Re: [HACKERS] PATCH: two slab-like memory allocators - Mailing list pgsql-hackers

On 03/06/2017 08:08 PM, Andres Freund wrote:
> Hi,
>
> On 2017-03-06 19:49:56 +0100, Tomas Vondra wrote:
>> 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.
>
> I'm not quite following here.  We only get a Datum and the knowledge
> that it's a pass-by-ref argument, so we really don't know that much.  We
> could create an "EmbeddedDatum" type that has a preceding chunk header
> (appropriately for the version), that just gets zeroed out at start.  Is
> that what you mean?
>

Yes, that's roughly what I meant.

>
>> 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?
>
> I don't recall any recent numbers.  I'm more than a bit doubful that it
> really matters - it's only used for the results of aggregate/window
> functions, and surely they've a good chunk of their own overhead...
>

And if the benefit is negligible, trying to keep the optimization might 
easily result in slowdown (compared to non-optimized code).

But I'm puzzled why we haven't seen any reports of failures? I mean, 
doing sum(int4) is not particularly extravagant thing, if there really 
is an issue, shouldn't we see a lot of reports? What are we missing?

regards

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