Re: [HACKERS] Block level parallel vacuum

On Tue, Feb 5, 2019 at 12:14 PM Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com> wrote:
>
>
> On Fri, Feb 1, 2019 at 8:19 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>>
>>
>> The passing stats = NULL to amvacuumcleanup and ambulkdelete means the
>> first time execution. For example, btvacuumcleanup skips cleanup if
>> it's not NULL.In the normal vacuum we pass NULL to ambulkdelete or
>> amvacuumcleanup when the first time calling. And they store the result
>> stats to the memory allocated int the local memory. Therefore in the
>> parallel vacuum I think that both worker and leader need to move it to
>> the shared memory and mark it as updated as different worker could
>> vacuum different indexes at the next time.
>
>
> OK, understood the point. But for btbulkdelete whenever the stats are NULL,
> it allocates the memory. So I don't see a problem with it.
>
> The only problem is with btvacuumcleanup, when there are no dead tuples
> present in the table, the btbulkdelete is not called and directly the btvacuumcleanup
> is called at the end of vacuum, in that scenario, there is code flow difference
> based on the stats. so why can't we use the deadtuples number to differentiate
> instead of adding another flag?

I don't understand your suggestion. What do we compare deadtuples
number to? Could you elaborate on that please?

> And also this scenario is not very often, so avoiding
> memcpy for normal operations would be better. It may be a small gain, just
> thought of it.
>

This scenario could happen periodically on an insert-only table.
Additional memcpy is executed once per indexes in a vacuuming but I
agree that the avoiding memcpy would be good.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center