Re: [HACKERS] PATCH: two slab-like memory allocators

On 03/06/2017 07:05 PM, Robert Haas wrote:
> On Mon, Mar 6, 2017 at 12:44 PM, Andres Freund <andres@anarazel.de> wrote:
>> On 2017-03-06 12:40:18 -0500, Robert Haas wrote:
>>> On Wed, Mar 1, 2017 at 5:55 PM, Andres Freund <andres@anarazel.de> wrote:
>>>> The issue was that on 32bit platforms the Datum returned by some
>>>> functions (int2int4_sum in this case) isn't actually a separately
>>>> allocated Datum, but rather just something embedded in a larger
>>>> struct.  That, combined with the following code:
>>>>         if (!peraggstate->resulttypeByVal && !*isnull &&
>>>>                 !MemoryContextContains(CurrentMemoryContext,
>>>>                                                            DatumGetPointer(*result)))
>>>> seems somewhat problematic to me.  MemoryContextContains() can give
>>>> false positives when used on memory that's not a distinctly allocated
>>>> chunk, and if so, we violate memory lifetime rules.  It's quite
>>>> unlikely, given the required bit patterns, but nonetheless it's making
>>>> me somewhat uncomfortable.
>>>>
>>>> Do others think this isn't an issue and we can just live with it?
>>>
>>> I think it's 100% broken to call MemoryContextContains() on something
>>> that's not guaranteed to be a palloc'd chunk.
>>
>> I agree, but to me it seems the only fix would be to just yank out the
>> whole optimization?
>
> Dunno, haven't looked into it.
>

I think it might be fixable by adding a flag into the chunk, with 'true' 
for regular allocations, and 'false' for the optimized ones. And then 
only use MemoryContextContains() for 'flag=true' chunks.

The question however is whether this won't make the optimization 
pointless. I also, wonder how much we save by this optimization and how 
widely it's used? Can someone point me to some numbers?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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