Re: [HACKERS] Block level parallel vacuum

On Fri, Oct 11, 2019 at 4:47 PM Mahendra Singh <mahi6run@gmail.com> wrote:
>
>
> I did some analysis and found that we are trying to free some already freed memory. Or we are freeing palloced memory in vac_update_relstats.
>     for (i = 0; i < nindexes; i++)
>     {
>         if (stats[i] == NULL || stats[i]->estimated_count)
>             continue;
>
>         /* Update index statistics */
>         vac_update_relstats(Irel[i],
>                             stats[i]->num_pages,
>                             stats[i]->num_index_tuples,
>                             0,
>                             false,
>                             InvalidTransactionId,
>                             InvalidMultiXactId,
>                             false);
>         pfree(stats[i]);
>     }
>
> As my table have 2 indexes, so we have to free both stats. When i = 0, it is freeing propery but when i = 1, then vac_update_relstats  is freeing memory.
>>
>> (gdb) p *stats[i]
>> $1 = {num_pages = 218, pages_removed = 0, estimated_count = false, num_index_tuples = 30000, tuples_removed = 30000, pages_deleted = 102, pages_free = 0}
>> (gdb) p *stats[i]
>> $2 = {num_pages = 0, pages_removed = 65536, estimated_count = false, num_index_tuples = 0, tuples_removed = 0, pages_deleted = 0, pages_free = 0}
>> (gdb)
>
>
> From above data, it looks like, somewhere inside vac_update_relstats, we are freeing all palloced memory. I don't know, why is it.
>

I don't think the problem is in vac_update_relstats as we are not even
passing stats to it, so it won't be able to free it.  I think the real
problem is in the way we copy the stats from shared memory to local
memory in the function end_parallel_vacuum().  Basically, it allocates
the memory for all the index stats together and then in function
update_index_statistics,  it is trying to free memory of individual
array elements, that won't work.  I have tried to fix the allocation
in end_parallel_vacuum, see if this fixes the problem for you.   You
need to apply the attached patch atop
v28-0001-Add-parallel-option-to-VACUUM-command posted above by
Sawada-San.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com