On Wed, Jul 12, 2017 at 1:08 PM, Claudio Freire <klaussfreire@gmail.com> wrote:
> On Wed, Jul 12, 2017 at 11:48 AM, Alexey Chernyshov
> <a.chernyshov@postgrespro.ru> wrote:
>> Thank you for the patch and benchmark results, I have a couple remarks.
>> Firstly, padding in DeadTuplesSegment
>>
>> typedef struct DeadTuplesSegment
>>
>> {
>>
>> ItemPointerData last_dead_tuple; /* Copy of the last dead tuple
>> (unset
>>
>> * until the segment is fully
>>
>> * populated). Keep it first to
>> simplify
>>
>> * binary searches */
>>
>> unsigned short padding; /* Align dt_tids to 32-bits,
>>
>> * sizeof(ItemPointerData) is aligned to
>>
>> * short, so add a padding short, to make
>> the
>>
>> * size of DeadTuplesSegment a multiple of
>>
>> * 32-bits and align integer components for
>>
>> * better performance during lookups into
>> the
>>
>> * multiarray */
>>
>> int num_dead_tuples; /* # of entries in the segment */
>>
>> int max_dead_tuples; /* # of entries allocated in the
>> segment */
>>
>> ItemPointer dt_tids; /* Array of dead tuples */
>>
>> } DeadTuplesSegment;
>>
>> In the comments to ItemPointerData is written that it is 6 bytes long, but
>> can be padded to 8 bytes by some compilers, so if we add padding in a
>> current way, there is no guaranty that it will be done as it is expected.
>> The other way to do it with pg_attribute_alligned. But in my opinion, there
>> is no need to do it manually, because the compiler will do this optimization
>> itself.
>
> I'll look into it. But my experience is that compilers won't align
> struct size like this, only attributes, and this attribute is composed
> of 16-bit attributes so it doesn't get aligned by default.
Doing sizeof(DeadTuplesSegment) suggests you were indeed right, at
least in GCC. I'll remove the padding.
Seems I just got the wrong impression at some point.