev;
> +   }
> +
> +  /* Now give back the fragmented pages to the OS, but keep the address
> +     space to reuse it next time. */
>
>   for (p = G.free_pages; p; )
>     {
> diff --git a/gcc/params.def b/gcc/params.def
> index 5e49c48..edbf0de 100644
> --- a/gcc/params.def
> +++ b/gcc/params.def
> @@ -561,6 +561,11 @@ DEFPARAM(GGC_MIN_HEAPSIZE,
>  #undef GGC_MIN_EXPAND_DEFAULT
>  #undef GGC_MIN_HEAPSIZE_DEFAULT
>
> +DEFPARAM(GGC_FREE_UNIT,
> +        "ggc-free-unit",
> +        "Continuous areas in OS pages to free back immediately",
> +        256, 0, 0)
> +
>  DEFPARAM(PARAM_MAX_RELOAD_SEARCH_INSNS,
>         "max-reload-search-insns",
>         "The maximum number of instructions to search backward when looking for equivalent reload",
> --
> 1.7.5.4
>
>