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

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

On Sun, Oct 23, 2011 at 12:23 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Fri, Oct 21, 2011 at 8:30 PM, Andi Kleen <andi@firstfloor.org> wrote:
>>> > 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
>>
>> If there was a spike in GC usage and we end up with lots of free
>> space in the free list afterward we free it back on the next GC cycle.
>> Then if there's a malloc or other allocator later it can grab
>> the address space we freed.
>>
>> That was done to address your earlier concern.
>>
>> This will only happen on ggc_collect of course.
>>
>> So one difference from before the madvise patch is that different
>> generations of free pages can accumulate in the freelist. Before madvise
>> the freelist would never contain more than one generation.
>> Normally it's sorted by address due to the way GC works, but there's no
>> attempt to keep the sort order over multiple generations.
>>
>> The "free in batch" heuristic requires sorting, so it will only
>> work if all the pages are freed in a single gc cycle.
>>
>> I considered sorting, but it seemed to be too slow.
>>
>> I can expand the comment on that.
>
> Ah, now I see ... but that's of course bad - I expect large regions to be
> free only after multiple collections.  Can you measure what sorting would
> make for a difference?

I wonder if the free list that falls out of a single collection is sorted
(considering also ggc_free) - if it is, building a new one at each collection
and then merging the two sorted lists should be reasonably fast.

>>
>>> 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?
>>
>> Not sure really about the hitrate. I haven't measured it. If hitrate
>> was a concern the free list should be probably split into an array.
>> I'm sure there are lots of other tunings that could be done on the GC,
>> but probably not by me for now :)
>
> Heh.  Yeah, I suppose the freelist could be changed into a list of
> allocation groups with free pages and a bitmap.
>
> Richard.
>
>>>
>>> I'm a bit hesitant to approve the new param, I'd be ok if we just hard-code
>>> quire-size / 2.
>>
>> Ok replacing it with a hardcoded value.
>>
>> -Andi
>>
>